The Human Body and Health

Genetics is the study of heredity, exploring how traits or characteristics are passed from parents to their offsprings through genes. Do you want to know the science behind why you might have your mom’s eye or your dad’s hair colour? Join me as we explore the concept of heredity.

Heredity is the process through which traits or characteristics are passed from parents to their offsprings. These traits are determined by genes which are tiny units made of DNA in our cells. Genes control everything from our eye colour to how our body functions, and they are inherited from both the mother and father.

In heredity, children get a mixture of genes from both parents. Some characteristics are controlled by dominant genes, which are more likely to show up in the child. While others are controlled by recessive genes which only show up if both parents pass them down.

Understanding heredity is important in genetics because it helps predict the likelihood of certain characteristics appearing in future generations.

KEY IDEAS

• Heredity and Genes: Heredity is the passing of traits from parents to offspring through genes. Genes are small units of DNA that determine characteristics like eye and hair colour.

• Dominance and Recessiveness: Traits are controlled by dominant and recessive genes.

Dominant genes are more likely to appear in the offspring, while recessive genes only appear if both parents pass them down.

• Mendel’s Law of Dominance: This law states that in a cross between two parents with contrasting traits, only the dominant trait will appear in the F1 generation. The recessive trait is masked but can reappear in later generations

• Mendel’s Law of Segregation: During the formation of gametes (reproductive cells), the two alleles for a trait separate from each other, so each gamete receives only one allele.

• Mendel’s Law of Independent Assortment: Alleles for different traits segregate independently during gamete formation. This means the inheritance of one trait does not affect another, leading to more genetic variation in offspring.

• Polygenic Inheritance: Many complex traits, such as human height and skin colour, are controlled by multiple genes. This results in a continuous range of phenotypes rather than distinct categories.

These terms provide a foundational understanding of genetics and are essential for studying the inheritance of traits and genetic variation.

1. Chromosomes: These are thread-like structures made up of nucleic acids (DNA) and proteins. They are mostly found in the nucleus of the cells. They carry the hereditary or genetic information in the form of genes. Humans have 46 chromosomes (23pairs).

They determine genetic traits and biological sex (XX for females, XY for males)

Figure 8.1: Chromosome

2. Gene: A small part of DNA that contains or gives instructions for building a specific protein or traits performing a specific function in an organism. Or basic units of inheritance responsible for the transmission of characters from parents to offspring.

Figure 8.2: A gene

3. Allele: Different forms or variants of a gene. Alleles can be dominant or recessive.

They are inherited in pairs – one from each parent.

Figure 8.3: An Allele

4. Genotype: The genetic makeup of an organism, often represented by the combination of alleles for a particular gene. They determine inherited traits

5. Phenotype: The observable physical or biochemical characteristics of an organism, which result from the interaction of its genotype with the environment (diet, lifestyle, climate).

6. Homozygous: When an individual has two identical alleles for a particular gene (e.g., AA or aa).

Figure 8.4: Homozygous

7. Heterozygous: When an individual has two different alleles for a particular gene (e.g., Aa).

8. Dominant: An allele that, when present, will determine the phenotype, even if a recessive allele is also present. They override or mask the effect of recessive allele in heterozygous pairing. Dominant alleles are typically represented with capital letters (e.g., A).

9. Recessive: An allele that is only expressed when two copies are present (homozygous).

Recessive alleles are typically represented with lowercase letters (e.g., a).

10. DNA (Deoxyribonucleic Acid): The molecule or a material in your cells that carries all your genetic information

11. Mutation: A permanent change in the DNA sequence of an organism. Mutations can be beneficial, harmful, or have no effect on an organism.

12. Genetic Trait: A specific characteristic or feature (ranging from physical features to behavioural tendencies) that is determined by one or more genes. They are passed from parents to offspring

13. Codominance: When two different alleles for a gene are both fully and equally expressed in the heterozygous phenotype. The offspring displays both parental traits simultaneously rather than a blended intermediate.

14. Incomplete Dominance: When the phenotype of a heterozygous individual is intermediate between the two homozygous forms. Neither allele is fully dominant over the other. This results in a heterozygous phenotype that is a blend that is distinct from homozygous parents. For example, in incomplete dominance, a red flower crossed with a white flower might produce pink flowers.

15. Pedigree: A chart or family tree that tracks the inheritance of a particular trait through generations.

16. Genetic Disorder: A condition caused by abnormal or mutated genes, leading to health problems or abnormal traits. Examples include cystic fibrosis, Down syndrome, and sickle cell anaemia.

17. Filial generation: The offspring of parents make up the filial generation. The first, second and third generations of offspring are known as the first, second and third filial generations respectively and denoted by the symbols F1, F2 and F3 respectively, too.

Heritable / Inheritable Characters Or Traits In Human Beings In both humans and plants, only traits that are part of the genetic makeup of the parents can be transmitted and expressed in the offspring. In humans, these inheritable traits include physical features such as skin colour, eye colour, hair colour and texture, body size and stature, shape of the head, ears, mouth, lips, and nose, as well as the length of the neck, arms, and legs. Other transmissible traits include blood group, baldness, tongue rolling ability, voice tone, intelligence, temperament, attitude, and certain genetic disorders like sickle cell anaemia and haemophilia. Some of these traits are controlled by single genes, while others are polygenic or influenced by environmental factors. These traits are passed on through genetic inheritance and can be selectively bred for desirable characteristics.

Activity 8.1 Match & Explain In a small group with 5 classmates, perform this activity.

What you need

• Manila Card/Printed cards

• Makers/Pens

• Scissors What to do

1. Revisit your Year 2 work on Reproduction in Humans.

2. Create 10 sets of 3 blank cards. For each set, write the name of a reproductive organ (e.g. uterus) or a process (e.g. ovulation) on one, the definition of this on another and a diagram demonstrating it on another.

3. Place the cards face up on the ground and mix them all up so that they are randomly distributed.

4. Present the cards to another team to solve by putting them back into their sets.

Equally, you should solve the cards that another team has made.

Key questions

a. What happens if ovulation does not occur?

b. Why is the uterus lining important?

Activity 8.2 Exploring Human Inheritance – Traits in Your Family In a small group with 4 classmates, perform this activity Aim: To investigate how certain human traits are inherited and apply knowledge of genetics to understand patterns of inheritance.

What you need

• Pen/pencil

• Worksheet or notebook

• Mirror (for observing your own traits)

• Optional: Photos of family members (for trait comparison) What to do

1. Trait Survey: Observe the following traits in yourself and at least 2 biological family members (e.g., parents or siblings) Trait You Family Member 1 Family Member 2 Earlobes (Attached/Free) Tongue rolling (Yes/No) Dimples (Yes/No) Widow’s peak (Yes/No) Hitchhiker’s thumb (Yes/No)

2. Record your data.

3. Fill in the table with what you observe.

Key questions (Write down your answer in your notebook)

a. Which of your traits are dominant?

b. Are there traits you have that neither parent has? If so, how might that be explained?

c. Using a Punnett square, predict the possible genotypes and phenotypes for one trait (e.g., tongue rolling) based on your parents’ traits.

Worked Example

If one parent is heterozygous (Rr) and the other is homozygous recessive (rr):

Genotype Ratio: 2 Rr: 2 rr Phenotype Ratio: 2:2

Inherited characters are determined by genes, which are segments of DNA located on chromosomes. In diploid organisms, with two sets of chromosomes, each gene exists in two copies, located at identical positions (called loci) on homologous chromosome pairs.

These organisms produce reproductive cells, or gametes, through a process called meiosis, which occurs in their reproductive organs.

During meiosis, the number of chromosomes is halved, resulting in haploid gametes: sperm cells in males and egg cells in females. Each gamete contains only one set of chromosomes and thus only one copy of each gene.

During sexual reproduction, a male and a female gamete fuse in a process called fertilisation to form a zygote, which is diploid, receiving one set of chromosomes from each parent.

This is how genes, and the traits they control, are transmitted from parents to offspring. The individuals that produce gametes make up the parental (P) generation, while their offspring are referred to as the first filial (F1) generation.

The process of mating or sexual union is called a cross (represented as “×”). A gene controlling a particular trait may exist in alternative forms called alleles, which may result in different expressions of the trait.

The science of heredity and genetic inheritance was founded by Gregor Mendel (1822– 1884), who is regarded as the father of genetics. Mendel conducted experiments using pea plants and developed the principles of inheritance using two main methods: monohybrid inheritance, which involves a single pair of contrasting traits, and dihybrid inheritance, which involves two pairs of contrasting traits.

Through his work, Mendel established the foundational laws of inheritance that explain how traits are passed from generation to generation.

Inheritance of one Gene (Monohybrid Inheritance) Gregor Mendel, often hailed as the “father of genetics,” laid the foundational principles of heredity through his experiments with garden pea plants (Pisum sativum).

His studies on monohybrid inheritance involved crossing plants that differed in only one specific, contrasting character at a time, such as tall versus short plant height. These experiments provided crucial insights into how traits are passed from one generation to the next.

The Parental (P) Generation Cross

Mendel began his experiments with true-breeding (or purebred) pea plants. A true-breeding plant consistently produces offspring identical to itself when self-pollinated over many generations, ensuring that it carries only one type of allele for the trait in question. For his monohybrid cross focusing on plant height, Mendel selected:

a. True-breeding tall pea plants: These plants always produce tall offspring when self-pollinated. Their genetic makeup for height can be represented as TT (homozygous dominant).

b. True-breeding dwarf pea plants: These plants always produced dwarf offspring when self-pollinated. Their genetic makeup can be represented as tt (homozygous recessive).

Mendel then performed an artificial cross-pollination (or hybridization) between these two contrasting true-breeding parental types. He carefully transferred pollen from the flowers of a tall plant to the stigma of a dwarf plant (and vice-versa, performing reciprocal crosses to ensure the results were consistent regardless of which parent contributed the pollen).

The First Filial (F1) Generation: A Glimpse of Dominance The seeds resulting from this initial cross were collected and planted to grow the F1 generation (First Filial generation). Mendel observed a striking and consistent result: all the plants in the F1 generation grew to be tall.

This observation was groundbreaking because the dwarf trait seemed to have disappeared entirely.

