Inheritance

Overview

Inheritance is the cornerstone of genetics, exploring how traits are passed from one generation to the next. For WJEC A-Level Biology, this topic is not just about memorising definitions; it's about application. You will be expected to construct and interpret complex genetic diagrams, analyse data using statistical tests, and explain deviations from expected Mendelian ratios. This unit connects directly to fundamental concepts in cell biology (meiosis), evolution (natural selection), and gene technology. Exam questions are typically structured problems requiring you to work through a scenario, often worth a high number of marks. Mastering the methodical approach outlined in this guide is therefore essential for exam success.

Key Concepts

Concept 1: Monohybrid and Dihybrid Inheritance

Monohybrid inheritance involves the study of a single gene. The fundamental tool for predicting outcomes is the Punnett square, which requires a systematic layout that examiners look for. You must be ableto predict phenotypic and genotypic ratios from crosses involving dominant, recessive, and codominant alleles. Codominance is a situation where both alleles are expressed in the phenotype of a heterozygote, such as in human ABO blood groups or roan cattle.

Dihybrid inheritance follows the inheritance of two different genes simultaneously. When these genes are on different chromosomes, they assort independently during meiosis. A cross between two parents heterozygous for both genes (e.g., AaBb x AaBb) produces the classic 9:3:3:1 phenotypic ratio. Recognising this ratio is a key skill, but more importantly, recognising when the ratio deviates from this is a trigger to consider linkage or epistasis.

Concept 2: Linkage (Autosomal and Sex-Linked)

Autosomal linkage occurs when two or more genes are located on the same autosome (non-sex chromosome). Because they are physically linked, they do not assort independently and are often inherited together. This drastically alters the expected 9:3:3:1 dihybrid ratio, resulting in a much higher proportion of offspring with the parental phenotypes and a much lower proportion of recombinant phenotypes. The frequency of recombination is a measure of the distance between the two linked genes.

Sex linkage refers to genes located on the sex chromosomes (X or Y). Most are X-linked. Since males (XY) have only one X chromosome, they will express a recessive X-linked allele even if they only have one copy. This is why conditions like haemophilia and red-green colour blindness are far more common in males. For exam questions, it is critical to use the correct notation, showing the alleles as superscripts on the X and Y chromosomes (e.g., X^H, X^h, Y).

Concept 3: Epistasis

Epistasis is a form of gene interaction where one gene masks or suppresses the expression of another gene at a different locus. This is a common source of modified dihybrid ratios. You need to be familiar with two main types:

• Recessive Epistasis (9:3:4 ratio): The homozygous recessive genotype at one locus (e.g., ee) masks the expression of alleles at a second locus (e.g., B/b). A classic example is coat colour in Labrador retrievers.
• Dominant Epistasis (12:3:1 ratio): A dominant allele at one locus (e.g., A) masks the expression of alleles at a second locus (e.g., B/b). An example is fruit colour in summer squash.

Concept 4: The Chi-Squared (X²) Test

The Chi-squared test is a statistical tool used to determine if the difference between observed and expected results in an investigation is statistically significant or simply due to chance. In genetics, it is used to test the 'goodness of fit' between your observed phenotypic ratios and the expected Mendelian ratios. A non-significant result supports your genetic hypothesis (e.g., that the genes assort independently), while a significant result suggests your hypothesis is incorrect and other factors (like linkage or epistasis) are at play.

Mathematical/Scientific Relationships

**1. The Chi-Squared Formula (Must memorise)**O - E
X² = Σ (-------)²
E

• X²: The Chi-squared value.
• Σ: The sum of.
• O: Observed frequency for a category.
• E: Expected frequency for a category.

2. Degrees of Freedom (df)

df = n - 1

• n: The number of phenotype categories.

Practical Applications

This topic is fundamental to modern medicine and agriculture. It is used for:

• Genetic Counselling: Assessing the risk of couples passing on inherited disorders like cystic fibrosis or Huntington's disease.
• Selective Breeding: Improving crop yields and livestock by selecting for desirable traits based on an understanding of their inheritance patterns.
• Conservation: Managing the genetic diversity of endangered species in captive breeding programmes to avoid inbreeding depression."