5.5 Environmental Effects on Phenotype

Genes plus environment

Phenotype = genotype + environment. An organism's genes set the possibilities, but the environment influences which phenotype actually appears. The same genotype can produce different phenotypes under different conditions.

Classic examples

Hydrangea flower color depends on soil pH. Fur color in Himalayan rabbits and Siamese cats depends on temperature (cooler body parts grow darker fur). Human height depends on genes *and* nutrition. The full range of phenotypes a genotype can produce across environments is its norm of reaction.

Key terms

Phenotype = genotype + environment

The observable trait depends on both genes and environmental conditions.

Norm of reaction

The range of phenotypes a single genotype can produce across environments.

Soil pH & hydrangeas

Hydrangea flower color shifts with the pH of the soil.

Temperature & fur color

Cooler body regions of Siamese cats/Himalayan rabbits grow darker fur.

Environmental influence

The same genotype can yield different phenotypes in different conditions.

Try an AP-style question

Which explanation fits best?

1. The shaved hair was replaced by a different gene
2. Black fur is dominant only on the back of the rabbit
3. The cold caused a mutation in the rabbit's skin cells
4. The pigment enzyme is active only at cooler temperatures