Mendel deduced that one trait must be “masking” or “dominating” the other. He termed the trait that was expressed in the F1 generation (tallness) the dominant trait, and the trait that disappeared (dwarfness) the recessive trait. Using his symbolic representation, if ‘T’ represents the allele for tallness and ‘t’ represents the allele for dwarfness:

a. The true-breeding tall parent contributed only ‘T’ alleles to its gametes.

b. The true-breeding dwarf parent contributed only ‘t’ alleles to its gametes.

c. Therefore, all F1 offspring inherited one ‘T’ allele from the tall parent and one ‘t’ allele from the dwarf parent, resulting in a heterozygous genotype of Tt.

d. Despite having the ‘t’ allele, all F1 plants were phenotypically tall, identical to the homozygous dominant (TT) parent, confirming the dominance of the ‘T’ allele.

The Second Filial (F2) Generation: The Principle of Segre- gation Unveiled To further investigate why the dwarf trait reappeared, Mendel allowed the plants of the F1 generation to self-pollinate. The seeds produced from these self-pollinating F1 plants were then collected and planted, giving rise to the F2 generation (Second Filial generation). The results from the F2 generation were even more revealing:

a. In the F2 generation, Mendel observed both tall plants and dwarf plants.

b. Crucially, he found that these traits appeared in a remarkably consistent ratio:

approximately three-quarters (¾) of the plants were tall, and one-quarter (¼) of the plants were dwarf. This is famously known as the 3:1 phenotypic ratio for a monohybrid cross.

This re-emergence of the recessive dwarf trait in the F2 generation, after its complete absence in the F1, led Mendel to propose the Law of Segregation. This law states that during gamete formation, the two alleles for a heritable character (e.g., T and t) separate (segregate) from each other so that each gamete carries only one allele.

When the F1 plants (Tt) self-pollinate, their gametes (T and t) randomly combine, leading to the observed 1:2:1 genotypic ratio (TT: Tt: tt) and the 3:1 phenotypic ratio in the F2 generation.

Mendel’s monohybrid crosses, with their simple design and clear mathematical ratios, provided the foundational evidence for the existence of discrete heritable factors (what we now call genes and alleles) and established the basic rules by which these factors are transmitted from one generation to the next.

The genotype for F1 is Tt The phenotype: all tall, but they are heterozygous (not pure breed). From this he concluded that T is the dominant allele which expresses itself irrespective of the second allele present.

In F2 generation, tall plants with genotype TT and Tt appeared while dwarf plants with tt gene appeared. He concluded that ‘t’ is the recessive allele and it can only express itself in the presence of the same recessive allele. Mathematically he calculated that in F2 generation ¼ tall plants were with genotype TT while ½were Tt and ¼ were tt. However, Tt and TT was tall, so phenotypically ¾ of the plants were tall while the only ¼ of them were phenotypically and genotypically dwarf.

The phenotypic ratio was calculated to be ¾ of tall: ¼of dwarf i.e., 3: 4. The genotypic ratio was calculated to be ¼: ½: ¼ (TT: Tt: tt), i.e. 1: 2: 1.

Mendel concluded all this with the help of Punnett square. The cross explained is a monohybrid cross as inheritance of only one trait is considered at one time only. It is monohybrid and an example of complete dominance.

If there is self-pollination to obtain second generation, we get the outcome below;

Genotypic ratio: 1: 2:1 Phenotype: 3: 1 Mendel’s Findings Gregor Mendel’s experiments with pea plants led to profound discoveries about heredity, which he explained through a series of observations and fundamental principles. His work, though initially overlooked, laid the foundation for modern genetics.

Summary of Mendel’s Observations and Ratios

Through his monohybrid crosses (as seen with tall and dwarf pea plants), Mendel made several crucial observations:

a. The F1 Generation’s Uniformity: When he crossed two true-breeding parents differing in a single contrasting trait (e.g., pure tall TT with pure dwarf tt), the entire First Filial (F1) generation uniformly resembled only one of the two parental traits. For instance, all F1 pea plants were tall, even though one parent was dwarf. This indicated that one trait was expressed while the other seemed to disappear.

b. The F2 Generation’s Re-emergence and Ratios: When the F1 generation plants (which were all phenotypically dominant) were allowed to self-pollinate, the Second Filial (F2) generation exhibited a predictable pattern

i. Phenotypic Ratio: Both the dominant and recessive forms of the trait reappeared. Roughly three-quarters (¾) of the F2 plants displayed the dominant phenotype (e.g., tall), while one-quarter (¼) displayed the recessive phenotype (e.g., dwarf). This established a consistent 3:1 phenotypic ratio (dominant: recessive).

ii. Genotypic Ratio: Further analysis (later confirmed by test crosses) revealed that the F2 generation possessed a specific genotypic ratio of 1:2:1. This ratio represented 1-part homozygous dominant (e.g., TT), 2 parts heterozygous (e.g., Tt), and 1-part homozygous recessive (e.g., tt). This showed that while the outward appearance (phenotype) was 3:1, the underlying genetic makeup (genotype) was more varied.

Interpretation of Mendel’s Results

Mendel’s genius lay not just in his careful experiments but also in his ability to interpret these numerical patterns. He proposed that hereditary characteristics are controlled by discrete “factors” (what we now call genes), which are transmitted from parents to offspring.

Each factor, or gene, contains specific information about a particular form of a trait. He understood that these factors exist in pairs within an individual and that they somehow separate and combine predictably.

Inheritance of Two Genes (Dihybrid Inheritance)

Following his groundbreaking work on monohybrid inheritance, Gregor Mendel further expanded his understanding of heredity by studying the simultaneous inheritance of two different traits at a time. This type of genetic investigation is known as a dihybrid cross, and it proved pivotal in formulating his second major law of inheritance.

The Parental (P) Generation Cross: Setting the Stage

Mendel began his dihybrid cross by selecting true-breeding pea plants that differed in two distinct pairs of contrasting characters. He chose:

a. Plants with Round and Yellow seeds: These were true-breeding for both traits, meaning they always produced round, yellow seeds when self-pollinated. Their genotype was represented as RRYY (homozygous dominant for both traits).

b. Plants with Wrinkled and Green seeds: These were true-breeding for their respective traits, always producing wrinkled, green seeds. Their genotype was represented as rryy (homozygous recessive for both traits).

Mendel then performed an artificial cross-pollination between these two contrasting parental types (RRYY x rryy).

The First Filial (F1) Generation: All Dominant Phenotypes

The seeds resulting from this cross were planted, yielding the F1 generation. All the plants in this F1 generation exhibited the dominant phenotype for both traits: they all produced Round and Yellow seeds.

This result was consistent with Mendel’s Law of Dominance, which he had established from his monohybrid crosses. Each F1 individual inherited one allele for seed shape (R) and one for seed colour (Y) from the RRYY parent, and one allele for seed shape (r) and one for seed colour (y) from the rryy parent. This made all F1 individuals heterozygous for both traits, with a genotype of RrYy.

Gamete Formation in the F1 Generation: The Law of In- dependent Assortment The most crucial stage for understanding the dihybrid cross lies in the formation of gametes by these F1 (RrYy) individuals. Here, Mendel’s observations led to his second major principle: The Law of Independent Assortment.

a. Principle: This law states that alleles for different traits (e.g., the alleles for seed shape, R/r, and the alleles for seed colour, Y/y) segregate independently of each other during gamete formation (meiosis). This means that the inheritance of one trait does not influence the inheritance of the other trait.

b. Mechanism: When an F1 individual (RrYy) produces gametes, the ‘R’ allele segregates from the ‘r’ allele, and the ‘Y’ allele segregates from the ‘y’ allele.

Crucially, these two segregation events happen independently. Therefore, a gamete can receive any combination of one allele from each gene pair. For

example, a gamete could receive ‘R’ along with ‘Y’ (RY). Or, it could receive ‘R’ along with ‘y’ (Ry). Similarly, it could receive ‘r’ along with ‘Y’ (rY). Or, it could receive ‘r’ along with ‘y’ (ry).

c. Gamete Ratios: Because of independent assortment, these four types of gametes (RY, Ry, rY, ry) are produced in approximately equal proportions, each representing 25% or ¼th of the total gametes produced by an F1 (RrYy) individual.

The Second Filial (F2) Generation (The Characteristic

9:3:3:1 Ratio) Mendel allowed the F1 (RrYy) plants to self-pollinate. The seeds produced from this self- pollination yielded the F2 generation, which displayed a variety of phenotypes, not just the two parental types. By counting the number of individuals in each phenotypic category, Mendel consistently observed a characteristic 9:3:3:1 phenotypic ratio:

a. 9 parts: Round and Yellow seeds (dominant for both traits)

b. 3 parts: Round and Green seeds (dominant for shape, recessive for colour)

c. 3 parts: Wrinkled and Yellow seeds (recessive for shape, dominant for colour)

d. 1 part: Wrinkled and Green seeds (recessive for both traits) This complex ratio, derived from the random combination of the four types of gametes (RY, Ry, rY, ry) in a 16-square Punnett square (an extension of the 2x2 monohybrid square), provided strong evidence for the independent assortment of different genetic traits.

It demonstrated that alleles for different genes are sorted into gametes independently of one another, leading to a wider range of phenotypic combinations in the offspring than would be possible if traits were inherited together. Mendel’s dihybrid cross was a massive achievement, revealing the elegant predictability of trait transmission a round yellow and wrinkled green seeds, F2 cross pollinated each other RY Ry rY ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy Interpretation of Mendel’s Results Gregor Mendel’s meticulous work with pea plants led to the formulation of fundamental principles that govern how traits are passed from one generation to the next. These principles, often referred to as Mendel’s Laws, alongside specific types of genetic crosses, are crucial for understanding inheritance patterns and determining the genetic makeup of individuals.

Mendel’s Laws of Inheritance

ₐ. The Law of Dominance: This law states that when an individual possesses two different alleles for a single inherited trait, one allele (the dominant allele) will mask the expression of the other allele (the recessive allele), determining the observable characteristic or phenotype. The recessive allele remains present in the genotype but is not expressed phenotypically. This principle is evident in the F1 generation of a monohybrid cross between two true-breeding parents. For instance, when a true-breeding tall pea plant (TT) is crossed with a true-breeding dwarf pea plant (tt), all F1 offspring are tall (Tt). The allele for tallness (T) is dominant and completely suppresses the expression of the allele for dwarfness (t) in the heterozygous state.

b. The Law of Segregation: This law states that during the formation of gametes (sperm and egg cells) in sexually reproducing organisms, the two alleles for a heritable character (e.g., T and t) separate (segregate) from each other so that each gamete receives only one allele. This segregation occurs during meiosis.

