A gene for the MN blood group has codominant alleles M and N. If both children are of blood type M, which of the following is possible? Both children are heterozygous for this gene. Each parent must be type M. Each parent is either M or MN. Neither parent can have the N allele.

Answer:

The correct answer would be "Each parent is either M or MN".  

Co-dominance is the condition in which both the alleles express themselves completely when present in the heterozygous condition.

The blood group of the offspring can be of M type in two conditions:

1. When both the parents are of M blood group  

2. When both the parents are heterozygous for the blood group that is, they both are MN.