Codominance and sex linkage
Codominance (Supplement)
- In codominance, both alleles in a heterozygous organism show in the phenotype.
- The ABO blood groups are the classic example. The alleles are $I^A$, $I^B$ and $I^o$, where $I^A$ and $I^B$ are codominant and $I^o$ is recessive:
| Genotype | Blood group |
|---|---|
| $I^A I^A$ or $I^A I^o$ | A |
| $I^B I^B$ or $I^B I^o$ | B |
| $I^A I^B$ | AB (both shown) |
| $I^o I^o$ | O |
Practice
In codominance:
Codominance means both alleles are expressed — e.g. group AB blood from alleles for A and B.
Practice
A person with genotype I-A I-B has blood group:
Because the A and B alleles are codominant, both show, giving group AB.
Sex linkage (Supplement)
- A sex-linked feature is controlled by a gene on a sex chromosome (usually the X).
- Males have only one X, so a recessive allele on it always shows — the feature is more common in males.
- Red-green colour blindness is an example: more common in boys than girls.
Practice
Red-green colour blindness is more common in boys because the recessive allele is on the X chromosome and males have only one X.
With only one X, a single recessive allele shows in males, so sex-linked recessive features are more common in boys.
You've got it
Key idea
- codominance: both alleles show — e.g. AB blood ($I^A I^B$) (Supplement)
- sex linkage: a gene on the X; recessive features are more common in males (only one X)
- red-green colour blindness is sex-linked