Question

A woman claimed that a certain man was the father of her child. When the case was brought to court, it was pointed out that the woman had blood type O, the child had blood type A, and the man had blood type O. Could he have been the father? What blood types could the biological father have had?

Answer 1

ABO blood group typing helps to identify the blood group of an organism. It is an example for co-dominance. Alleles are the alternate forms of a gene. There are three different alleles for the human blood group type, such as IA , IB and IO (for simplicity we can also represent them as A, B and O respectively). The common genotypes and corresponding phenotypes of human ABO blood typing are listed below.

IA IA and IAIO = blood group A

IB IB and IBIO = blood group B

IAIB    = blood group AB

IO IO =blood group O

During inheritance, each biological parent donates one of their two ABO alleles to their child. So in the given problem the mother's blood group is 'O' and the child's blood type is 'A'. Therefore,

the genotype of the mother whose blood type is O = IO IO

the genotype of the child whose blood type is A    = IAIO

(if blood type is A then the possible genotypes are IA IA and IAIO. But the genotype of this child's mother is IO IO and the mother should donate one IO to the child)

The blood group of the suspected father is O and its genotype is IO IO.

The possible blood type this mother and father's offsprings could have can be determined using a cross between them.

Parents : Mother (IO IO) X Father (IO IO)

Gametes: IO    IO   

F1 generation:    IO IO

From this cross, we can understand that two parents with O blood group will have offsprings with only the blood type O. So we can conclude that this person could not be the biological father of this child.

Since the mother is 'O' blood type (IO IO) the possible blood type of the biological father could be 'A' with the genotypes IA IA or IAIO

The possible crosses are

1) Parents:    Mother (IO IO) X Father (IA IA)

   Gametes:    IO IA

F1 generation:    IA IO (blood type A)

2) Parents:    Mother (IO IO) X Father (IAIO)

   Gametes:    IO IA IO

F1 generation:    IA IO ( A type) and   IOIO (O type)