Inheritance and Monohybrid Crosses for the ESAT

Chromosomes and Genes

In eukaryotic cells, the nucleus serves as the control centre and the primary site of genetic material. This material is stored as linear chromosomes, which are long, thread-like structures composed of DNA wrapped around proteins. In humans, most body cells contain 23 pairs of chromosomes. One pair determines biological sex, while the remaining 22 pairs are known as autosomes.

A gene is a specific, short section of a chromosome that codes for a single protein. These proteins determine an organism's characteristics. For instance, a gene might determine hair colour or the ability to taste certain chemicals.

Exercise 13: Which type of human cell has no nucleus and therefore no chromosomes?

Solution: Mature red blood cells have no nucleus or chromosomes.

Alleles and Genotypes

Genes often exist in different versions called alleles. For example, the autosomal gene controlling the ability to taste the bitter chemical PTC has two main alleles: the 'tasting' allele and the 'non-tasting' allele.

1. Dominant Alleles: These are represented by capital letters (e.g. $T$). A dominant allele always determines the phenotype if at least one copy is present.
2. Recessive Alleles: These are represented by lowercase letters (e.g. $t$). A recessive allele only determines the phenotype if two copies are present in the cell.

The specific combination of alleles an individual possesses is their genotype.

• Homozygous: Having two identical alleles for a gene (e.g. $TT$ or $tt$).
• Heterozygous: Having two different alleles for a gene (e.g. $Tt$).

The phenotype refers to the observable characteristics resulting from the genotype, though environmental factors can sometimes influence this.

Exercise 14: Which two genotypes could give a person the ability to taste PTC (where $T$ is the dominant tasting allele)?

Solution: The genotypes $TT$ (homozygous dominant) and $Tt$ (heterozygous) both result in the ability to taste PTC.

The Mechanism of Inheritance

Inheritance occurs because offspring receive one copy of every gene from each parent. One allele is delivered by the sperm cell (father) and one by the egg cell (mother).

Exercise 15: A mother has no hair on her finger joints (recessive, $h$). A father has hair on his finger joints (dominant, $H$). Their child has no hair on her finger joints. Deduce the father's genotype.

Solution: The child is homozygous recessive ($hh$) because she shows the recessive phenotype. She must have inherited one $h$ allele from each parent. Since the father has the dominant phenotype (hair), he must possess at least one $H$ allele. Therefore, his genotype must be heterozygous ($Hh$).

Monohybrid Genetic Crosses

A monohybrid cross examines the inheritance of a single gene. Scientists use genetic diagrams and Punnett squares to predict the outcomes of these crosses.

Constructing a Genetic Diagram

Consider the ability to roll the tongue, where the rolling allele ($T$) is dominant and the non-rolling allele ($t$) is recessive. If we cross a homozygous tongue roller ($TT$) with a non-tongue roller ($tt$):

1. Identify parental phenotypes: Tongue roller and Non-tongue roller.
2. Identify parental genotypes: $TT$ and $tt$.
3. Identify gametes: One parent produces only $T$ gametes; the other produces only $t$ gametes.
4. Show fertilisation: All offspring will be $Tt$.

In this specific cross, $100\%$ of the offspring are heterozygous tongue rollers.

Using Punnett Squares

A Punnett square is a grid used to calculate the probability of genotypes. Below is a square for two heterozygous rollers ($Tt \times Tt$).

From this cross, $75\%$ of offspring are predicted to be tongue rollers and $25\%$ non-rollers. This is a phenotypic ratio of $3:1$.

Exercise 16: Draw a Punnett square for a heterozygous tongue roller and a non-tongue roller. State the ratio.

Solution:

The ratio of tongue rollers to non-tongue rollers is $1:1$ (or $50\%$ each).

Inherited Conditions

Genetic conditions can be caused by dominant or recessive alleles.

Polydactyly (Dominant Condition)

Polydactyly causes extra fingers or toes and is caused by a dominant allele. Only one copy is needed to show the condition.

In the diagram above, a heterozygous parent and a homozygous recessive parent have a $0.5$ (or $50\%$) probability of having a child with polydactyly at each conception.

Cystic Fibrosis (Recessive Condition)

Cystic fibrosis affects mucus production and is caused by a recessive allele ($f$). A carrier is a heterozygous individual ($Ff$) who does not have the condition but can pass the allele to offspring.

Exercise 17: Two carriers have four children, none of whom have cystic fibrosis. Why does this not match the expected $25\%$ probability?

Solution:

Fertilisation is a random process. In a small sample size (four children), the actual outcomes often deviate from the mathematical probability. The $1:4$ ratio is an average expected over a very large number of offspring.

Family Pedigree Diagrams

Pedigree diagrams track the inheritance of a trait through generations. They allow scientists to deduce genotypes.

In this pedigree for cystic fibrosis, individuals 1 and 2 must be carriers because they are unaffected but have an affected child (person 3). Person 3 must be $ff$.

Exercise 18: In a family tree, two parents showing 'Characteristic A' have a child who does not. Is the allele dominant or recessive? What is the probability their next child has it?

Solution:

The allele is dominant. If it were recessive, two affected parents could only produce affected children. Since the child is unaffected ($aa$), the parents must be heterozygous ($Aa$). The probability for the next child showing the characteristic is $0.75$ (or $75\%$).

Multiple Gene Inheritance

While some traits result from a single gene, most human phenotypes are polygenic, meaning they are controlled by multiple genes working together. Height is a classic example. Because so many genes contribute to the final phenotype, there is a continuous range of possible heights rather than just two distinct categories.