For example, a heterozygous F1 plant (Tt) will produce two types of gametes:

50% carrying the ‘T’ allele and 50% carrying the ‘t’ allele. The alleles do not blend or permanently influence each other; they remain discrete units and simply separate during gamete formation. This random segregation ensures that each offspring receives a unique combination of alleles, leading to the reappearance of recessive traits in the F2 generation.

c. The Law of Independent Assortment: This law states that alleles for different genes (i.e., genes controlling different traits) segregate independently of each other during gamete formation. This means that the inheritance of one trait does not influence the inheritance of another distinct trait.

This principle applies to dihybrid crosses (where two or more traits are studied simultaneously). For example, in a cross-involving seed colour (yellow/green) and seed shape (round/wrinkled), the way the alleles for colour separate into gametes is independent of how the alleles for shape separate. An individual heterozygous for both traits (RrYy) will produce gametes containing all possible combinations of alleles (RY, Ry, rY, ry) in equal proportions, as if each gene were inherited alone. This independence leads to greater genetic variation in offspring.

Determining Parental Genotypes: Back Cross and Test

Cross In genetics, it is often crucial to determine the specific genotype of an individual, particularly if they exhibit a dominant phenotype, as this phenotype can result from either a homozygous dominant (e.g., TT) or a heterozygous (e.g., Tt) genotype. This uncertainty can be resolved using specific types of genetic crosses.

1. Back Cross

A back cross is a general term for a cross where the F1 progeny (first filial generation offspring) is mated back to either one of its parentals (P) genotypes.

Back crosses are commonly used in breeding programs to introduce a specific trait from a wild relative into a cultivated crop, or to rapidly develop inbred lines (a group of genetically identical homozygous organisms produced by repeated self-fertilization between close relatives over several generations).

F1 x Dominant Parent (e.g., Tt x TT): This type of back cross is used to quickly increase the proportion of offspring carrying the dominant allele, often seen in livestock breeding to improve desirable traits.

F1 x Recessive Parent (e.g., Tt x tt) : This specific type of back cross is also known as a Test Cross (see below) and is particularly informative for determining genotype.

2. Test Cross

A test cross is a specialised type of back cross where an individual displaying the dominant phenotype (but of unknown genotype, e.g., T?) is crossed with a homozygous recessive individual (e.g., tt) for the same trait. The primary purpose of a test cross is to determine the unknown genotype (whether homozygous dominant or heterozygous) of the parent showing the dominant phenotype.

If the unknown parent is homozygous dominant (TT), all F1 offspring from the test cross (TT x tt) will be heterozygous (Tt) and will express the dominant phenotype (100% dominant phenotype).

If the unknown parent is heterozygous (Tt), then crossing Tt x tt will produce offspring in a 1:1 phenotypic ratio of dominant to recessive traits (e.g., 50% tall (Tt) and 50% dwarf (tt)).

The appearance of any recessive offspring immediately reveals that the unknown parent must have been heterozygous. Test crosses are widely used in agriculture (plant and animal breeding) to confirm the genetic purity of breeding lines or to identify desirable heterozygotes.

Polygenic Inheritance

Polygenic Inheritance: The Basis of Continuous Variation

Beyond the simple inheritance patterns described by Mendel’s monohybrid and dihybrid crosses, many complex traits in organisms are influenced by the combined action of multiple genes. This phenomenon is known as polygenic inheritance.

Polygenic inheritance occurs when the expression of a single phenotypic trait is controlled by the collective effect of more than one gene. These genes are often located at different loci (positions) on the same or different chromosomes. Because multiple genes are involved, this mode of inheritance is also referred to as quantitative inheritance or multiple factor inheritance.

Characteristics of Polygenic Traits

Polygenic traits exhibit several distinctive features that differentiate them from Mendelian traits (which are typically controlled by a single gene):

1. Continuous Variation: Unlike Mendelian traits that usually fall into distinct categories (e.g., tall or dwarf, yellow or green), polygenic traits show a continuous range or spectrum of phenotypes within a population. This variation can be measured and often follows a bell-shaped curve (normal distribution). There are many intermediate forms between the two extremes.

2. Additive Effect of Alleles: The genes involved in polygenic inheritance typically contribute to the phenotype in an additive manner. This means that each “contributing allele” (often the dominant allele) adds a small, incremental, or cumulative effect to the phenotype. The more contributing alleles an individual possesses, the more pronounced the trait becomes. There is generally no complete dominance or recessiveness among the individual alleles across all the genes involved; rather, their effects sum up.

3. Multifactorial Influence (Gene-Environment Interaction): Many polygenic traits are also multifactorial traits. This implies that while multiple genes play a significant role, environmental factors (such as nutrition, lifestyle, exposure to sunlight, disease) can also interact with the genetic predisposition to modify the final phenotype. The environment can either enhance or diminish the expression of the genetic potential.

Examples of Polygenic Inheritance in Humans

Human traits provide excellent illustrations of polygenic inheritance.

1. Height: Human height is a classic example of a polygenic trait. We do not observe just two distinct categories like “tall” and “short” in the population. Instead, there’s a wide range of heights, from very short to very tall, with many intermediate measurements.

This continuous variation reflects the influence of hundreds of genes, each contributing a small amount to an individual’s adult height.

An individual’s height depends on the cumulative effect of the “tall” alleles inherited from both parents across these numerous genes (Additive Effect).

While genetics sets the potential range, environmental factors like adequate nutrition during growth, lack of chronic illness, and general health significantly influence whether an individual reaches their full genetic height potential

2. Skin Colour: Human skin pigmentation is another well-known example of polygenic inheritance, determined by the amount and type of melanin produced in the skin.

It is influenced by multiple gene pairs (often modelled with three, but potentially up to six or more genes in reality) that contribute to melanin production. Using a simplified model with three gene pairs (A/a, B/b, C/c), where uppercase alleles (A, B, C) contribute to darker skin (more melanin) and lowercase alleles (a, b, c) contribute to lighter skin (less melanin):

a. The genotype aabbcc would represent the very lightest skin colour (zero contributing alleles). The genotype AABBCC would represent the very darkest skin colour (six contributing alleles). All other combinations represent intermediate shades.

b. For example, a person with genotype AaBbCc (three contributing alleles) would have a medium skin tone. A person with AABbcc (four contributing alleles) would be darker than medium but lighter than AABBCC. The effect of each contributing allele is additive, meaning the total number of uppercase alleles directly correlates with the degree of skin pigmentation.

c. This additive effect across multiple genes results in the vast and continuous spectrum of human skin colours observed globally, from very light to very dark, with countless intermediate shades (Continuous Spectrum). Environmental factors like sun exposure (tanning) can further modify the expressed phenotype of skin colour, highlighting its multifactorial nature.

Have you ever wondered why certain diseases seem to “run in families”? Maybe you’ve heard someone say, “Diabetes runs in my family” or “My mom and grandma both had breast cancer.” These are examples of what scientists call hereditary diseases; health conditions that are passed from parents to their children through genes.

To understand hereditary diseases, we first need to understand a bit about genetics. Every human has DNA, which contains genes; the instructions that determine everything from our eye colour to how our bodies work. We inherit half of our genes from our mother and the other half from our father. Sometimes, genes can carry mutations, or small changes, that may cause a disease. If these faulty genes are passed on to children, they can also inherit the risk of developing the same disease.

KEY IDEAS

• How genes and inheritance work

• Examples of common hereditary diseases

• Types of Hereditary diseases The Role of Genes in Heredity Heredity is the process by which traits and characteristics are passed from parents to their children. This happens through genes, which are small sections of DNA found in our chromosomes. Genes carry the instructions for how our bodies grow, develop, and function.

Each person inherits two copies of every gene; one from their mother and one from their father. Sometimes, changes or mutations in these genes can lead to genetic disorders.

These disorders can be passed down from generation to generation, depending on how the faulty gene is inherited.

Examples of Hereditary Diseases

a. Cystic Fibrosis: a recessive disorder (genetic condition that occurs when an individual inherits two nonworking copies of a gene – one from each parent) that affects the lungs and digestive system.

b. Huntington’s Disease: a dominant disorder (genetic disorder caused by the mutation in one copy of a gene from either parent) that affects the brain and movement. The mutation is caused on a non-sex chromosome

c. Haemophilia: an X-linked disorder (genetic condition caused by the mutations in genes on the X chromosomes) that affects the blood’s ability to clot.

Types of Hereditary Diseases

₁. Single-Gene (Monogenic) Disorders: These are caused by changes (mutations) in just one gene. The disorder can be inherited in different ways:

a. Dominant: One copy of the faulty gene causes the disease. Example: Huntington’s disease

b. Recessive: Two copies of the faulty gene are needed. Example Cystic fibrosis

c. X-linked: The gene is on the X chromosome. Examples Haemophilia, Duchenne muscular dystrophy etc.

2. Chromosomal Disorders: These are caused by problems (abnormalities) with whole chromosomes; such as missing, extra, or broken chromosomes. Examples are Down syndrome and Turner syndrome.

3. Multifactorial (Complex) Disorders: These are caused by a combination of genes and environmental factors, such as lifestyle or diet. They don’t follow simple inheritance patterns. Examples are heart disease diabetes and some types of cancer, etc.

There are various inheritance patterns that determine how hereditary diseases are passed on.

In some cases, a person can carry a gene with a defect without experiencing any symptoms;

this is known as being a carrier. This happens because the normal version of the gene can compensate for the defective one. However, if a child inherits two defective copies of a gene (one from each parent), they are likely to develop the associated disorder.

This explains why sometimes a child can be born with a genetic disease even when neither parent shows any signs of illness. The condition only manifests when both parents unknowingly carry the same defective gene and pass it on to their child. This pattern is common in recessive genetic disorders.

Examples of Hereditary Diseases

There are many well-known examples of hereditary diseases. These include:

a. Hereditary Hemochromatosis: a disorder causing the body to absorb too much iron, leading to organ damage.

b. Down Syndrome: a chromosomal condition caused by an extra copy of chromosome 21, leading to developmental delays and physical characteristics.

c. Sickle Cell- anaemia: a blood disorder where red blood cells are abnormally shaped, leading to pain and reduced oxygen transport.

d. Turner Syndrome: a chromosomal condition affecting females, where one of the X chromosomes is missing or incomplete, impacting growth and development.

