5.4 Multiple Alleles and Blood Types
Notes
Some genes have more than two alleles in a population, even though each person carries only two.
- Human ABO blood type has three alleles: $I^A$, $I^B$ and $i$.
- $I^A$ and $I^B$ are codominant; both are dominant over $i$.
| Blood type | Possible genotypes |
|---|---|
| A | $I^AI^A$ or $I^Ai$ |
| B | $I^BI^B$ or $I^Bi$ |
| AB | $I^AI^B$ |
| O | $ii$ |
Practice
Which genotype gives type AB blood?
$I^A$ and $I^B$ are codominant, so both show.
A mother is type O ($ii$) and the father is type AB ($I^AI^B$). Which blood types can their children have?
Children get $i$ from mom and $I^A$ or $I^B$ from dad: $I^Ai$ (A) or $I^Bi$ (B).
Two parents with type A blood have a child with type O blood. What are the parents' genotypes?
A type O child ($ii$) must get an $i$ from each parent.
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