If a Mother is A positive blood group and a Father is A positive blood group can they have a child that is O negative blood group?

That is always possible if both the parents are heterozygous for Rh antigen gene. It means that both the parents. There are 2 different alleles for the Rh factor known as Rh+ and Rh-.Someone who is "Rh positive" or "Rh+" has at least one Rh+ allele, but could have two. Their genotype could be either Rh+/Rh+ or Rh+/Rh-. Someone who Rh- has a genotype of Rh-/Rh-. If their genotypes are Rh+/Rh-, then their is possibility of of Rh- also.

Rh factor

Possible genotypes

Rh+

Rh+/Rh+

Rh+/Rh-

Rh-

Rh-/Rh-

Prof. A.L. Bhatia

http://www.competition-India.blogspot.com/

http://www.albhatia.in

Type O blood is also recessive to type A, so both parents could be heterozygous type A's. Their genotypes would have to be A/O.