Some Other Types of Hereditary Diseases and

Organs Affected

1. Cystic Fibrosis

Cystic Fibrosis (CF), also known as mucoviscidosis, is a serious, inherited genetic disorder that profoundly impacts the body’s exocrine glands. It leads to the production of abnormally thick, sticky mucus and excessively salty sweat. This viscous mucus obstructs various ducts and passageways, particularly in the respiratory, digestive, and reproductive systems, leading to a range of complications. CF is caused by a defect in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene.

This gene provides instructions for creating the CFTR protein, which acts as a channel controlling the movement of chloride ions and water in and out of cells. In individuals with CF, the mutated CFTR gene produces a faulty protein that either doesn’t work correctly or isn’t produced at all. This dysfunction disrupts the delicate balance of salt and water transport, causing secretions (like mucus, sweat, and digestive juices) to become dehydrated, thick, and sticky.

Instead of acting as lubricants, these secretions plug up tubes and ducts, leading to significant organ damage over time.

2. Organs Affected

a. Lungs: The thick mucus clogs the small airways, making it difficult to breathe and creating an ideal environment for bacterial infections. This leads to chronic lung infections, inflammation, and progressive lung damage, including bronchiectasis (widening of the airways).

b. Pancreas: The mucus blocks the ducts that carry digestive enzymes from the pancreas to the small intestine. This prevents proper digestion and absorption of fats, proteins, and vitamins, leading to malnutrition, poor growth, and sometimes cystic fibrosis-related diabetes (CFRD).

c. Intestines: Blockages can occur, particularly in newborns (meconium ileus) and older children/adults (distal intestinal obstruction syndrome), causing severe constipation and abdominal pain.

d. Liver: Blockage of bile ducts can lead to inflammation and scarring (cirrhosis) in a small percentage of individuals.

e. Sweat Glands: The faulty CFTR protein also affects sweat glands, leading to abnormally salty sweat. This is a key diagnostic indicator.

f. Reproductive System: In men, thick mucus can block or cause the absence of the vas deferens (the tube that carries sperm), leading to infertility in almost all cases. Women with CF may experience reduced fertility but can often conceive and have successful pregnancies.

3. Down Syndrome

Down syndrome, also known as Trisomy 21, is a genetic condition caused by the presence of extra genetic material from chromosome 21. This chromosomal abnormality results in a unique set of physical characteristics, intellectual disabilities, and developmental delays, the severity of which can vary significantly among individuals. It is the most common chromosomal disorder leading to intellectual disability in children.

Causes of Down Syndrome

Down syndrome is caused by abnormal cell division that leads to an extra full or partial copy of chromosome 21. While the exact reason for these cell division errors is not fully understood, they can occur in one of three ways:

a. Trisomy 21 (Nondisjunction): This is the most common cause, accounting for about 95% of Down syndrome cases. In Trisomy 21, an individual has three copies of chromosome 21 in every cell of their body, instead of the usual two. This occurs due to an error in cell division during the formation of the sperm or egg cell (meiosis), where a pair of chromosomes 21 fails to separate. When this egg or sperm fuses with a normal gamete, the resulting embryo has three copies of chromosome 21.

b. Mosaic Down Syndrome: This is a rare form of Down syndrome, affecting about 1% of individuals with the condition. In mosaic Down syndrome, a person has a mix of cells, some with the usual two copies of chromosome 21 and others with three copies. This “mosaic” pattern is caused by an abnormal cell division error that occurs after fertilization, early in embryonic development. Individuals with mosaic Down syndrome may have fewer characteristics of Down syndrome than those with Trisomy 21 because of the presence of some normal cells.

c. Translocation Down Syndrome: This form accounts for about 4% of Down syndrome cases. In translocation Down syndrome, part of chromosome 21 breaks off during cell division and attaches (translocate) to another chromosome, most commonly chromosome 14, 21, or 22. Individuals with translocation Down syndrome have the usual two full copies of chromosome 21, but they also have extra genetic material from chromosome 21 attached to another chromosome.

This can occur spontaneously or, rarely, can be inherited from a parent who is a “balanced carrier” of the translocation. A balanced carrier has the rearranged chromosome but no extra or missing genetic material, so they do not have Down syndrome themselves, but they can pass on the unbalanced translocation to their children. Only about one-third of translocation Down syndrome cases are inherited.

4. Asthma Asthma is a long-term (chronic) condition that affects the lungs. It makes it hard to breathe because the airways (tubes that carry air in and out of the lungs) become;

a. Swollen (inflamed)

b. Tight (the muscles around the airways squeeze)

c. Full of mucus (a sticky substance) What Triggers Asthma?

_(A) _(trigger) is something that causes asthma symptoms to start or get worse.

Common triggers:

a. Dust, pollen, mould, pet hair

b. Cold air

c. Smoke or pollution

d. Exercise

e. Strong smells (like perfume or cleaning chemicals)

f. Stress or strong emotions

g. Infections (like a cold or flu)

h. Some medicines (like aspirin) Common Symptoms of Asthma:

ₐ. Shortness of breath

b. Wheezing (whistling sound when breathing)

c. Chest tightness

d. Coughing, especially at night or early morning Asthma symptoms can come and go. Sometimes, the symptoms get worse very quickly — this is called an asthma attack Types of Asthma Asthma is not the same for everyone. There are different types, depending on what causes it or how it shows up.

a. Allergic Asthma: Caused by allergies (e.g., dust, pollen, animals)

i. Usually starts in childhood

ii. Often linked to other allergic problems like eczema or hay fever

b. Non-Allergic Asthma: Not caused by allergies.

i. Triggered by smoke, cold air, stress, or illness

ii. More common in adults

c. Exercise-Induced Asthma

i. Triggered by physical activity or sports

ii. You may feel breathless or start coughing during or after exercise

d. Occupational Asthma

i. Caused by breathing in chemicals or dust at work

ii. Gets better when you’re away from work

e. Cough-Variant Asthma

i. The main symptom is a dry cough

ii. There’s usually no wheezing

f. Aspirin-Induced Asthma

i. Triggered by taking aspirin or certain painkillers

ii. Often happens with sinus problems or nasal polyps

g. Nocturnal Asthma: Symptoms get worse at night while you are sleeping

h. Severe Asthma

i. Hard to control even with strong medication

ii. Needs special treatment from a doctor

Activity 8.3 Exploring a Sex-Linked Recessive Hereditary Disorder

Perform this activity in a group of five members or alone.

Aim: To analyse real-life case studies to understand the symptoms, progression and impact of hereditary disease What you need

• A printout of a simplified real-life story with details of family history, early symptoms, diagnosis process and impact on lifestyle and family dynamics.

• Notebooks

• Pen/Pencils What to do

1. Analyse this real-life scenario carefully about a 15-year-old boy

2. Use these questions as a guide, discuss and write down your findings for presentation after analysing the scenario.

a. What clues suggested the 15-year-old boy might have a genetic disorder?

b. How is haemophilia inherited? Draw a Punnett square using an X-linked trait?

c. How did his condition affect his daily life and emotional well-being?

d. How did the family respond, and what support systems helped them?

e. What advice would you give to the boy or his family?

The Scenario: Background & Family History A 15-year-old boy from the Eastern Cape. He lives with his mother, grandmother, and younger sister. His family never knew that haemophilia ran in their bloodline, but his mother recalls that her younger brother died from internal bleeding after a minor accident when he was a teenager. At the time, no one knew the cause.

Because haemophilia is inherited through the X chromosome, his mother is likely a carrier and unknowingly passed the faulty gene to her son.

Early Symptoms

When he was about 2 years old, his mother noticed that he would

a. Bleed from the gums during brushing that took a long time to stop.

b. Have painful swelling in his knees and elbows after playing or falling.

At age 4, he suffered a nosebleed that lasted for almost an hour. That is when his mother decided to take him to a doctor specialist.

Diagnosis Process

The doctor rans blood tests and found that his blood lacked a protein called Factor VIII, which is needed for proper blood clotting. He was diagnosed with Haemophilia A – a lifelong, genetic condition. They were referred to a genetic counsellor who explained to the mother that;

a. The disease is X-linked recessive.

b. Since her son is male (XY), he got the faulty gene from his mother (a carrier).

c. His sister may also be a carrier, and might one day pass it on to her children.

Impact on Lifestyle

The young boy now has to make many adjustments:

a. He avoids contact sports and rough play.

b. He receives regular injections of clotting factor at a clinic.

c. He wears a medical bracelet in case of emergencies.

d. His family has learned to treat cuts quickly and monitor swelling and bruises closely.

He sometimes feels isolated when his friends play soccer, and he can’t join. He also misses school during hospital visits, which affects his academics.

Impact on Family Dynamics

His diagnosis changed the way the whole family lives:

a. His mother became more protective and sometimes overly cautious, which frustrates him

b. The family had to budget for transport and occasional private healthcare when government clinics were full. c. His sister became more curious about genetics and now wants to become a nurse.

d. The family is now more educated about hereditary diseases and is considering genetic testing and counselling for future family planning.

Activity 8.4 Understanding Heredity Disease through Research and Real–life Stories Aim: To research hereditary diseases What you need

• For research (Notebooks, test books or internet)

• For presentation (Poster papers, makers.

What to do

1. In groups of 3-5, choose a hereditary disease, for example: Cystic Fibrosis, Sickle Cell, Anaemia, Haemophilia, etc.

2. Each group must research genetic causes, inheritance patterns, symptoms, age of onset, and available treatments.

3. Write down your findings to create a visual aid for presentation.

Diabetes Mellitus (DM) is a chronic metabolic disorder characterised by high levels of glucose (sugar) in the blood. This condition arises either because the body does not produce enough insulin or because the body cannot effectively use the insulin it produces.

Glucose is a primary source of energy for the body’s cells, and insulin is the hormone that allows glucose to enter the cells. When insulin is lacking or not functioning properly, glucose accumulates in the bloodstream instead of being used by the cells for energy.

The Role of Insulin

_(Insulin) is a hormone produced by the beta cells in the pancreas (specifically in the islets of Langerhans). It plays a central role in regulating blood glucose levels.

