In some cases the gene comes in three different alleles. One example is human bl

Question

In some cases the gene comes in three different alleles. One example is human blood groups which is affected by a single group with three alleles: O, A and B. O is recessive, so the only way to be blood group O is to have two O alleles. A and B are dominant, so AA and AO individuals are blood group A, while BB and BO individuals are blood group B. A and B are "codominant" with each other, this means that AB individuals end up in the AB blood group.

Image two parents, one of whom is blood group A and the other is blood group B, but you don't know whether the parents are heterozygotes or homozygotes (assume there is a 50/50 chance of either zygosity).

Starting from the same two parents, again not knowing whether they are heterozygotes or homozygotes, what is the probability that their child is blood group O?

Explanation / Answer

1a. When mother and father are both homozygotes

AB blood groups will only be obtained.

b. When mother is a heterozygote and father is a homozygote.

The BO genotype will give B blood groups, while AB will have AB blood group (50%:50%)

c. When mother is a homozygote and father is a heterozygote.

The AO genotypes will give A blood groups, while the AB genotypes will have AB blood group (50%:50%).

d. When both mother and father are heterozygotes.

The blood groups will be AB, A, B and O in equal proportion (25%)

Reversing the parental blood groups (father with A and mother with B) will also give the same ratio.

Hence, the only possibility of obtaining an O blood group (25%) is when the parents are heterozygotes. The chance is 1 in 4 offsprings (1/4) in these case.

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