512 inheritance cells — an AB and an O parent can have neither an AB nor an O child
Each parent passes on one of the two alleles they carry, so the child has four possible genotypes — count the four squares and you have the probability. A and B beat neither each other nor lose to anything but O; for Rh, a single D makes you positive. A parent’s blood type does not pin down their genotype (an A parent may be AA or AO), so the probabilities here are given for a fixed pair of parental genotypes. Eight fathers × eight mothers × eight children: 512 cells.
Possible children
Father \ Mother
| O+ | O− | A+ | A− | B+ | B− | AB+ | AB− | |
|---|---|---|---|---|---|---|---|---|
| O+ | 2 | 2 | 4 | 4 | 4 | 4 | 4 | 4 |
| O− | 2 | 1 | 4 | 2 | 4 | 2 | 4 | 2 |
| A+ | 4 | 4 | 4 | 4 | 8 | 8 | 6 | 6 |
| A− | 4 | 2 | 4 | 2 | 8 | 4 | 6 | 3 |
| B+ | 4 | 4 | 8 | 8 | 4 | 4 | 6 | 6 |
| B− | 4 | 2 | 8 | 4 | 4 | 2 | 6 | 3 |
| AB+ | 4 | 4 | 6 | 6 | 6 | 6 | 6 | 6 |
| AB− | 4 | 2 | 6 | 3 | 6 | 3 | 6 | 3 |
Which children which parents can have, worked out with Punnett squares. An AB and an O parent can have neither an AB nor an O child.
The Punnett square
Put the father’s two alleles across the top and the mother’s down the side, fill in the four squares, and that is every genotype the child can have. Translate the four into phenotypes and count: there is the probability. ABO and Rh sit on different chromosomes and are inherited independently, so you count each separately and multiply — four squares times four squares, sixteen in all.
Rh+ parents can have an Rh− child
A single D already reads as Rh positive, so an Rh+ person may be DD or Dd. If both parents are Dd, one child in four is dd — Rh negative. The reverse has no escape hatch: Rh− is only ever dd, so two Rh− parents can have only Rh− children.
Swapping father and mother
Neither ABO nor Rh sits on a sex chromosome, so swapping father and mother changes nothing. There are 512 cells, but only about half of them are distinct answers.
There are very rare exceptions
People with the Bombay phenotype test as group O even though they carry A or B genes. There is also cis-AB, where A and B sit on the same chromosome and pass together — cases of an O child from an AB parent have been reported. These are very rare, but they mean a result that contradicts this chart is not by itself evidence of anything.
Blood type is not a parentage test
All this chart can do is exclude. It can say a combination is impossible; it cannot say that a possible one means the child is yours — far too many people share any given blood type. And the rare exceptions above mean even the exclusions are not certainties. Parentage is settled by DNA testing.
How to read it
- There are three ABO alleles — A, B and O — and each person carries two of them.
- A and B do not lose to each other and beat only O, which is why AB exists.
- An A person may be AA or AO; an O person can only be OO.
- For Rh, D beats d: Rh+ is DD or Dd, Rh− is dd.
- Pair the father’s two with the mother’s two, count the four squares, and you have the probability.
- ABO and Rh sit on different chromosomes, so count them separately and multiply.
Common questions
Q. Can two A parents have an O child?
Yes. An A person may be AA or AO, and if both parents are AO then one child in four is OO — group O. Both parents look like A while quietly carrying an O.
Q. Why can an AB and an O parent not have an AB child?
An AB child needs an A from one side and a B from the other, and the O parent has only O to give. The same reasoning rules out an O child, since the AB parent carries no O. That leaves A and B.
Q. Can Rh+ parents have an Rh− child?
Yes. Rh+ may be DD or Dd, so if both parents are Dd, one child in four is dd. The reverse does not happen: two Rh− parents cannot have an Rh+ child.
Q. Can this chart settle parentage?
No. It can name impossible combinations, but a possible one proves nothing, and rare exceptions like the Bombay phenotype or cis-AB mean even the exclusions are not certainties. Parentage is settled by DNA testing.