6.2.6

• codominant - where both alleles present in the genotype of a heterozygous individual contribute to the individual’s phenotype
• Codominant alleles

  where both alleles present in the genotype of a heterozygous individual contribute to the individual’s phenotype, they are described as codominant

  →the two alleles are responsible for two distinct + detectable gene products

  the phenotype of heterozygotes is different from the phenotype of the homozygotes
• Codominant inheritance in animals

  coat colour in shorthorn cattle is an example as the one gene for coat colour has two alleles: C^R (red) + C^W (white)

  • cattle that are homozygous for the red-coat allele, C^R, has a red coat
  • cattle homozygous for the white-coat allele
  • heterozygous cattle, genotype C^RC^W, have both making roan colour
• Codominant inheritance in humans

  MN blood groups

  The MN blood group system is controlled by a single gene with two alleles, G^M + G^N

  • the gene codes for a particular gene on the surface on erythrocytes
  • the G^M allele codes for one version of the protein + the G^N alle codes for a slightly different version of the protein

  → these alleles are codominant

  figure 4 shows the probability of inheritance of the MN blood group system in children of a couple, where both are heterozygous, having blood group MN

  
• ABO blood groups

  you will notice from that information that the alleles I^A + I^B are codominant to each other

  • an individual of genotype I^A I^B are codominant expresses both + has both types of isoagglutinogen protein on their erythrocytes
  • the inheritance of these blood groups also shows dominance, as both I^A + I^B are dominant to the allele I^o
• Sickle cell anaemia

  caused by a mutation in the gene that codes for the beta-globin chain of haemoglobin

  • the mutant allele is given the symbol Hb^s + the normal allele is given the symbol Hb^N
  • in heterozygous people, at least half the haemogloblin in their red blood cells is normal + half is abnormal

  → by they don’t suffer from sickle cell anaemia