The ABO blood group assorts independently of the Rhesus (Rh) blood group and the MN blood group. Three alleles, IA , IB , and i, occur at the ABO locus. Two alleles, R, a dominant allele producing Rh+, and rh, a recessive allele for Rh−, are found at the Rh locus, and codominant alleles M and N occur at the MN locus. Each gene is autosomal. A child with blood types A, Rh−, and M is born to a woman who has blood types O, Rh−, and MN and a man who has blood types A, Rh+, and M.
Determine the genotypes of each parent.
What is the mother’s blood type? The father's blood type?

What proportion of children born to a man with genotype IAIB Rr MN and a woman who is IAi Rr NN will have blood types B, Rh−, and MN?

Part 1:

The mother’s genotype is ii, rr, MN

The father’s genotype is IAIA/IAi, Rr, MM

Part 2:

Approximately 1/32 (3.125%) of children can be expected to have the stated blood types.

Explanation:

Part 1:

The mother has blood types 0, Rh- and MN. Therefore, her genotype has to be ii, rr, MN.

The father has ABO type A. His ABO genotype may therefore be either IAIA or IAi, as both are compatible with type A offspring with a type 0 mother. His Rh genotype must be rr, since his Rh- child necessarily has to have inherited one r allele from each parent. Finally, his MN genotype has to be MM since both alleles are codominant.

Part 2:

Using the given data, we build the following Punnett chart:

                IA R N     IA r N     i R N     i r N    

IA R M

IA R N

IA r M

IA r N

IB R M

IB R N

IB r M                                                    X

IB r N                                                              

Note that, of all 32 possible combinations, only a single one of them (marked with “X” produces a genotype which matches the desired blood type, as follows:

genotype: IBi, rr, MN

blood type: B, Rh-, MN