Blood Types and Genetics

Factual Questions
1

Is it possible for two A blood type parents to have an O child? And if so, how does that work. I had always assumed that an A- mother and an A+ father have two possibities when the child is born, A- or A+. I read a post on a pregnancy message board talking about rhesus factors where one woman said that she was A- and hubby was A+ and they have two O+ daughter and one A+ son. Does her hubby need to worry that the first two daughters aren’t his?

Thanks:confused:

2

Yes. The “O” gene is recessive. So a person with type A or B can also have an “O” gene from the other parent.

3

This also means that both parents have to be carrying the recessive “O” gene.

4
  • and - refer to Rh type. Somebody who is Rh- is homozygous recessive. Rh+ could be homozygous dominant or heterozygous, so a + and - mating could produce either + or - children.

Blood type genotypes vs. phenotypes:

Genotype Phenotype
aa A
a0 A
bb B
b0 B
ab AB
00 O

Blood typing is an extremely simple genetic system.

5

A person with type A blood has one type A gene and one type O gene. A person with type B blood has one B gene and one O gene. Thus two people with type A or B blood (or one of each) have a 25% chance of having a child with type O blood (two O genes). A mixed couple with an A parent and a B parent also have a 25% chance of having an AB child (one A gene and one B gene).

6

Thank you…that’s good to know…so if the O gene is recessive and my bf is O+ and I know for a fact that both my parents were A, does that mean I’ll have an A baby?

7

Knowing only that both your parents were A type means that you are either A or O type. If you are O, you and your boyfriend have a 100% chance of having an O baby. If you are A, you and he have a 50% chance of having an O baby.

8

If one of your A parents was O recessive, you could be carrying the O recessive gene. If you are the chances only 1/4 for an A child (with a recessive O gene), vs. 3/4 for an O child.

9

Q.E.D. I am A, sorry, forgot to put that in my post. So even though my parents were both A, that doesn’t mean that I am AA, I could be AO? I think I’m getting it now…sorry to be so dense…thank you for your time.

10

Yes. One or both of your parents could be AO, and you could have gotten the recessive gene. If you’re O, you know you are OO (no A or B genes), or if you’re AB, you know you are AB (no O genes). A’s and B’s could have recessive O’s.

In the above example, it shoukld be 50/50 of either A or O. My mistake, the four options are AO, OO, AO, or AO. :smack:

11

To elaborate a little more:

Since you don’t know your parents’ genotype, but only their phenotype (A), several possibilities exist:

If they are both genotype AA, then you would have a 100% chance of being AA.

If one is AA and the other AO, then you would have a 50% chance of being AA and 50% of being AO.

If both are AO, then you have a 25% chance of being AA, 50% of being AO, and 25% of being OO. (Of course, since you already know you are A phenotype, in this scenario you actually have a 33.3% chance of being AA and 66.7% chance of being AO).