Functions of Insulin

ₐ. Facilitates glucose uptake by cells (especially muscle and fat cells).

b. Stimulates the liver to store excess glucose as glycogen.

c. Inhibits glucose production by the liver.

d. Promotes the storage of fats and protein synthesis.

When insulin is absent or not working properly, the following occurs:

a. Glucose stays in the blood which result in hyperglycaemia (high blood sugar).

b. Cells are starved of energy despite plenty of glucose being available.

c. Over time, this leads to serious damage to the heart, blood vessels, eyes, kidneys, and nerves.

Types of Diabetes Mellitus

There are several types of diabetes, but the main ones are:

A. Type 1 Diabetes Mellitus (T1DM)

_(Cause): Autoimmune destruction of pancreatic beta cells → little or no insulin production.

Onset: Usually develops in childhood or adolescence, but can occur at any age.

Insulin-dependent: Patients need lifelong insulin therapy.

Symptoms Of Type 1 Diabetes Mellitus

a. Rapid weight loss

b. Fatigue

c. Frequent urination (polyuria)

d. Excessive thirst (polydipsia)

e. Increased hunger (polyphagia) B. Type 2 Diabetes Mellitus (T2DM) Cause: The body becomes resistant to insulin, and the pancreas cannot make enough insulin to overcome this resistance.

Onset: Most common in adults over 40, but increasingly seen in children due to obesity and sedentary lifestyle.

Risk Factors

a. Obesity

b. Physical inactivity

c. Family history

d. Unhealthy diet Managed by

a. Lifestyle changes (diet, exercise)

b. Oral medications

c. Sometimes, insulin therapy if required Gestational Diabetes Mellitus (GDM) Occurs during pregnancy, usually in the second or third trimester.

Cause: Hormones from the placenta interfere with the action of insulin.

Usually resolves after birth, but it increases the mother’s risk of developing type 2 diabetes later.

Risks to the baby

a. High birth weight

b. Preterm birth

c. Low blood sugar after birth Managed by

a. Lifestyle changes (diet, exercise)

b. Insulin if necessary Sickle cell disease Sickle cell disease (SCD), also known as sickle cell anaemia, is a group of inherited blood disorders. This means it’s passed down from parents to their children through genes. At its core, SCD affects haemoglobin, the protein in red blood cells responsible for carrying oxygen throughout the body. SCD is caused by a mutation (a change) in the HBB gene. This gene provides instructions for making beta-globin, a component of haemoglobin. A specific mutation in the HBB gene leads to the production of an abnormal form of haemoglobin called haemoglobin S (HbS).

Normally, healthy red blood cells contain normal haemoglobin (HbA), are round, flexible, and can easily move through even the smallest blood vessels to deliver oxygen. In individuals with SCD, the presence of HbS causes red blood cells to become rigid, sticky, and take on a characteristic “sickle” or C-shape, especially when they release oxygen.

These sickled cells have several problems:

a. They are stiff and sticky: They cannot move easily through small blood vessels and tend to clump together, blocking blood flow.

b. They die prematurely: Sickled cells have a much shorter lifespan (10-20 days) compared to normal red blood cells (100-120 days). This leads to a chronic shortage of red blood cells, a condition called anaemia.

The blockage of blood flow and the chronic anaemia are responsible for the wide range of symptoms and complications associated with SCD, including severe pain episodes (vaso- occlusive crises), organ damage (spleen, kidney, liver, lungs), increased risk of infection, and stroke.

Inheritance Pattern: Autosomal Recessive

Sickle cell disease is inherited in an autosomal recessive pattern. This means that a child will only develop SCD if they inherit two copies of the mutated HBB gene, one copy from each parent.

Normal Gene (HbA): Represents the gene for normal haemoglobin.

Sickle Cell Gene (HbS): Represents the gene for abnormal haemoglobin.

If both parents have Sickle Cell Trait (HbAS)

a. Each parent carries one normal gene (HbA) and one sickle cell gene (HbS). They do not have SCD themselves, but they are “carriers” of the gene.

b. People with sickle cell trait usually do not experience symptoms, except in rare, extreme conditions like severe dehydration or intense physical activity.

For each child they have, there is a;

i. 25% chance (1 in 4) that the child will inherit two normal genes (HbAA) and will not have SCD or be a carrier.

ii. 50% chance (1 in 2) that the child will inherit one normal gene and one sickle cell gene (HbAS), meaning they will also have sickle cell trait and be a carrier.

iii. 25% chance (1 in 4) that the child will inherit two sickle cell genes (HbSS), meaning they will have sickle cell disease.

This is the most common way SCD is inherited.

If one parent has Sickle Cell Disease (HbSS) and the other has Sickle Cell Trait (HbAS)

a. The parent with SCD has two sickle cell genes (HbSS).

b. The parent with sickle cell trait has one normal and one sickle cell gene (HbAS).

For each child they have, there is a;

i. 50% chance (1 in 2) that the child will inherit one HbS gene from the parent with SCD and one HbS gene from the carrier parent, resulting in HbSS (sickle cell disease).

ii. 50% chance (1 in 2) that the child will inherit one HbS gene from the parent with SCD and one HbA gene from the carrier parent, resulting in HbAS (sickle cell trait).

iii. No children will be completely unaffected (HbAA) in this scenario.

If one parent has Sickle Cell Disease (HbSS) and the other has normal haemoglobin (HbAA)

a. The parent with SCD has two sickle cell genes (HbSS).

b. The parent with normal haemoglobin has two normal genes (HbAA).

c. All children (100% chance) will inherit one HbS gene from the affected parent and one HbA gene from the unaffected parent, meaning all their children will have sickle cell trait (HbAS). None will have SCD.

If both parents have normal haemoglobin (HbAA), All children (100% chance) will have normal haemoglobin (HbAA) and will not have SCD or be carriers.

Spinal Muscular Atrophy (SMA)

The term “progressive muscular atrophies” can be broad, but the description provided strongly points to Spinal Muscular Atrophy (SMA) as a key example of a severe, heritable muscle disorder affecting infants. Atrophy refers to the wasting away or decrease in the size of a body part, in this case, muscles, due to a lack of normal development or use. SMA is a group of genetic diseases that affect the motor neurons in the spinal cord. Motor neurons are nerve cells that send signals from the brain and spinal cord to the muscles, controlling muscle movement. In SMA, these motor neurons progressively degenerate and die, leading to muscle weakness and wasting (atrophy). It affects movement Causes of Spinal Muscular Atrophy (SMA) ₐ. Genetic Cause: SMA is primarily caused by a mutation in the Survival Motor Neuron 1 (SMN1) gene. This gene is responsible for producing the SMN protein, which is crucial for the health and survival of motor neurons. When the SMN1 gene is mutated or missing, insufficient SMN protein is produced, leading to the degeneration of motor neurons.

b. Inheritance Pattern: SMA is an autosomal recessive disorder. This means that an individual must inherit two copies of the mutated SMN1 gene (one from each parent) to develop the condition. If a person inherits only one mutated copy, they are a carrier and typically do not show symptoms, but they can pass the gene on to their children.

Albinism Albinism is another group of recessively inherited genetic disorders that affects the production of melanin, the pigment responsible for the colour of skin, hair, and eyes. It is not contagious and does not inherently affect intelligence or physical health beyond the issues directly related to pigmentation.

Genetic Basis and Types of Albinism

Albinism results from mutations in specific genes that are involved in the production of melanin. There are different types of albinism, classified based on the gene affected and the degree of pigment reduction. The most common type is oculocutaneous albinism (OCA), which affects the skin, hair, and eyes. Ocular albinism (OA) primarily affects the eyes.

a. Inheritance Pattern: Most forms of albinism, including the common types of OCA, are inherited in an autosomal recessive pattern. This means that both parents must be carriers of the mutated gene (each having one normal copy and one mutated copy) for their child to have albinism. If a child inherits two copies of the mutated gene (one from each carrier parent), they will develop albinism.

b. Melanocytes: The disorder affects melanocytes, which are specialized cells located primarily in the skin, hair follicles, and eyes. These cells produce melanin.

In albinism, either the melanocytes cannot produce melanin at all, or the amount of melanin produced is significantly reduced, or the quality of melanin is altered.

Characteristics and Associated Issues

Individuals with albinism typically exhibit striking physical characteristics due to the lack or reduction of melanin:

a. Skin Colour: The skin can appear very pale or strikingly white, sometimes with a pinkish hue due to visible blood vessels. It does not tan and remains abnormally sensitive to sunlight. It may indeed appear wrinkled prematurely due to sun damage.

b. Hair Colour: Hair can range from very white to light yellow, light brown, or reddish, depending on the type of albinism and the individual’s ethnic background.

c. Eye Colour: Eyes can appear very light blue, grey, green, or, less commonly, reddish-pink (due to the visible blood vessels in the iris when pigment is absent).

d. Vision Problems: This is a hallmark of albinism and is often the most impactful health concern. Melanin plays a crucial role in the normal development of the optic nerves and retina.

The Test Cross: Determining Unknown Genotypes

Beyond predicting offspring from known parents, Punnett’s work (and Mendel’s prior insights) also provided a method to determine the genotype of an organism that displays a dominant phenotype but whose underlying genotype is unknown (e.g., is a yellow-seeded pea plant YY or Yy). This method is called the test cross.

Principle: A test cross involves crossing the individual with the unknown dominant phenotype with an individual that is homozygous recessive for the same characteristic.

The homozygous recessive individual’s genotype (yy) is known, and it will only contribute recessive alleles (y) to its gametes.

Scenario 1: Unknown Dominant Individual is Homozygous Dominant (YY) Cross: YY (unknown) x yy (test cross parent) Punnett Square: All offspring will be Yy.

Phenotypes: All F1 offspring will express the dominant trait (e.g., all yellow seeds). This indicates the unknown parent was YY.

Scenario 2: Unknown Dominant Individual is Heterozygous (Yy) Cross: Yy (unknown) x yy (test cross parent) Punnett Square: Offspring will be 50% Yy and 50% yy.

Phenotypes: The F1 offspring will exhibit a 1:1 ratio of the dominant phenotype to the recessive phenotype (e.g., 1 yellow seed: 1 green seed). This indicates the unknown parent was Yy.

