Genes, inheritance and selection Study Guide

Single gene inheritance, also known as Mendelian inheritance, refers to the inheritance pattern of traits determined by a single gene. Each gene has different forms called alleles. Individuals inherit two alleles for each gene, one from each parent. These alleles can be dominant or recessive.

A dominant allele will express its characteristic whenever it is present, even if only one copy is inherited. It masks the effect of a recessive allele. A recessive allele will only express its characteristic if two copies are inherited (one from each parent), and no dominant allele is present.

We use genetic diagrams, often called Punnett squares, to predict the probabilities of offspring inheriting certain traits. In these diagrams, dominant alleles are typically represented by an uppercase letter (e.g., 'A'), and recessive alleles by a lowercase letter (e.g., 'a').

An individual's genotype is the combination of alleles they possess for a particular gene (e.g., AA, Aa, aa). An individual's phenotype is the observable characteristic resulting from their genotype (e.g., brown eyes, blue eyes). If an individual has two identical alleles (e.g., AA or aa), they are homozygous for that gene. If they have two different alleles (e.g., Aa), they are heterozygous for that gene.

For example, if brown eye colour (B) is dominant over blue eye colour (b):

• A person with genotype BB would have brown eyes (homozygous dominant).
• A person with genotype Bb would also have brown eyes (heterozygous, as B is dominant).
• A person with genotype bb would have blue eyes (homozygous recessive).

By understanding these principles and using Punnett squares, we can predict the likelihood of offspring inheriting specific genetic traits from their parents.