6.2.4 - Multiple Alleles

• Over time, a huge number of changes can occur in a gene; as a result, many genes have more than two alleles.
• When a gene has 3 or more potential alleles in the gene population which could present at the locus, the gene is said to have multiple alleles:

  • Any individual can only possess two alleles, one on each gene locus in a pair of homologous chromosomes.
• Inheritance of ABO Blood groups is a good example of multiple alleles. It also demonstrates both dominance and codominance.
• Codominance - Both alleles present in the genotype of a heterozygous individual contribute to the phenotype.
• There are 3 alleles responsible for blood group inheritance:

  • The 4 blood groups, A, B, AB and O are determined by 3 alleles on chromosome 9.
  • The gene encodes an isoagglutinogen on the surface of erythrocytes.
• The alleles present in the human gene pool are IA, IB and IO:

  • IA and IB are codominant, IO is recessive.
  • If both IA and IB are present in the genotype, both with contribute to the phenotype.
  • Any individual will have 2/3 alleles in their genotype.
• Coat colours in rabbits are determined by 1 gene with 4 alleles:

  • Agouti (grey base, yellow band, white tip) is dominant to all other alleles, C.
  • Chinchilla (silvery grey hairs, no yellow band) Cch, is dominant to Himalayan (White but black feet, ears, nose and tail), Ch.
  • Albino (white) is recessive to all other alleles.