5.3 Incomplete Dominance and Codominance
Notes
Not all traits follow simple dominant-recessive patterns.
- Incomplete dominance: the heterozygote shows a blend of the two traits. Example: red snapdragon x white snapdragon gives pink.
- Crossing two pinks gives 1 red : 2 pink : 1 white.
- Codominance: both alleles are fully shown at the same time. Example: a red cow x white cow gives a roan cow with both red and white hairs.
- Polygenic traits are controlled by many genes, giving a range of phenotypes (height, skin color).
Practice
Two pink snapdragons (incomplete dominance) are crossed. What fraction of offspring will be pink?
RW x RW gives 1 RR (red) : 2 RW (pink) : 1 WW (white).
A chicken with black feathers and a chicken with white feathers produce chicks with both black and white feathers in a speckled pattern. This is:
Both alleles show fully in separate feathers.
Human height varies across a wide range rather than falling into a few groups. This is because height is:
Many genes, plus environment, produce a continuous range.
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