The test cross is a powerful analytical tool that further supports Mendel’s postulate of segregation, as the appearance of recessive offspring directly reveals the presence of a recessive allele in the previously unknown parent. This technique is widely used in plant and animal breeding to ensure desired genetic lines.

Pedigree Analysis: Tracing Inheritance in Humans

While test crosses are valuable in experimental settings, they are unethical and impractical for humans. Instead, geneticists use pedigree analysis to study the inheritance patterns of human genetic diseases and other traits within families.

Organisation and Symbols of a Pedigree

A pedigree is a standardised diagram that represents the ancestral history of a genetic trait within a family.

Generations:

Each horizontal row represents a single generation, with the oldest generation at the top of the pedigree and the most recent generation at the bottom. Generations are typically numbered with Roman numerals (I, II, III...).

Individuals:

Males: Represented by squares (◻).

Females: Represented by circles (○).

Affected Individuals: Shaded (or filled) shapes (e.g., blue or black) indicate individuals who express the trait or disease.

Unaffected Individuals: Unshaded (or clear) shapes indicate individuals who do not express the trait.

Carriers: Half-shaded or a dot within the shape can sometimes indicate individuals who are heterozygous carriers for a recessive trait but do not show the phenotype.

Relationships:

Marriage/Mating: A single horizontal line connects individuals who have had children together.

Offspring: Children are connected to their parents’ horizontal mating line by a vertical line, typically arranged in birth order from left to right.

Twins: Indicated by two vertical lines diverging from the same point on the parental line.

Identical (monozygotic) twins have a horizontal line connecting their two vertical lines.

Analysing Inheritance Patterns with Pedigrees: Geneticists use pedigree analysis to deduce genotypes and predict the likelihood of a trait appearing in future generations. Key patterns emerge:

Recessive Traits: Affected individuals often have unaffected parents (who must be carriers).

The trait may skip generations. Affected individuals are usually homozygous recessive (aa).

Example: Alkaptonuria: Individuals with alkaptonuria cannot properly metabolise phenylalanine and tyrosine, leading to symptoms like darkened skin, brown urine, and joint damage. In a pedigree, unaffected individuals (e.g., AA or Aa) might have affected offspring (aa), indicating that both unaffected parents must be heterozygous carriers (Aa).

If an individual is unaffected but has an affected sibling, their genotype could be AA or Aa, often represented as A?

Dominant Traits: Affected individuals usually have at least one affected parent. The trait appears in every generation. Affected individuals transmit the trait to approximately half their offspring if heterozygous.

Activity 8.5 Video on Hereditary Disorders

Watch the video linked below.

https://www.youtube.com/watch?v=UC-YwiB_FFg&t=4s

1. Explain how polygenic inheritance differs from Mendelian inheritance. Use the

example of human skin colour to describe how multiple genes can lead to a continuous range of phenotypes.

2. Analyse the relationship between Mendel’s Law of Segregation and the Law of Independent Assortment. How do these two laws work together during gamete formation to ensure genetic variation in the F2 generation of a dihybrid cross?

3. Explain why a person with sickle cell trait (HbAS) is considered a carrier and typically does not show severe symptoms of sickle cell disease.

4. Haemophilia is an X-linked recessive disorder. A woman is a carrier, and a man is unaffected. Analyse the genetic risks for their children. Construct a Punnett square to show the possible genotypes and phenotypes of their offspring and explain why the sons and daughters have different probabilities of being affected or being carriers.

In this section, we will explore the world of human genetics, where you will learn how your unique traits are passed down from your parents. We will explore the principles of Mendelian genetics to understand how we inherit traits like dimples, tongue rolling, and attached earlobes. We will also cover sex determination and the role of the X and Y chromosomes in deciding if you are male or female.

KEY IDEAS

• Mendelian Crossings and Sex Determination: Human sex is determined by the inheritance of sex chromosomes (X and Y) from parents, a process that follows Gregor Mendel’s principles of inheritance.

• Chromosomal Basis of Sex: Humans have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes. The chromosomal basis of sex in humans depends on the 23ʳᵈpair of chromosomes. Females have two X chromosomes (XX), and males have one X and one Y chromosome (XY).

• Role of the Father’s Gamete: The father’s sperm, which can carry either an X or a Y chromosome, determines the sex of the child at the moment of fertilization.

• Sex-Linked Inheritance: Genes on sex chromosomes (X or Y) follow different inheritance patterns than genes on autosomes, affecting males and females differently.

• X-Linked Traits: Conditions caused by genes on the X chromosome are more common in males. Females with one affected X chromosome are typically carriers and often don’t show symptoms.

• Y-Linked Traits: Genes on the Y chromosome are passed exclusively from a father to all of his sons. Daughters cannot be affected or be carriers.

Mendelian crossing is based on the work of Gregor Mendel, who discovered how traits are passed from parents to their children through genes. One example of this in humans is how biological sex (male or female) is determined.

Humans have 23 pairs of chromosomes in each cell. One of these pairs is called the sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).

When a baby is made; the mother always passes on an X chromosome. The father can pass on either an X or a Y:

a. If the father gives an X → the baby is female (XX)

b. If the father gives a Y → the baby is male (XY) This follows Mendel’s law of segregation, which says that each parent passes on only one of their two chromosomes to their child. So, the sex of the baby depends on which sex chromosome the father passes on to him or her.

Genetic Control of Sex Determination

In higher organisms like humans, a person’s sex is usually determined at the moment of fertilization. This is because of special chromosomes called sex chromosomes, which carry genes that control how sexual characteristics develop. Some animals, such as certain fish, use different genes on autosomes (non-sex chromosomes) to decide sex. However, in most complex organisms, the most common and well-understood way sex is determined is through chromosomal sex determination.

The XX-XY System in Humans

Humans have 23 pairs of chromosomes, making a total of 46 chromosomes in each body (somatic) cell. Out of these, 22 pairs are called autosomes. These chromosomes carry genes for most of the body’s traits and are the same in both males and females. The remaining one pair is called the sex chromosomes, which determine a person’s biological sex.

a. Females have two X chromosomes (XX)

b. Males have one X and one Y chromosome (XY)

Figure 9.1: Pairs of chromosomes Humans and many other mammals use the XX-XY system to determine sex:

a. Females have two X chromosomes → XX

b. Males have one X chromosome and one Y chromosome → XY What Happens During Fertilisation?

Sex is determined at the moment of fertilisation, when a sperm cell from the father joins with an egg cell from the mother.

Egg cells (from the mother) always carry an X chromosome, because females are XX.

Sperm cells (from the father) can carry either an X or a Y chromosome, because males are XY.

This means:

a. If a sperm with an X chromosome fertilizes the egg (which has an X), the baby will be XX → female.

b. If a sperm with a Y chromosome fertilizes the egg, the baby will be XY → male.

So, the father’s sperm determines the sex of the child.

There is about a 50% chance of having a boy and a 50% chance of having a girl in each pregnancy.This scientific explanation proves that the mother does not control the sex of the baby; a belief that was wrongly accepted in the past.

Parental phenotypes Female Male Parental genotype XX x XY Meiosis Gametes X X x X Y Random fertilization Offspring genotypes XX XY XX XY offspring phenotypes Girl Boy Girl Boy Other Types of Sex Determination Systems While the XX-XY system is common, other mechanisms exist:

1. ZW-ZZ System: Found in birds, some fish, and some insects (e.g., butterflies). In this system, females are the heterogametic sex (ZW) and males are homogametic (ZZ).

2. XO System: Seen in many insects (e.g., grasshoppers). Females are XX, but males have only one X chromosome (XO), lacking a Y chromosome.

3. Haplodiploidy System: Characteristic of social insects like bees, ants, and wasps.

Males develop from unfertilized eggs (haploid, having one set of chromosomes), while females develop from fertilized eggs (diploid, having two sets of chromosomes).

4. Environmental Sex Determination: In some reptiles (e.g., crocodiles, turtles), the temperature during egg incubation determines the sex of the offspring.

Sex-Linked Inheritance: Genes on Sex

Chromosomes Sex linkage refers to the inheritance patterns of genes located specifically on the sex chromosomes (X or Y), rather than on the autosomes. These genes are called sex-linked genes or X-linked/Y-linked genes, and their inheritance patterns differ significantly from autosomal traits because males and females have different numbers of X and Y chromosomes.

X-Linked Inheritance _(The) X chromosome is much larger than the Y chromosome and contains a significantly greater number of genes (over 900 genes). Genes found on the X chromosome can affect both males and females. X-linked traits can be either recessive or dominant.

X-Linked Recessive Inheritance

₁. Common in Males: X-linked recessive conditions are much more prevalent in males than in females. This is because males have only one X chromosome. If a male inherits a mutated recessive allele on his X chromosome, he will express the trait because he has no second X chromosome to provide a normal, dominant allele to mask it.

2. Females as Carriers: Females have two X chromosomes. If a female inherits one mutated recessive allele on one X chromosome and a normal, dominant allele on the other X chromosome, she will usually be a carrier. She typically won’t show symptoms herself (or may have milder symptoms due to skewed X-inactivation, where one of the X chromosomes is preferentially inactivated), but she can pass the mutated allele to her children.

Inheritance Patterns

₁. Affected Son from a Carrier Mother: A son born to a carrier mother and an unaffected father has a 50% chance of inheriting the mutated and thus being affected.

2. Daughters of Carrier Mothers: Daughters of a carrier mother have a 50% chance of being carriers themselves. They rarely show the full condition unless they inherit the recessive allele from both parents (which is rare, requiring an affected father and a carrier/affected mother) or exhibit skewed X-inactivation.

3. Affected Father: An affected father cannot pass the X-linked trait to his sons (as sons inherit the Y chromosome from their father). However, all his daughters will be carriers because they inherit his X chromosome.

Examples of X-linked recessive conditions include ₁. Red-green colour blindness: Difficulty distinguishing between shades of red and green.

2. Haemophilia A and B: Blood-clotting disorders where the blood does not clot properly.

3. Duchenne Muscular Dystrophy: A progressive muscle-wasting disease. It manifests very early in childhood causing delayed walking and difficulty in running

4. Glucose-6-phosphate dehydrogenase (G6PD) deficiency: A disorder where the body lacks enough of an enzyme needed for th survival of red blood cells. This condition can lead to haemolytic anaemia, often triggered by certain foods, drugs, or infections.

