How does an A plus and an A- have a O child?

Answer 1

AO and AO through sexual reproduction at 25%.

Firstly, the positive and negative in A+ and A- is indicative of the Rh factor which is inconsequential in forming a O child. (It is important as concerns the difference between O+ and O-, but not in determining O vs. A, B, or AB.)

The phenotype (exhibited characteristic) of A-type blood can be caused by two allele combinations: AA and AO. Since the O in the AO is recessive, the dominant A is exhibited. (Note that the AA does not exhibit a "greater" amount of A-type blood.) When genes are passed from father/mother to child (in sexual reproduction) the gametes are formed using only one half of the alleles. (Therefore an AA person will always donate a single A and an AO person will donate an A half of the time and an O the other half of the time.)

In order to achieve the result requested, both parents must carry the recessive O gene in order for the child to get both recessive genes and exhibit O characteristics. Therefore both parents must be AO. Two AO parents will produce 25% AA children, 50% AO children, and 25% OO children where AA and AO exhibit as A and OO exhibits as O.