Blood Groups

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No particular reason, it just has the least incidence in the population.

It is only the "A" and "B" combination of alleles that give rise to group "AB", whereas A can be "AA" or "AO" and B can be "BB" or "BO".

AB should be as common as "OO" but there is only in fact 1 allele that gives "B", and 2 that give "A" where as there are 3 alleles that code for "O" meaning that there is a higher probability of having blood group "OO" than "AB" (and as a result, AB is the rarest group).

http://en.wikipedia.org/wiki/ABO_blood_group_system#Distribution_and_evolutionary_history

Reply 3

Renal

Anyway, the proper answer is easy. In order to have an AB kid you need mum to be AB or A and dad to be AB or B. If you go back to the genotype tables you'll be able to work out the likelihood from there. However, fundamentally, it's because AB is very rare and B is rare - there are not many of the right genes circulating in the population.