What is the genetic basis of the A/B/B+/O/etc. blood type system? Are there definitive loci that correspond to each or can multiple different genotypes produce the same antigen profile? Also, is the blood type of a child necessarily one of the blood types of the parents, or can an A/B female cross to A/B male produce a child with blood type O?
I can answer only half your question.
It is not necessary that the child has to have the blood group of parents. The inheritance is like this : 1.There are 3 alleles Ia , Ib and Io.
2.Ia and Ib are codominant i.e. they will both be expressed if present together.
3.Ia and Ib are dominant over Io. So, blood group O can only be expressed if the genotype is IoIo. IaIa or IaIo is blood group A and IbIb or IbIo is blood group B.
5.IaIb is blood group AB
So in the case of both the parents being AB blood group,the child can not have O group.S/he can have any other blood group.
The inheritance of negative and positive blood group follows simple mendelian inheritance.
Please feel free to correct me!
There is only one locus involved in ABO blood type, the ABO gene on chromosome 9. It has three alleles: A, B, and O.
We have two copies of every gene, one from each parent. Because the O allele is recessive to A and B, there are blood types with multiple genotypes. AO and AA will both produce blood type A, and BO and BB will both produce blood type B. Only OO will produce blood type O, and AB will get you blood type AB. The specific cross you mentioned (AB x AB) will never produce a child with blood type O, because neither of the parents had any O alleles for the child to inherit. However, any other blood type is possible with this particular cross.
The + or - you sometimes see is a separate system called the rhesus blood type system. The genetics involved in that are actually a bit complicated, but they don't involve the ABO gene at all.