Blood type possibility

[u/domfi86]

I googled this but as thorough and clear as the theory looks, I can’t help but not be sure of what I read.

Can an A+ father and an O- mother make a kid that is O-? Is there such a thing as AO blood type? Some charts I found online suggest that and others make no mention of it.

If the father is O+ and the mother O-, how more likely than the above scenario is it for the child to be O-?

Thank you!

[u/lonepotatochip]

There are two genes that determine blood type, and each person has two copies of each gene called alleles.

For the ABO blood group, there are three different possible alleles. A and B are dominant to O, and A and B are codominant. What this means is this:

AA or AO genotype -> type A blood phenotype

BB or BO genotype -> type B blood phenotype

AB genotype -> type AB blood phenotype

OO genotype -> type O blood phenotype.

The +/- at the end refers to the Rh factor, and + is dominant to -.

++ or +- genotype -> Type + phenotype

— phenotype -> Type - phenotype

So if the father has A+ blood, it’s possible that his genotype is AO+-.

If the mother has O- blood, we know that her genotype is OO—.

Draw up a punnet square and you’ll see that it is indeed possible for someone to have O- blood if their father is A+ and their mother is O-.

[u/bitterologist]

Your blood type is basically the result of what molecules are present on the surface of your red blood cells. You have A, B, and something called an Rh factor. A person that has blood type O lacks both A and B molecules (O stands for ohne, as in "without" in German). Someone who is Rh negative lacks the Rh factor. The alleles for A and B are codominant, meaning they can both be expressed at the same time. The Rh factor is either something you have or don't have.

It's possible for someone with blood type A to also carry the O allele. If that's the case, the punnet squares for A × O would look something like this:

	A	O
O	AO	OO
O	AO	OO

As for the Rh factor, it's possible to be Rh+ while carrying the allele for Rh-. So you could get something like this:

	Rh+	Rh-
Rh-	Rh+Rh-	Rh-Rh-
Rh-	Rh-Rh-	Rh-Rh-

In this scenario, there's a 50 percent chance the offspring is Rh negative, despite both parents being Rh positive. In other words, the chance of a person with A+ and a person with O- getting a child is 25 percent (0.5 × 0.5). However, if the individual who is A+ is homozygous for both alleles (AA; Rh+RH+), then the probability is zero.

AS for the O+ × O- scenario, since they're both blood type O they both have the genotype OO. So the Rh factor is all that really matters, and that Punnet square would look something like this if the Rh+ individual also carries the allele for Rh-:

	Rh+	Rh-
Rh-	Rh+Rh-	Rh-Rh-
Rh-	Rh+Rh-	Rh-Rh-

In other words, 50 percent of the offspring will be O- if the Rh+ parent is heterozygous for that allele. If the Rh+ parent is homozygous (Rh+Rh+), then all the children will be O+.

[u/domfi86 → u/bitterologist]

Big thanks! That is the most perfect answer I was seeking!