Blood type is one of the most straightforward examples of Mendelian genetics in human biology — but the rules trip people up constantly. Your ABO blood type and Rh factor are determined by genes inherited from both parents, and the possible combinations follow predictable patterns.
How ABO Blood Type Is Inherited
The ABO blood group is controlled by a single gene on chromosome 9 with three alleles — A, B, and O. You inherit one allele from each parent, giving you two alleles total. Your blood type (phenotype) depends on which combination you carry:
- AA or AO → Blood type A
- BB or BO → Blood type B
- AB → Blood type AB
- OO → Blood type O
Key point: A and B are codominant, and both are dominant over O. The O allele is "silent" — it produces no antigen. Two parents who are both type A can carry the hidden AO genotype and produce an O-type child. This surprises people who assume blood type is a simple either/or trait.
Which Blood Types Can Two Parents Produce?
- O + O: O only (no A, B, or AB possible)
- A + O: A or O (no B or AB possible)
- B + O: B or O (no A or AB possible)
- A + A: A or O (no B or AB possible)
- B + B: B or O (no A or AB possible)
- A + B: A, B, AB, or O (all four types possible)
- AB + O: A or B only (no O or AB possible)
- AB + A: A, B, or AB (O not possible)
- AB + B: A, B, or AB (O not possible)
- AB + AB: A, B, or AB (O not possible)
Notable: two AB parents cannot produce a type-O child. An AB parent and an O parent can never produce AB or O children — only A or B.
The Rh Factor: Positive or Negative
The Rh system is separate from ABO and adds the "+" or "−" to your blood type. Rh positive means you carry at least one copy of the RhD antigen gene; Rh negative means you carry no copies. Rh positive (D+) is dominant over Rh negative (D−):
- Two Rh-positive parents who are both heterozygous (D/d) have a 25% chance of producing an Rh-negative child
- Two Rh-negative parents will always produce Rh-negative children
- One Rh-positive, one Rh-negative parent: child is 50–100% likely positive depending on whether the positive parent is homozygous or heterozygous
Why Rh Status Matters in Pregnancy
If an Rh-negative mother carries an Rh-positive baby (which happens when the father is Rh-positive), small amounts of fetal blood can cross the placenta. The mother's immune system may produce anti-D antibodies. In a subsequent pregnancy with an Rh-positive fetus, these antibodies can cross the placenta and attack the baby's red blood cells — causing hemolytic disease of the fetus and newborn (HDFN), which ranges from mild jaundice to life-threatening anemia.
Prevention: Rh-negative pregnant women are given Rh immunoglobulin (RhoGAM) at 28 weeks and within 72 hours of delivery to prevent sensitization. This is one of the most successful preventive interventions in modern obstetrics.
Can a Child Have a Different Blood Type Than Both Parents?
Yes — and this is common, not unusual. A type-A parent and a type-B parent can produce a type-O child. Two type-A parents can produce a type-O child. What matters is the underlying genotype (which alleles the parents carry), not just the phenotype (which blood type they express). Historically, blood typing was used in paternity exclusion — it could rule out a biological relationship but could not confirm one. DNA testing has replaced it for this purpose.
Why Does Blood Type Compatibility Matter for Transfusions?
Your immune system produces antibodies against blood type antigens you do not carry: type A has anti-B antibodies, type B has anti-A, type O has both anti-A and anti-B, and type AB has neither. If you receive incompatible blood, these antibodies attack the donor red blood cells — causing an acute hemolytic transfusion reaction that can be fatal.
Type O negative blood is the "universal donor" for red blood cells because it has no A, B, or Rh antigens and is safe for any recipient in emergencies. AB positive is the universal recipient for red blood cells and the universal donor for plasma.
How Is Blood Type Tested?
Blood typing requires a simple blood draw. The lab performs ABO forward typing (testing your red cells against anti-A and anti-B serum) and reverse typing (testing your plasma against known type-A and type-B red cells). Results are available within minutes. Speedy Sticks can collect blood typing samples at your home and route them to your preferred lab. Schedule your visit here.
This content is for informational purposes only and is not a substitute for professional medical advice.