Inheritance of two genes: independent assortment

In a dihybrid cross Mendel crossed round-yellow (RRYY) with wrinkled-green (rryy) peas. All F₁ seeds were round and yellow (RrYy).

Selfing the F₁ produced an F₂ ratio of 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green, from 16 combinations of gametes.

Looked at one character at a time, round : wrinkled and yellow : green are each 3:1, showing that each gene still segregates on its own.

Law of independent assortment: if a hybrid carries two pairs of traits together, one pair segregates independently of the other. So an RrYy plant produces four kinds of gametes, RY, Ry, rY and ry, in equal proportion.

A dihybrid test cross (RrYy × rryy) gives four phenotypes in a 1:1:1:1 ratio.

Mendel's work, published in 1865, went unrecognised for years, partly because factors were abstract, not physically observed, and his mathematical approach was new to biologists. It was rediscovered independently in 1900 by de Vries, Correns and von Tschermak.