Understanding Blood Group Inheritance
Quick Answer: The ABO blood group is determined by three alleles: IA, IB, and i. IA and IB are codominant, while i is recessive. Rh factor is determined separately, with Rh+ dominant over Rh−.
Key Genotypes & Phenotypes
Type A: IAIA or IAi. Type B: IBIB or IBi. Type AB: IAIB. Type O: ii. Rh+: ++ or +−, Rh−: −−. The two genes assort independently.
Solved Examples
A x B parents
Possible genotypes: A: IAi, B: IBi
Offspring: A, B, AB, O (all 25%)
All four types possible
Possible genotypes: A: IAi, B: IBi
Offspring: A, B, AB, O (all 25%)
All four types possible
O x AB parents
O: ii, AB: IAIB
Offspring: A (50%), B (50%)
No O or AB children
O: ii, AB: IAIB
Offspring: A (50%), B (50%)
No O or AB children
Rh+ x Rh-
If Rh+ is heterozygous, 50% Rh+, 50% Rh-
If homozygous, all Rh+
Dominant inheritance
If Rh+ is heterozygous, 50% Rh+, 50% Rh-
If homozygous, all Rh+
Dominant inheritance
FAQs
Can two O parents have an A child?
No, both O parents have genotype ii, so they can only pass i alleles. All children will be O.
Why is AB called the universal recipient?
AB individuals have both A and B antigens on red cells and no anti-A or anti-B antibodies, so they can receive blood from any ABO type.
Does Rh factor affect pregnancy?
Yes, if an Rh- mother carries an Rh+ baby, she may develop antibodies that attack fetal red blood cells in subsequent pregnancies (erythroblastosis fetalis).