Question

8. A man has blood type A and his wife has type B. A physician types the blood of their four children and is amazed to find one of each of the four blood types among them. He is not familiar with genetics and calls upon you for an explanation. Provide one.

Answer 1

Answer:

A blood test is nothing but to determine whether the A and/or B characteristics are present in a blood sample. It is not possible to determine the exact genotype from a blood test result of either type A or type B. The classification of human blood (ABO blood group) based on the inherited properties of red blood cells (erythrocytes) and that is determined by the presence or absence of the antigens A and B.

In given example man has blood type A, he will have at least one copy of the A allele, but he can have two copies of the allele A. Therefore his blood group genotype is either AA or AO.

Wife is having blood type B. Similarly she will have at least one copy of the allele B, she can have two copies of the allele B. therefore her blood group genotype is either BB or BO.         

           

Parents	Blood type	possible genotypes
Mother	B	BB (homozygous) or BO (heterozygous)
Father	A	AA (homozygous) or AO (heterozygous)

Their four children are having one of each of the four blood types among them. So children genotypes are as follows:

Children No.	Blood type	Possible genotypes
1	A	AA AO
2	B	BB BO
3	AB	AB
4	O	OO

Both A and B alleles are dominant over O. As a result, individuals who have an AO genotype will have an A phenotype (like father in given problem). If individuals who have a BO genotype for blood group B, will have a B phenotype (like mother in given problem). People who are type O have OO genotypes it means they inherited a recessive O allele from both parents. The A and B alleles are codominant. Therefore, if an A is inherited from one parent and a B from the other, the phenotype will be AB.

Each biological parent donates one of their two ABO alleles to their child. A mother who is blood type B can pass a B or an O allele to her child. A father who is blood type A could pass either an A or an O allele to his child. If each of the four children is having one of each of the four blood types among them this is only possible when both the parents are heterozygous for their blood group. It means fathers genotype is AO and mother’s genotype is BO then only all the four children will have one of each of the four blood types among them. Using a Punnett square you can figure it out the child’s possible blood types based on the parents’ blood types (see figure).

Parent’s possible genotype and their cross (AA or AO) X (BB or BO)

It means fathers genotype is AO and mother’s genotype is BO then only all the four children will have one of each of the four blood types among them. Now we know that parents are heterozygous for their blood types and that is the reason their 4 children’s are having one of the four blood types among them.

Children’s possible Blood groups are: AB, AO, BO, OO (1:1:1:1)

If both the parents are homozygous for their blood group (AA x BB), then all the children’s will have AB blood group (see the 1^st Punnett square).

Father can pass on either his A or his O gene version. Mother can also either pass on her B or her O. Because of this, you can see that there’s 1 in 4 or 25% chance for a child to have blood type O. Table shows that all possible genotype (in Black) and phenotypes (in red) in the offspring’s.

A blood group type AB or type O is more informative because child with blood type AB must have both the A and B alleles. The genotype must be AB. Child with blood type O has neither the A nor the B allele. Therefore genotype must be OO.