Originally formulated in Gregor Johann Mendel's first law of segregation, segregation occurs during meiosis when a pair of alleles is split to form a single gamete (sex cell). Within meiosis, homologous chromosome pairs separate from each other, and consequently, a gamete receives only one of each type of chromosome. Occurring on the chromosomes, allele pairs split as the chromosomes split, naturally transmitting only one allele of a pair to a gamete.

When coupled with the observation of homologous chromosomes separating during meiosis, Mendel's law of segregation first indicated that genes were located on chromosomes.