If someone has blood type A+: which are his possible genotypes and who can he donate blood to?
The ABO blood type system relies upon the co-dominant alleles IA and IB, which are expressed any time they are present and are dominant over i which is recessive. If someone has blood type A, that means in their genotype they have at least one IA allele, for otherwise they would not express phenotype A. In addition we can infer, they do not have allele IB in their genotype, because if they had it, they would express it. So the possible genotypes of someone whose phenotype is A+ are: IA IA (homozygote: both alleles are the same) and IA i (heterozygote: alleles are different). In both cases , A would be expressed and B would not, so the phenotype would be A. Regarding the +/- blood type, this trait is determined by the presence of the allele Rh which is dominant over rh (which is recessive). Whenever Rh is present, it is expressed and the phenotype is +. Since for the studied individual the phenotype is +, we know Rh is present. But we do not know if the individual is heterozygote or homozygote: so there are two possible genotypes RhRh and Rhrh.
Regarding blood donation, the key idea to consider is that one can only donate to people that have the same antigens (traits like +, A or B) than one has. This includes people who have the same antigens plus some extra ones. The reason for this is that the body only rejects antigens that are foreign to theirs. An individual with phenotype A+ has got antigens A and +. So he cannot donate to people who do not have antigens A and +: that rules out A- (they do not have +), B- (they do not have A or +), AB- (they do not have +), 0- (they do not have A or +), 0+ (they do not have A) and B+ (they do not have A). So the studied individual can donate to: A+ (share A and + and have no extra antigen) and AB+ (they share A and + and have the extra antigen B)
