6.2.4

• Genes with multiple alleles

  over time a huge number of changes can occur anywhere within a gene; as a result many genes have more than 2 alleles

  • when 3 or more alleles at a specific gene locus are known, then gene is said to have multiple alleles

  → however any individual can only possess two alleles, one on each gene locus, in a pair of homologous chromosomes
• Human blood groups

  inheritance of human ABO blood groups demonstrates both dominance + codominance

  • the 4 blood groups (phenotypes) are all determined by 3 alleles of a single gene on chromosome 9

  →the gene encodes an isoagglutinogen, I, on the surface of erythrocytes

  • the alleles present in the human gene pool are IA, IB + IO

  → IA + IB are both dominant to IO, which is recessive

  • IA + IB are codominant; so if they are both present in the genotype, they will both contribute to the phenotype

  → any individual will have only two of the three alleles within their genotype