Genetics study guide

Genetic variants in the coding region of a gene change the DNA base sequence. This can alter the sequence of codons in messenger RNA, leading to differences in which amino acids are joined together during protein synthesis. Because the order of amino acids determines how a protein folds and what shape it takes, even a single change can alter the protein’s three-dimensional structure. This can change or reduce the protein’s activity, often leading to noticeable changes in an organism’s phenotype—its observable characteristics. The process highlights a direct link from genetic code, through protein structure, to phenotypic traits.

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.