In Mendel's peas one allele fully hid the other. Many genes do not work that way. In monohybrid crosses the heterozygote looked like the dominant parent; here it looks different.
Key idea
Incomplete dominance: the heterozygote is in between. Red (RR) × white (rr) snapdragons give pink (Rr). Codominance: both alleles show fully, side by side. In roan cattle, a heterozygote has a coat of red hairs mixed with white hairs.
Why it works
An allele usually stands for a version of a protein. In snapdragons, one copy of the red allele makes only about half as much red pigment as two copies, so the flower is pale: pink. In codominance, each allele makes its own working product, so both products show, as in AB blood (see Multiple Alleles: Blood Types).
Worked example: Pink × pink
Two pink snapdragons (Rr) are crossed. What fraction of the offspring are red, pink and white?
- Boxes: RR, Rr, Rr, rr.
- Each genotype has its own color, so count genotypes.
Answer: ¼ red, ½ pink, ¼ white.
Worked example: Pink × white
A pink snapdragon is crossed with a white one. What do the offspring look like?
- Rr × rr: gametes R, r and r.
- Boxes: Rr, Rr, rr, rr.
Answer: Half pink, half white.
Worked example: Which pattern?
A red-coated bull is crossed with a white-coated cow. The calf's coat has red hairs and white hairs mixed together. Which pattern is this?
- The calf is not a single in-between color.
- Both parents' colors show at once, each in its own hairs.
Answer: Codominance.
| Pattern | Heterozygote looks | Example |
|---|---|---|
| Complete dominance | like the dominant parent | purple pea flowers |
| Incomplete dominance | in between | pink snapdragons |
| Codominance | both traits at once | roan coat; AB blood |
Common mistake
Pink flowers can look like proof that traits blend. Test it: cross two pinks. If traits blended, every offspring would be pink forever. Instead red and white come back in a 1 : 2 : 1 ratio, so the R and r alleles stayed separate the whole time. Only the look of the heterozygote is in between.