Question

Codominance is a form of inheritance in which two different alleles for a gene are both expressed, and neither allele is dominant over the other. One example of codominance is blood type in humans; the alleles for type A blood and type B blood are codominant. If a man who is homozygous for type A blood and a woman who is homozygous for type B blood have children, what would you expect to find in the children?
Half will have type A and half will have type B blood.
The children will all have type B blood.
The children will all half type A blood.
The children will all have type AB blood.

Answer 1

First we establish a table of genotypes and phenotypes:

genotype  phenotype
AA             type A
BB             type B
AB             type AB

Homozygous type A   has genotype AA
Homozygous type B   has genotype BB

Now we make the punnett square

           A             A
B        AB          AB
B        AB          AB

So we see that all (100%) of outcomes are type AB.