Explain why polygenic traits are expressed through quantitative variation, using skin color as an example.

Answer:

Skin color is controlled by many genes, and alleles that contribute to darker or lighter pigment have additive effects. The total combination of alleles determines the shade, so people in the same family can have a range of skin colors. Someone who inherits more dark-pigment alleles may be darker than their parents, and the range of outcomes often forms a bell-shaped curve.