The Genetics of Blood Type: An Inheritance Guide
A baby's blood type is inherited from their parents through a combination of genes. Understanding how this works involves two key systems: the ABO system and the Rh factor. Our calculator uses these genetic rules to determine all possible blood types for your child.
The ABO Blood Group System
The ABO system is based on three different alleles (versions of a gene): A, B, and O. Every person inherits one allele from each parent.
- Alleles A and B are co-dominant. If you inherit both, you'll have type AB blood.
- Allele O is recessive. This means you must inherit an O from both parents to have type O blood. If you inherit an A and an O, your blood type will be A.
Here’s how the parent genotypes translate to blood types:
- Type A: Genotype is either AA or AO.
- Type B: Genotype is either BB or BO.
- Type AB: Genotype is AB.
- Type O: Genotype is OO.
What is the Rh Factor (+ or -)?
The Rh factor is a separate protein found on red blood cells. Its inheritance is simpler:
- Positive (+) is the dominant allele. You only need to inherit one positive allele from a parent to be Rh-positive. Your genotype could be (+,+) or (+,-).
- Negative (-) is the recessive allele. You must inherit a negative allele from both parents to be Rh-negative. Your genotype is (-,-).
This is why two Rh-positive parents can have an Rh-negative child (if they both carry the recessive negative allele), but two Rh-negative parents can only have an Rh-negative child.
Why It Matters
While this calculator is for informational fun, knowing blood types is medically important. During pregnancy, if a mother is Rh-negative and the baby is Rh-positive, it can cause a condition called Rh incompatibility. Doctors test for this and can provide treatment to ensure a healthy pregnancy. For a deeper dive into how traits are passed down, explore our more Advanced Baby Genetics Calculator.