Blood Type Genetics: Learn the Science
Understand how blood type inheritance works through clear explanations of genetics, dominant and recessive genes, and Punnett square calculations.
How Blood Type Inheritance Works
Your blood type is determined by genes you inherit from your parents. Each parent passes down one gene (called an allele) for your ABO blood type and one for your Rh factor. The combination of these alleles determines your blood type.
This follows the principles of Mendelian genetics, named after Gregor Mendel, who discovered the fundamental laws of inheritance in the 1860s by studying pea plants.
The ABO Blood Group System
Three Alleles: A, B, and O
The ABO system is controlled by three possible alleles:
- ACodes for A antigens on red blood cells
- BCodes for B antigens on red blood cells
- OCodes for no antigens (recessive)
Dominance Rules
A and B are co-dominant: If you inherit both A and B alleles, you'll have AB blood type (both antigens are expressed).
O is recessive: The O allele is only expressed when you have two O alleles (OO). If you have A and O, you'll be type A. If you have B and O, you'll be type B.
Genotype vs. Phenotype
| Genotype (genes) | Phenotype (blood type) |
|---|---|
| AA or AO | Type A |
| BB or BO | Type B |
| AB | Type AB |
| OO | Type O |
The Rh Factor
The Rh factor is a separate protein that can be present on red blood cells. It's controlled by a different gene from the ABO system.
Simple Dominance
- Rh+ (positive) is dominant: If you inherit at least one Rh+ allele, you'll be Rh positive.
- Rh- (negative) is recessive: You must inherit two Rh- alleles to be Rh negative.
Rh Inheritance Examples
• Rh+ × Rh+: Can have Rh+ or Rh- children (if both parents carry the recessive allele)
• Rh+ × Rh-: Can have Rh+ or Rh- children (depending on parent's genotype)
• Rh- × Rh-: Can ONLY have Rh- children
Punnett Square Tutorial
A Punnett square is a diagram that predicts the possible genetic outcomes of offspring based on the parents' genes. Let's walk through an example:
Example: Parent 1 (Type A) × Parent 2 (Type B)
Let's say Parent 1 has genotype AO (appears as Type A) and Parent 2 has genotype BO (appears as Type B).
| B | O | |
|---|---|---|
| A | AB | AO |
| O | BO | OO |
Possible Outcomes:
- • AB (25%): Type AB blood
- • AO (25%): Type A blood
- • BO (25%): Type B blood
- • OO (25%): Type O blood
Result: This couple could have children with any of the four major blood types!
💡 Key Insight
Each child has a 25% chance of each blood type in this example. However, that doesn't mean a family with 4 children will have exactly one of each type—probability doesn't guarantee specific outcomes, it just shows what's possible.
Genetics Glossary
Allele
A variant form of a gene. For blood type, A, B, and O are different alleles of the same gene.
Dominant
An allele that is expressed even when only one copy is present. A and B are dominant over O; Rh+ is dominant over Rh-.
Recessive
An allele that is only expressed when two copies are present. O and Rh- are recessive.
Co-dominant
When both alleles are expressed equally. A and B alleles are co-dominant, which is why AB blood type exists.
Genotype
The actual genetic makeup (the alleles you have). Examples: AA, AO, BO, OO.
Phenotype
The observable trait (what you see). Examples: Type A, Type B, Type O, Type AB.
Homozygous
Having two identical alleles for a gene. Examples: AA, BB, OO.
Heterozygous
Having two different alleles for a gene. Examples: AO, BO, AB.
Antigen
A protein marker on the surface of red blood cells that determines blood type. A, B, and Rh are examples of antigens.
Antibody
A protein in blood plasma that attacks foreign antigens. Type A blood has anti-B antibodies, Type B has anti-A antibodies.
Frequently Asked Questions
How is blood type inherited from parents?
Blood type is inherited through genes from both parents. Each parent passes one ABO allele (A, B, or O) and one Rh allele (+ or -) to their child. The combination determines the child's blood type. For example, if one parent passes an A allele and the other passes an O allele, the child will have type A blood (AO genotype).
What blood type will my baby have?
Your baby's blood type depends on the combination of alleles inherited from both parents. Use a Punnett square or blood type calculator to determine all possible blood types. For example, two type A parents (genotype AO) can have children with type A (75% chance) or type O (25% chance) blood.
Can two O positive parents have an A positive child?
No. Two O positive parents can only have O positive or O negative children. Type O means both parents have OO genotype and can only pass O alleles to their children. Since O is recessive, children cannot have type A, B, or AB blood.
What does it mean if blood type is dominant or recessive?
A dominant allele (A or B) is expressed even with just one copy, while a recessive allele (O) requires two copies to be expressed. For example, AO genotype appears as type A because A is dominant over O. Only OO genotype shows as type O blood. A and B are co-dominant, so AB genotype shows both antigens.
Why do I have a different blood type than my parents?
You inherit one allele from each parent, and these combine to form your blood type. If both parents carry a recessive O allele, you might have type O even if they're type A or B. For example, parents with genotypes AO and BO can have children with types A, B, AB, or O—all four blood types are genetically possible.
How does Rh factor inheritance work?
Rh factor follows simple dominant/recessive inheritance. Rh positive (+) is dominant, so you only need one + allele to be Rh positive. Rh negative (-) is recessive, requiring two - alleles. Two Rh negative parents can only have Rh negative children, while two Rh positive parents might have Rh negative children if both carry a hidden - allele.
Practice Your Knowledge
Ready to apply what you've learned? Try our interactive tools:
Educational Purpose: This content provides a simplified overview of blood type genetics for educational purposes. Real genetics can involve rare variations and exceptions. For medical advice or genetic counseling, consult a healthcare professional.