Pea seeds are round or wrinkled, with nothing in between. But people are not "tall" or "short"; heights spread over a smooth range. Dihybrid crosses already showed how two genes combine. Add more genes and the steps blur into a curve.
Key idea
A polygenic trait is shaped by many genes, each adding a little, and usually by the environment too. The result is a continuous range that often forms a bell-shaped curve.
Why it works
Imagine wheat kernel color set by genes A and B, where each capital allele adds one dose of red pigment. AaBb × AaBb offspring have 0, 1, 2, 3 or 4 doses. There are more ways to get 2 doses (AAbb, aaBB and four kinds of AaBb boxes) than 0 or 4, so the middle shades are most common: 1 : 4 : 6 : 4 : 1 out of 16. With three genes there are 7 shades, 1 : 6 : 15 : 20 : 15 : 6 : 1 out of 64.
Worked example: Counting classes
Kernel color is set by 4 genes like those above. In a cross of two plants heterozygous for all four, how many color classes appear?
- Each offspring has 8 alleles of these genes.
- The number of capital alleles can be 0, 1, 2, … 8.
Answer: 9 classes (2 × 4 + 1).
Worked example: The rare extreme
In AaBb × AaBb (two genes), what fraction of the offspring are the darkest shade?
- Darkest means 4 capital alleles: AABB only.
- AA has chance ¼ and BB has chance ¼.
Answer: ¼ × ¼ = 1/16.
The environment matters. Two cuttings of the same plant, which have identical genes, grow to different heights in rich and poor soil. In people, height depends on many genes and also on nutrition and health during childhood. Genes set a range; conditions decide where in the range a person lands.
Common mistake
It is natural to expect every trait to sort into neat boxes, as in Mendel's peas. Most traits do not. Mendel chose traits controlled mostly by one gene. Height, crop yield and the color of wheat kernels involve many genes plus the environment, so they vary smoothly.