law of independent assortment

Imagine choosing an outfit by flipping one coin for the shirt colour and a separate coin for the trouser colour. Neither flip influences the other, so all four shirt-trouser combinations turn up in equal proportions. The law of independent assortment says that the alleles for two different traits are dealt into gametes by separate, independent coin flips.

Mendel reached this conclusion by following two traits at once in his pea crosses — for example seed colour and seed shape. The way one trait was inherited gave no clue about the other, and the offspring appeared in the characteristic 9:3:3:1 ratio that independent assortment predicts.

The physical reason is that, during meiosis, each pair of homologous chromosomes lines up and orients at random with respect to every other pair. There is an important limit, though: the law holds cleanly only for genes on different chromosomes (or far apart on the same one). Genes that sit close together tend to be inherited as a package — a phenomenon called linkage — and assort far from independently.