Prep Guide: Genetics

Key Concepts (Recall & Understanding)

Mendel's Laws:
- Law of Dominance: In a heterozygote, one trait (dominant) will conceal the presence of another trait (recessive) for the same characteristic.
- Law of Segregation: The two alleles for each trait separate (segregate) during gamete formation.
- Law of Independent Assortment: Alleles of different genes are inherited independently of one another.
Key Terms:
- Allele: An alternative form of a gene.
- Homozygous: Having two identical alleles for a trait (e.g., TT, tt).
- Heterozygous: Having two different alleles for a trait (e.g., Tt).
- Phenotype: The observable physical characteristics (e.g., tall, short).
- Genotype: The genetic makeup of an organism (e.g., TT, Tt, tt).
Genetic Crosses:
- Monohybrid Cross: A cross involving one trait. Phenotypic ratio in F2 is 3:1.
- Dihybrid Cross: A cross involving two traits. Phenotypic ratio in F2 is 9:3:3:1.
Sex Determination: In humans, XX results in a female, and XY results in a male. The sperm determines the sex of the child.
Sex-Linked Inheritance: Traits determined by genes on the sex chromosomes (usually the X chromosome). Examples include haemophilia and colour blindness.

Application Corner

In pea plants, tall (T) is dominant to dwarf (t). If a heterozygous tall plant (Tt) is crossed with a dwarf plant (tt), what is the probability of producing a dwarf offspring?
- Answer: 50%. The cross is Tt x tt. The possible genotypes for the offspring are Tt and tt. The probability of tt (dwarf) is 1/2 or 50%.
A man with colour blindness (an X-linked recessive trait) marries a woman who is a carrier for the trait. What is the probability that they will have a colour-blind son?
- Answer: 50%. The man's genotype is XcY, and the woman's is XcX. A son receives the Y chromosome from his father and one X from his mother. He has a 50% chance of inheriting her Xc chromosome, which would make him colour-blind (XcY).

Analytical Thinking

Odd One Out: Homozygous, Heterozygous, Phenotype, Genotype.
- Odd One: Phenotype.
- Category: The rest are terms describing the genetic makeup (alleles) of an individual.
Scenario: A farmer crosses two plants, one with red flowers and one with white flowers. All the offspring have pink flowers. Which of Mendel's principles is not demonstrated by this cross, and what is this pattern of inheritance called?
- Answer: The Law of Dominance is not demonstrated. This pattern of inheritance is called incomplete dominance, where neither allele is completely dominant over the other, resulting in a blended phenotype.

Haemophilia is an X-linked recessive disorder that impairs the body's ability to make blood clots. It became famous as the "royal disease" because it affected many members of the European royal families, descended from Queen Victoria of England, who was a carrier. Because it is an X-linked trait, her male descendants had a 50% chance of inheriting the disease, while her female descendants had a 50% chance of being carriers. This case study is a classic example of human sex-linked inheritance, demonstrating how a single faulty gene on the X chromosome can be passed down through generations, with different consequences for males and females.

Assertion-Reason Practice

Assertion (A): The phenotypic ratio of a dihybrid cross in the F2 generation is 9:3:3:1. Reason (R): The alleles of two different genes are inherited independently of each other.

(a) Both (A) and (R) are true and (R) is the correct explanation of (A). (b) Both (A) and (R) are true but (R) is not the correct explanation of (A). (c) (A) is true but (R) is false. (d) (A) is false but (R) is true.

Answer: (a) Both (A) and (R) are true and (R) is the correct explanation of (A). The 9:3:3:1 ratio is a direct result of the independent assortment of the alleles for the two traits.

HOTS (Higher-Order Thinking Skills) Question

In a dihybrid cross, if two genes are located very close to each other on the same chromosome, they are said to be "linked." How would this linkage affect the results of the cross compared to what Mendel predicted with his Law of Independent Assortment? Would you still expect to see a 9:3:3:1 phenotypic ratio?

Answer: If the genes are linked, they will not assort independently. Instead, they will tend to be inherited together. This would drastically change the F2 phenotypic ratio from the expected 9:3:3:1. You would see a much higher proportion of the parental phenotypes and a much lower proportion of the recombinant (new combination) phenotypes. The 9:3:3:1 ratio is only observed when the genes for the two traits are on different chromosomes or are far apart on the same chromosome, allowing for frequent crossing over.

Specimen Prep - Genetics