Genetics study guide

Phenotypic variation—the differences in observable traits like height, colour, or body shape—results from both genetic and environmental factors. Genetic variation is produced when inherited DNA differs due to mutation (random changes in genetic code) or is shuffled through sexual reproduction, leading to new and unique trait combinations in offspring, such as blood type or fur colour. Environmental variation arises when conditions like diet, climate, or experience affect how traits develop, resulting in acquired characteristics such as muscle size from exercise or plant height influenced by sunlight. Some features result from both genetic and environmental interactions, further increasing the diversity of traits seen within a population.

The inheritance of the ABO blood groups is a classic example of both multiple alleles and codominance. Unlike traits determined by two alleles, the ABO system involves three different alleles for a single gene: Iᔧ, Iᔥ, and i. The Iᔧ allele codes for the A antigen on red blood cells, while the Iᔥ allele codes for the B antigen. The allele i codes for no antigen. Alleles Iᔧ and Iᔥ are codominant with each other, meaning that if both are present in an individual (genotype IᔧIᔥ), both A and B antigens will be expressed, resulting in blood type AB. However, both Iᔧ and Iᔥ are dominant over the i allele. This means a person with genotype Iᔧi will have blood type A, and a person with genotype Iᔥi will have blood type B. Individuals with genotype ii will have blood type O. This genetic system explains the four primary blood types: A, B, AB, and O, and their specific inheritance patterns.