If child has O blood type, what blood type is parent be?

Since AA, AO and OA are blood type A, and OO is blood type O, thus their child has 6.25% chance to be blood type O and 93.75% chance to be blood type A. The +/- is called the rhesus factor, with + being dominant, and - being recessive. So if both parents are -, the kids are always -, otherwise the kids might be + or -. read more

In this case, the child is "pure-bred" for the A gene. The child can only pass on an A gene to his offspring. If one parent contributes and A gene and the other contributes an O gene, the child will have A blood type, but, like his parents, he'll also have both A and O genes that he may pass along to his children. read more