1. Blood Types The gene for producing blood has three variants:
Blood cells include the A antigen
Blood cells include the B antigen
Blood cells have neither (inactive "O" version).
Separately: do you have the Rh factor? (+ or -)

2. Blood Types Frequencies
frequency (US)
note O-
6.6 %
universal donor O+
37.4 % A-
6.3 % A+
35.7 % B-
1.5 % B+
8.5 %
AB-
0.6 %
AB+
3.4 %
universal recipient

3. Genes
Each chromosone has one slot filled with A, B, or O. Separate slot: Rh+ or Rh-.
If mother phenotype is B- and father phenotype AB-, what can children be?
Mother genotype: BB or BO; --.
Father genotype: AB; --.
If mother BB, children are AB-- or B--.
If mother BO, children are AB--,BB--,AO--,BO--.

4. Why it matters
If your blood does not have the A antigen but you get a transfusion that does, your body sees the A as "enemy" – rejects.
What if pregnant, and blood type of child differs from mother?!
A/B/O okay -- antibodies don't pass placenta.
But anti-Rh+ antibodies can cross; dangerous if: mother Rh-, and mother previously exposed to Rh+, and baby is Rh+.