Parents pass on alleles, not their blood type
- The idea
- Predict
- See it
- Why
- Check
The idea
“A parent hands down their blood type, and O goes with anything.”
Kids often have a parent's blood type, so it looks like the type itself gets passed down. A parent actually passes on one allele.
The short version
The mistake: A parent hands down their blood type, and O goes with anything.
What's actually true: Each parent passes one of their two blood type alleles, so a child can have a type neither parent has.
- You get one ABO allele from each parent. The common ones are I-A, I-B and i.
- I-A and I-B are codominant, so I-A I-B is type AB. Both are dominant to i, so type O is always ii.
- In meiosis the two alleles separate, so each egg or sperm carries one. This is segregation.
- Type O isn't a blank. An ii parent can only pass on i, so the other parent decides the child's type.
On the exam: Write each parent's possible genotypes, then pass one allele from each.