Someone recently asked me how O can be both a recessive allele and the world's most common blood type. She had never heard of the Punnet Square, which, in my day, we all had to learn in 7th grade Science. I explained to her that two parents carrying recessive O alleles can produce an O child, even though neither is O; O's can show up as the children of 2 A's, 2 B's, etc.

Not everyone knows how to make or use the Punnet Square, but many wonder about the possible blood types for offspring of parents of the various types. Another question often posed is that of a parent's blood type when the type of the child and the other parent are known. Here's the handy-dandy Results Sheet for your reference:

Parents

O+O = 100% O

O+A

O+Ao = 50% A, 50% O
O+Aa = 100% A

O+B

O+Bo = 50% B, 50% O
O+Bb = 100% B

O+AB = 50% A, 50% B

A+A

Ao+Ao = 75% A, 25% O
Ao+Aa = 100% A
Aa+Aa = 100% A

A+B

Ao+Bo = 25% A, 25% B, 25% AB, 25% O
Ao+Bb = 50% B, 50% AB
Aa+Bo = 50% A, 50% AB
Aa+Bb = 100% AB

A+AB

Ao+AB = 50% A, 25% B, 25% AB
Aa+AB = 50% A, 50% AB

B+B

Bo+Bo = 75%B, 25% O
Bo+Bb = 100% B
Bb+Bb = 100% B

B+AB

Bo+AB = 25% A, 50% B, 25% AB
Bb+AB = 50% B, 50% AB

AB+AB = 25% A, 25% B, 50% AB

Note that these are odds, i.e., likelihoods, not actual results. For example, the four children of a Bo/Bo couple could be all B's, even though the likelihood of each child's being B was only 75%.