Inheritance of blood groups

Codominance

Some alleles are both expressed in the same phenotype, this is called codominance.

Both codominant alleles are shown with upper case letters in genetic diagrams, but the letters used are different. For example, feather colour in hens may be white, black or speckled (it has both white feathers and black feathers).

The alleles can be shown as W for white and B for black.

There are three possible genotypes: WW, BB and BW.

There are also three possible phenotypes: WW = white, BB = black, and BW = speckled.

Blood groups

The gene controlling human ABO blood groups has three alleles, not just two:

  • IA and IB are not dominant over one another
  • both are dominant over IO

This table shows the possible genotypes (alleles present) and phenotypes (blood group):

GenotypePhenotype
IA IA or IA IOA
IB IB or IB IOB
IA IBAB
IO IOO

So to inherit blood group A you must receive an IA allele from one parent and either an IA or an IO allele from the other. To inherit blood group B you must receive an IB allele from one parent and either an IB or an IO allele from the other. To inherit blood group AB you must receive an IA allele from one parent and an IB allele from the other. To inherit blood group O you must receive an Io from both parents.