I was watching a Ted-Ed video about blood types. It said that an O kid is only possible for O & O parents. I want to make sure of that fact, and to know whether it's related to only and specifically parents or not.

If an O+ individual has an O+ mother what are the probabilities for the blood type of his biological father?

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  • I guess I found the video you've mentioned; there "O and O parents" refers to the allele that each parent transfers. As stated in my answer, this allele can be present in O, A or B blood types.
    – practiZ
    Jan 19, 2019 at 14:20
  • Well thanks for the clarification, that cleared it out for me.
    – user597368
    Jan 19, 2019 at 14:38

1 Answer 1


If a child has blood type O, it means both his alleles are recessive (since A and B are dominant over O); one of them he gets from their mother, and the other one from their father. That means that the father must have at least one recessive allele that he transfers to the offspring; so the possibilities of the father's genotype are OO, AO or BO.

As for the Rh inheritance, it is pretty much the same: positive allele is dominant over negative. So in order for somebody's phenotype to be expressed as positive, the child only needs one positive allele. We only know that the mother is Rh+, so likewise her genotype is either Pos/Pos or Pos/Neg. Therefore she either donates her Pos allele to the offspring - in this case, it absolutely does not matter what the child receives from his father; or she transfers her Neg - here, the child has to get a Pos from his father, which leaves the father with possible genotypes Pos/Pos or Pos/Neg.

Some pretty accurate additional info at:

  • So the genotype is only what matters, I mean can the father in this case have A or B blood types?
    – user597368
    Jan 19, 2019 at 8:51
  • @user597368 blood types are named after genotypes that code for them: blood type A has either AA or A0 genotype, B is either BB or B0, 0 is 00, and AB is pretty much self explanatory. And generally speaking - yes - just as anything in your body, the expression of certain trait depends on its gene.
    – practiZ
    Jan 19, 2019 at 9:54
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  • 2
    This is not necessarily the case (if a child has blood type O, it means both his alleles are recessive). What it means is that the child's blood doesn't have the oligosaccharide antigens of any of the many A or B types. There are many reasons this could occur, most commonly, the genes for the enzymes that produce those antigens are not present (i.e., both alleles are recessive), but it's not necessarily that simple. This is why blood type is not a good way to rule out paternity.
    – De Novo
    Jan 24, 2019 at 0:32

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