How is the genetics of human ABO blood groups an example of “Codominance involving multiple alleles”?
The human ABO blood groups are determined by a single gene, designated I, that codes for a glycosyl transferase enzyme. Table 3.3 (page 115) shows the six different possible genotypes (column 1), resulting from the fact that there are three alleles of this gene in the human population. The A and B alleles are both dominant, and the O allele is recessive. Thus, the six diploid genotypes result in four phenotypes (Column 3: blood types A, B, AB, and O). (For our purposes, you don’t have to worry about columns 4, 5, and 6 in this table.) 3. What is meant by the term “Epistasis”? Figure 3.23 shows a classic case of epistasis, which signifies some alteration in the basic 9:3:3:1 phenotypic ratio expected from a dihybrid cross. In the case shown, we are looking at a single phenotypic trait (pea flower color) that is determined by two independent genes. We get the dominant phenotype in plants that have at least one dominant allele of EACH of the two genes; otherwise we get the recessive pheno