Example: Inheritance of Haemophilia

Let’s use the Punnett square you provided to illustrate haemophilia inheritance: (XH = normal allele, Xh = haemophilia allele) X XᴴXʰ XᴴXᴴXᴴXᴴXʰ Y XᴴY XʰY This Punnett square shows that if the mother is a carrier and the father is unaffected:

1. There’s a 50% chance for daughters to be carriers and a 50% chance to be unaffected.

2. There’s a 50% chance for sons to be affected and a 50% chance to be unaffected. This explains why it primarily affects males.

X-Linked Dominant Inheritance

₁. Rare: X-linked dominant conditions are less common than X-linked recessive ones.

2. Inheritance Patterns:

a. Affected Mother: If an affected mother has children with an unaffected father, there is a 50% chance for both sons and daughters to be affected. However, some X-linked dominant conditions can be lethal to male embryos, making them appear almost exclusively in females.

b. Affected Father: If an affected father has children with an unaffected mother, all his daughters will be affected because they inherit his X chromosome. None of his sons will be affected, as they inherit his Y chromosome.

Examples of X-linked dominant conditions include ₁. Rett syndrome: A severe neurodevelopmental disorder (often lethal in males).

2. Fragile X syndrome (in some cases): While primarily X-linked recessive, some female carriers can show milder symptoms due to X-inactivation. It causes intellectual disability.

3. Vitamin D-resistant rickets (X-linked hypophosphatemia). This genetic disorder causing chronic severe bone softening, bow legs and some dental issues.

Y-Linked Inheritance (Holandric Inheritance)

Y-linked traits, also known as holandric traits, are determined by genes located exclusively on the Y chromosome. The Y chromosome is much smaller than the X chromosome and contains very few genes (estimated around 200).

1. Inheritance Pattern: Y-linked conditions are passed only from father to son and will affect all sons of an affected father. Daughters will never be affected and cannot be carriers, as they do not inherit a Y chromosome.

2. Limited Recombination: The Y chromosome generally does not undergo genetic recombination (crossing over) with other chromosomes, except for small regions called pseudo autosomal regions. This means that Y-linked genes are typically passed down unchanged from father to son through generations.

3. Difficulty in Detection: Due to the Y chromosome’s small size and limited gene content, Y-linked traits were historically difficult to confirm. However, advancements in DNA sequencing have allowed for more accurate identification of Y-linked genes.

4. Examples: Most well-established Y-linked genes are involved in male sex determination and fertility. A common example is the SRY gene (Sex-determining Region Y gene), which triggers male development. Other Y-linked genes are associated with conditions like certain forms of male infertility due to Y-chromosome deletions.

5. Hypertrichosis pinnae auris (hairy ears): Was once thought to be a Y-linked trait but later discredited. This highlights the challenges in definitively confirming Y-linkage without modern genetic tools.

Characteristics of Y-linked inheritance ₁. Occurs only in males.

2. Appears in all sons of males who exhibit the trait.

3. Is absent from daughters of trait carriers.

Other Complex Inheritance Patterns

Beyond simple Mendelian inheritance and sex-linked patterns, many traits are influenced by multiple genes and environmental factors:

a. Sex-Influenced Traits: Sex-influenced traits are autosomal traits whose expression or dominance is influenced by an individual’s biological sex. This means an allele might be dominant in one sex but recessive in the other, often due to differences in sex hormones.

Example: Pattern Baldness in Humans: The gene for pattern baldness is located on an autosome. It acts as a dominant trait in males (requiring only one copy of the allele for expression) but a recessive trait in females (requiring two copies for significant expression, and even then, expression may be less severe than in males). This is influenced by the presence of male hormones like testosterone.

b. Sex-Limited Traits: Sex-limited traits are autosomal traits that are expressed in only one of the sexes, even though the genes for these traits are present in both males and females. The expression is effectively “limited” by the presence or absence of specific sex hormones or anatomical structures.

Example: Milk Production in Mammals: Genes for milk production are present in both male and female mammals. However, only females express the trait of lactation, as it’s dependent on female hormones and mammary gland development.

Other Examples: Age of onset of menstruation, width of pelvis, and specific patterns of body hair distribution.

c. Polygenic Inheritance (Multifactorial Traits): Polygenic inheritance describes traits that are determined by the cumulative effects of multiple genes (alleles at many loci) rather than a single gene. These traits often show continuous variation within a population, meaning there’s a gradual range of phenotypes rather than distinct categories.

d. Continuous Traits: Unlike “discontinuous traits” (like blood types A, B, O, AB, which fall into clear categories), polygenic traits are “continuous.” You can’t easily put individuals into distinct groups.

e. Additive Effect: In polygenic inheritance, the effect of individual alleles is often small, and they typically do not display simple dominance or recessiveness.

Instead, each “contributing allele” adds a small, additive effect to the phenotype.

f. Multifactorial Traits: Many polygenic traits are also multifactorial, meaning they are influenced by both multiple genes and environmental factors. Environmental factors can significantly modify the expression of these genetic predispositions.

Examples of Polygenic Inheritance

₁. Skin Colour: Human skin colour is primarily determined by at least three, and likely more, genes that control the production of melanin. Individuals inherit a combination of “contributing” alleles (leading to darker skin) and “non-contributing” alleles (leading to lighter skin). The additive effect of these alleles results in the wide spectrum of human skin tones.

2. Human Height: Height is an incredibly complex multifactorial trait controlled by over 400 genes. This complexity makes predicting an offspring’s height challenging.

Environmental factors during growth, such as nutrition, health, and exposure to disease, also play a significant role in determining final adult height. Two short parents can have a tall child, and vice versa, due to the complex interplay of numerous genes and environmental influences.

3. Weight, Body Build, Intelligence: These are also examples of complex multifactorial traits influenced by many genes and environmental factors.

Have you ever wondered why you have your mother’s eyes or your father’s nose? Or why some siblings look so similar while others appear quite different? The answers lie in the fascinating field of genetics, the study of heredity, which is how traits are passed from parents to their offspring.

As you have already learnt, centuries ago, a curious monk named Gregor Mendel conducted groundbreaking experiments with pea plants. His meticulous work, long before the discovery of DNA, laid the foundation for our understanding of how traits are inherited. His principles, now known as Mendelian genetics, provide a powerful framework for predicting the patterns of inheritance for many simple traits.

This week, we will dive into selected human traits that often follow Mendelian patterns of inheritance. You will continue to learn to use the tools of Mendelian crossing, like the Punnett Square, to predict the likelihood of offspring inheriting specific characteristics. By observing traits in yourselves, your families, and even your classmates, you’ll gain a deeper appreciation for the genetic blueprint that makes each of us unique.

KEY IDEAS

• Explanation of polygenic inheritance.

• Prediction of inherited traits.

• The meaning of genotype and phenotype.

• What sex-inked traits are.

Many human traits are complex and influenced by multiple genes and environmental factors (polygenic or multifactorial traits). However, some fascinating traits are widely accepted to follow simple Mendelian inheritance patterns, making them excellent examples for our study. Some common examples are:

1. Earlobe attachment

a. Phenotypes: Free earlobes versus attached earlobes.

b. Inheritance pattern: Free earlobes are generally considered dominant over attached earlobes.

c. Alleles: Allele for free earlobes, F, is dominant while allele for attached earlobes, f, is recessive.

2. Genotypes and phenotypes

a. FF: Free earlobes (homozygous dominant)

b. Ff: Free earlobes (heterozygous)

c. ff: Attached earlobes (homozygous recessive)

Figure 9.2: Attached ear lobe (left), free ear lobe (right)

3. Widow’s peak hairline

a. Phenotypes: Widow’s peak (V-shaped hairline) versus straight hairline.

b. Inheritance pattern: Widow’s peak is generally considered dominant over a straight hairline.

c. Alleles

i. W: Allele for Widow’s peak (dominant).

ii. w: Allele for straight hairline (recessive).

d. Genotypes and phenotypes

i. WW: Widow’s peak (homozygous dominant).

ii. Ww: Widow’s peak (heterozygous).

iii. ww: Straight hairline (homozygous recessive).

4. Tongue rolling

a. Phenotypes: Ability to roll tongue into a U-shape versus inability to roll tongue.

b. Inheritance pattern: Tongue rolling is generally considered dominant over non- rolling.

c. Alleles

i. R: Allele for tongue rolling (dominant).

ii. r: Allele for non-rolling (recessive).

d. Genotypes and phenotypes

i. RR: Tongue Rolling (homozygous dominant)

ii. Rr: Tongue Rolling (heterozygous)

iii. rr: Non-rolling (homozygous recessive).

5. Dimples

a. Phenotypes: Presence of dimples versus absence of dimples.

b. Inheritance pattern: Presence of dimples is generally considered dominant over the absence of dimples.

c. Alleles

i. D: Allele for Dimples (dominant).

ii. d: Allele for No Dimples (recessive).

d. Genotypes and phenotypes

i. DD: Dimples (homozygous dominant).

ii. Dd: Dimples (heterozygous).

iii. dd: No dimples (homozygous recessive).

6. Phenylthiocarbamide (PTC) tasting

a. Phenotypes: Ability to taste PTC versus inability to taste PTC

b. Inheritance pattern: Ability to taste PTC is generally considered dominant over non-tasting. (PTC is a harmless chemical often found on special paper strips used for testing).

c. Alleles

i. T: Allele for PTC taster (dominant).

ii. t: Allele for PTC non-taster (recessive).

d. Genotypes and phenotypes:

i. TT: PTC Taster (homozygous dominant).

ii. ii Tt: PTC Taster (heterozygous).

iii. tt: PTC Non-taster (homozygous recessive).

The Punnett Square

This is a simple yet powerful tool for visualizing and predicting the outcomes of genetic crosses.

Steps for performing a monohybrid cross using the example of earlobe attachment:

Scenario: A man who is heterozygous for free earlobes (Ff) marries a woman who is also heterozygous for free earlobes (Ff). What are the possible genotypes and phenotypes of their children, and what are the probabilities?

Step 1: Identify the trait and its alleles

a. Trait: Earlobe attachment.

b. Alleles: F (Free earlobes - dominant), f (Attached earlobes - recessive)

Step 2: Determine the genotypes of the parents (P generation)

a. Man’s genotype: Ff

b. Woman’s genotype: Ff

Step 3: Determine the possible gametes each parent can produce Each parent, being heterozygous (Ff), can produce two types of gametes: one carrying the F allele and one carrying the f allele.

a. Man’s gametes: F, f

b. Woman’s gametes: F, f

Step 4: Draw a Punnett Square and fill it in

a. Place the possible gametes from one parent along the top row.

b. Place the possible gametes from the other parent along the left column.

c. Fill in each box by combining the alleles from the corresponding row and column.

