Blood group of parents: father A mother B — possible blood groups of children

Question

Father has blood group A and mother has blood group B. What are all the possible blood groups of their children?

Solution — Step by Step

The ABO blood group system is controlled by a single gene with three alleles: $I^A$ , $I^B$ , and $i$ .

Blood group A: genotype $I^A I^A$ (homozygous) or $I^A i$ (heterozygous)
Blood group B: genotype $I^B I^B$ (homozygous) or $I^B i$ (heterozygous)
Blood group AB: genotype $I^A I^B$ (co-dominance)
Blood group O: genotype $ii$ (recessive)

$I^A$ and $I^B$ are co-dominant — both are expressed when present together, giving blood group AB. $i$ is recessive to both.

Father (Blood group A): either $I^A I^A$ or $I^A i$

Mother (Blood group B): either $I^B I^B$ or $I^B i$

Since the question asks for ALL possible blood groups of children, we need to consider all combinations of parental genotypes. The most interesting case (and the one that gives maximum diversity) is when both parents are heterozygous:

Father: $I^A i$ × Mother: $I^B i$

Cross: $I^A i \times I^B i$

	$I^B$	$i$
$I^A$	$I^A I^B$	$I^A i$
$i$	$I^B i$	$ii$

Offspring genotypes: $I^A I^B$ : $I^A i$ : $I^B i$ : $ii$ = 1 : 1 : 1 : 1

$I^A I^B$ → Blood group AB
$I^A i$ → Blood group A
$I^B i$ → Blood group B
$ii$ → Blood group O

All four blood groups (A, B, AB, O) are possible in a 1:1:1:1 ratio, when both parents are heterozygous.

If one or both parents are homozygous, the range is more limited. For example, $I^A I^A \times I^B I^B$ gives ONLY $I^A I^B$ children (all AB).

Why This Works

The ABO system involves co-dominance between $I^A$ and $I^B$ alleles. When both are present, both glycoproteins are produced on red blood cells, giving blood group AB. This is unlike typical dominance where one allele “wins.”

The key insight: blood group O is a recessive phenotype requiring homozygosity ( $ii$ ). Even if neither parent has blood group O, they can carry the $i$ allele silently (as heterozygotes) and produce O-type children. This is why blood typing can sometimes yield “surprising” results in families.

Alternative Method — Other Parental Genotype Combinations

If both parents are homozygous: $I^A I^A \times I^B I^B$ → only AB children (no A, B, or O possible).

If father is $I^A i$ and mother is $I^B I^B$ : possible children are $I^A I^B$ (AB) and $I^B i$ (B). No A or O possible.

If father is $I^A I^A$ and mother is $I^B i$ : possible children are $I^A I^B$ (AB) and $I^A i$ (A). No B or O possible.

Common Mistake

Students often think a person with blood group A has genotype $I^A I^A$ only. They forget the heterozygous possibility $I^A i$ . This matters: a person with genotype $I^A i$ carries the $i$ allele silently and CAN pass it to children, potentially producing an O-type child even if neither visible parent is type O. Always consider both homozygous and heterozygous possibilities unless the question specifies “pure breeding.”

Question

Solution — Step by Step

Why This Works

Alternative Method — Other Parental Genotype Combinations

Common Mistake

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