independent assortment

Meaning: Mendel's second law. In metaphase I, each homologous pair orients randomly at the plate, so which parental chromosome goes to a given pole does not depend on how other pairs orient. Alleles of genes on different chromosomes therefore enter gametes independently. Genes far apart on one chromosome also assort nearly independently because crossing over separates them.

Equation: Gamete combinations = 2ⁿ, where n is the number of homologous pairs (a unitless count). Humans, with n = 23, give 2²³ ≈ 8.4 million combinations, before crossing over is considered.

On the MCAT: A dihybrid cross AaBb × AaBb gives a 9:3:3:1 phenotype ratio when both genes show complete dominance and assort independently. A testcross AaBb × aabb gives 1:1:1:1. Offspring counts that depart from these ratios point to linkage. It is also tested as a source of genetic variation, alongside crossing over and random fertilization.

Common confusion: Do not mix it up with segregation, Mendel's first law, which says the two alleles of one gene separate into different gametes. Independent assortment compares different genes. Genes close together on one chromosome are linked and tend to be inherited together, so recombinant offspring fall below 50%, whereas independent assortment gives 50%.