Gametes (X) F f F FF Ff f fF ff

Step 5: Determine the predicted genotypes and phenotypes of the offspring (First filial generation, F1) and their ratios/probabilities.

a. Genotypes

i. FF: 1 out of 4 squares (1/4 or 25%)

ii. Ff: 2 out of 4 squares (2/4 or 1/2 or 50%)

iii. ff: 1 out of 4 squares (1/4 or 25%)

iv. Genotypic Ratio: 1 FF: 2 Ff: 1 ff

b. Phenotypes

i. Free earlobes (FF and Ff): 3 out of 4 squares (3/4 or 75%)

ii. Attached earlobes (ff): 1 out of 4 squares (1/4 or 25%)

c. Phenotypic ratio: 3 Free earlobes: 1 attached earlobes

Activity 9.1 My family’s genetic traits Aim: To use Mendelian genetics to verify observable traits in oneself and one’s family.

Important ethical note

a. This activity is for educational purposes only.

b. Do not attempt to diagnose any medical conditions or make judgments about individuals based on these simple traits.

c. Always ask for permission before observing or discussing traits with family members, and respect their privacy.

What you need

• Pen/Pencil

• This reference material

• Observation skills

• Optional: PTC taste strips (if available for testing PTC tasting) What to do

a. Observe your own traits: For each trait listed in the table below, determine your own phenotype.

b. Observe your family traits: With permission, observe the same traits in your parents and siblings. If you don’t know or cannot observe a trait for a family member, leave it blank.

Fill in the Table: Record your observations in the “Phenotype” column Trait Dominant phenotype Recessive phenotype My phenotype (circle one) Parent 1 phenotype Parent 2 phenotype Sibling 1 phenotype Sibling 2 phenotype Earlobe attachment Free Attached Free/ attached Widow’s peak Widow’s peak Straight hairline Widow’s peak/ Straight hairline Tongue rolling Can roll tongue Cannot roll tongue Can roll/ cannot roll Dimples Present Absent Present/ Absent PTC tasting Can taste Cannot taste Can taste/ cannot taste Analysis Using the information from week 14 and week 16, answer the following questions.

If you need to make assumptions about genotypes (e.g., if a parent has a dominant phenotype but their genotype isn’t known from observations), state your assumptions clearly.

a. Your genotypes: Based on your phenotype and your parents’ phenotypes, try to determine your most likely genotype for each trait. Explain your reasoning.

b. Example: If you have attached earlobes (ff), both your parents must carry the ‘f’ allele. If they both have free earlobes, they must both be heterozygous (Ff).

c. Parental genotypes: For any trait where you and your parents have a dominant phenotype but an offspring has a recessive phenotype (e.g., parents both have free earlobes, but a sibling has attached ones), what must the parents’ genotypes be? Show a Punnett Square to support your answer.

d. Predicting offspring: If two individuals, both heterozygous for tongue rolling (Rr x Rr), were to have children, what is the probability that their child would not be able to roll their tongue? Show your Punnett Square.

e. Class data (Optional): If your teacher facilitates, collect class data for each trait and discuss the observed frequencies versus the Mendelian ratios (e.g., why might the 3:1 ratio not be perfectly observed in a small sample like your class?).

Explanation of related terms While Mendelian crosses are powerful for understanding basic inheritance, it’s important to remember that human genetics is often more complex.

1. Polygenic traits: Many human traits (like height, skin colour, and intelligence) are influenced by multiple genes acting together, not just one.

2. Environmental Influences: The environment can also play a significant role in how genes are expressed (e.g., nutrition affecting height potential).

3. Incomplete Dominance: In some cases, the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes (e.g., red and white flowers producing pink offspring).

4. Codominance: Both alleles are fully expressed in the heterozygous phenotype (e.g., ABO blood groups, where A and B alleles are codominant).

5. Sex-linked traits: Traits determined by genes located on the sex chromosomes (X or Y), leading to different inheritance patterns in males and females (e.g., colour blindness, haemophilia, baldness).

Sample determination of polygenic and sex-linked traits

1. Height (tall/dominant versus short/recessive) trait Assumption: Tall (T) is dominant and short (t) is recessive.

A heterozygous tall person (Tt) is crossed with another heterozygous tall person (Tt).

Punnett Square

Gametes (X) T t T TT Tt t Tt tt Genotypes: 1 TT (homozygous dominant), 2 Tt (heterozygous), 1 tt (homozygous recessive).

Genotypic ratio: 1TT (25%) : 2Tt (50%) : 1tt (25%) Phenotypes: 3 out of 4 offsprings are tall while 1 out of 4 is short.

Interpretation There is a 75% chance of a tall child and 25% chance of a short child.

Now, let us explain height as polygenic inheritance using Punnet Square.

Assume 2 gene pairs, each with a dominant and recessive allele:

Thus, let A = tall, a = short, B = tall, b = short.

Each dominant allele contributes to tallness. The more the dominant alleles, the taller is the individual.

Parent Genotypes: Aa Bb × Aa Bb We will create a 16-square Punnett Square using all combinations.

Step 1: Possible gametes from each parent are as follows:

Gametes of parent AaBb: AB, Ab, aB, ab

Step 2: Punnett Square

Gametes AB Ab aB ab AB AABB AABb AaBB AaBb Ab AABb AAbb AaBb Aabb aB AaBB AaBb aaBB aaBb ab AaBb Aabb aaBb aabb Count dominant alleles and interpret phenotype. This is tabulated as follows:

Genotype Dominant alleles Phenotype AABB 4 Very tall AABb 3 Tall AaBB 3 Tall AaBb 2 Medium height AAbb 2 Medium height Aabb 1 Short aaBB 2 Medium height aaBb 1 Short aabb 0 Very short

2. Skin colour A simplified model using one gene pair.

Note: In reality, skin colour is a polygenic trait influenced by multiple genes. But for teaching Mendelian basics, a simplified dominant/recessive model is used.

Assumption: Dark skin (D) is dominant, light skin (d) is recessive.

At crossing: Heterozygous parents Dd × Dd Punnett Square Gametes D d D DD Dd d Dd dd Genotypes:1 DD (homozygous dark), 2 Dd (heterozygous dark), 1 dd (homozygous light).

Genotypic ratio: 1DD (25%) : 2Dd (50%) : 1dd (25%) Phenotypes: 3 dark-skinned, 1 light-skinned Interpretation: Dark skin appears more frequently due to dominant allele.

Skin colour can be explained as a polygenic inheritance using Punnett Squares as follows:

We will make the following assumptions:

a. Two genes, A and B, control skin pigment.

b. Dominant alleles, A and B, contribute to dark pigment.

c. Recessive alleles, a and b, contribute to light pigment.

d. The total number of dominant alleles determines skin shade.

Parental Genotypes: AaBb × AaBb Each parent can pass on four possible gametes, AB, Ab, aB, ab Using a 16-Square Punnett grid:

Gametes AB Ab aB ab AB AABB AABb AaBB AaBb Ab AABb AAbb AaBb Aabb aB AaBB AaBb aaBB aaBb ab AaBb Aabb aaBb aabb A table of dominant allele count and phenotype is as follows:

Genotype Dominant alleles Skin colour phenotype AABB 4 Very dark AABb 3 Dark AaBB 3 Dark AaBb 2 Medium tone AAbb 2 Medium tone aaBB 2 Medium tone Aabb 1 Light aaBb 1 Light aabb 0 Very light Interpretation:

a. Skin colour shows a range, not just light or dark.

b. Most offsprings are expected to fall into the medium tone category (2 dominant alleles).

c. More dominant alleles yield more melanin which produces a darker skin.

3. Predicting baldness, a sex-influenced trait

a. Assumption: Baldness is dominant in males (B), recessive in females (b).

b. Genotypes:

i. BB = bald in both sexes.

ii. Bb = bald in males, not bald in females.

iii. bb = not bald in both sexes.

Example: A heterozygous bald man (Bb) marries a heterozygous woman (Bb).

Punnett Square

Gametes B b B BB Bb b Bb bb Genotypes:1 BB: 2 Bb: 1 bb Phenotypes

a. Males

i. BB and Bb = Bald (3 out of 4)

ii. bb = Not bald (1 out of 4)

b. Females:

i. BB = Bald (1 out of 4)

ii. Bb = Not bald (2 out of 4)

iii. bb = Not bald (1 out of 4) Interpretation: This shows how the same genotype (Bb) expresses differently in males and females, due to influence of sex hormones.

Explanation of haemophilia using a Punnett Square Haemophilia is a sex-linked recessive genetic disorder, most commonly carried on the X chromosome. It affects blood clotting and occurs more frequently in males because they have only one X chromosome (XY), while females have two (XX).

Key points

1. The haemophilia gene is represented by Xʰ (affected) and a normal X by X.

2. Since males inherit only one X from their mother, if that X carries the haemophilia gene, they will have the disorder.

3. Females need two copies of the haemophilia gene to be affected (XʰXʰ), which is rare.

If they have one affected X (XʰX), they are carriers.

Punnett Square

Let’s consider a carrier mother (XʰX) and a normal father (XY).

Gametes X (father) Y (father) X (mother) XX XY Xʰ(mother) XʰX XʰY Offspring outcomes

1. XX (normal girl) – not affected

2. XʰX (carrier girl) – not affected, but carries the gene

3. XY (normal boy) – not affected

4. XʰY (haemophilic boy) – affected

1. Describe the inheritance pattern for an X-linked dominant trait and explain why all daughters of an affected father will also be affected.

2. A man has a family history of a Y-linked trait. His wife is unaffected. Analyse the probability that their son will inherit the trait and explain the reasoning behind your conclusion.

3. A man with a straight hairline (ww) and a woman with a Widow’s peak (Ww) have children. Use a Punnett Square to predict the percentage of their children who will have a Widow’s peak.

4. Analyse a scenario where two parents both have normal hair, but one of their children is bald. Use a Punnett Square to determine the likely genotypes of the parents and explain how the trait of baldness is inherited.