Law of Independent Assortment

Gregor Mendel’s first experiments showed that units called genes help pass traits from parents to offspring.

Each organism has two copies of each gene, called alleles, with one inherited from each parent.

Mendel’s next experiment used dihybrid crosses of pea plants that differed in two traits, such as height and flower color, to test whether the inheritance of one trait would affect the inheritance of another.

If the alleles for the two traits were inherited together as a single unit in the F1 generation, the F2 offspring would always show either both dominant or both recessive phenotypes and would never show a mix of the two.

When Mendel crossed the F₁ dihybrid pea plants, he found that among every 16 F₂ offspring, about nine showed both dominant phenotypes—tall plants with purple flowers. Three showed a dominant height trait and a recessive flower color trait—tall plants with white flowers. Another three showed a recessive height trait and a dominant flower color trait—short plants with purple flowers. One showed both recessive phenotypes—short plants with white flowers.

When each trait is considered separately, such as plant height in the F2 generation, the ratio of dominant tall plants to recessive short plants remains 3:1. The same pattern appears for flower color, with a 3:1 ratio of purple flowers to white flowers.

All four phenotype combinations could appear only if the alleles for height assorted independently from the alleles for flower color.

These results formed the basis of Mendel’s Law of Independent Assortment, which states that alleles for genes on different chromosomes, or far apart on the same chromosome, sort into gametes independently of one another.

The 9:3:3:1 phenotypic ratio shows 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.