B− × B+ parents, A+ child — impossible

Probability by parental genotype

No combination of parental genotypes produces a A+ child.

All eight children for these parents

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.

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.

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.

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.