ABO Blood Groups

The alleles and how they interact

The ABO blood group gene has three alleles: I^A (produces antigen A), I^B (produces antigen B), and i (produces neither antigen). I^A and I^B are codominant with each other, and both are dominant over the recessive allele i.

Worked problem 2 (medium): predicting children's blood groups

A father with blood group A, genotype I^A i, has children with a mother with blood group B, genotype I^B i.

Parental genotypes: I^A i x I^B i.

Gametes: father produces I^A and i; mother produces I^B and i.

Reading the result

Reading the genotypes against the phenotype table gives a possible blood group ratio of 1 AB : 1 B : 1 A : 1 O among the children. This shows that heterozygous parents of blood groups A and B can, together, produce children of all four possible blood groups.

How it is examined

SPM questions typically give the blood groups of two parents (sometimes stating whether they are homozygous or heterozygous) and ask you to determine the genotypes, draw the cross, and state the possible blood groups of the children.

Worked problem 1 (easy): group O mother x homozygous group A father

A mother of blood group O (genotype i i) has children with a father who is homozygous group A (genotype I^A I^A). The mother's gametes all carry i, and the father's gametes all carry I^A, so every child receives one allele of each type.

Worked problem 3 (SPM level): group AB father x heterozygous group A mother

A father of blood group AB (genotype I^A I^B) has children with a mother of blood group A who carries a recessive allele (genotype I^A i). The father produces I^A and I^B gametes, and the mother produces I^A and i gametes.

Reading each genotype against the phenotype table gives 2 group A (I^A I^A and I^A i) : 1 group AB (I^A I^B) : 1 group B (I^B i), a phenotype ratio of 2 A : 1 AB : 1 B. No child can be group O, because the father has no i allele to pass on, so no offspring can be i i.

Practice problems

Work out each cross on paper, then check your reasoning against the answers below.

• 1. A father is blood group AB and a mother is blood group O. State the possible blood groups of their children and the ratio.
• 2. Both parents are blood group A and heterozygous (I^A i). State the possible blood groups of their children and the ratio.
• 3. A man of blood group B (I^B i) has children with a woman of blood group A (I^A i). What is the probability that a child is blood group O?

Answers

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions