Prawo niezależnego asortymentu

Gregor Mendel's first experiments determined that there are units, genes, that are responsible for passing on traits from parent to offspring. Each organism has two copies of each gene, called alleles, one inherited from each parent. Mendel's next experiment used dihybrid crosses of pea plants that differed by two traits, for example, height and flower color, to examine whether the inheritance of one trait would influence the inheritance of another.

If the alleles for the two traits being examined in a dihybrid cross were inherited from the parental generation as a unit, when the F1 generation reproduced, the F2 offspring would always display either both dominance or both recessive phenotypes, never a combination of the two. When Mendel crossed the dihybrid F1 pea plants he found that for every 16 F2 offspring, about nine had both of the dominant phenotypes, three had one dominant and one recessive genotype, three more had the reverse pairing of recessive and dominant phenotypes and one had both recessive phenotypes. The ratio of dominant to recessive phenotypes for each trait separately is still three to one in the F2 generation.

All four combinations of phenotypes could only occur if the dominant and recessive alleles for height were not linked to the dominant and recessive alleles for flower color. These observations provided the basis for Mendel's Law of Independent Assortment which states that the alleles for different genes are segregated into gametes independently of one another. The nine, three, three, one phenotypic ratio indicates that each dihybrid parent is equally likely to pass on all possible combinations of dominant and recessive alleles. Tall with purple flowers, tall with white flowers, short with purple flowers or short with white flowers.