Mendel’s First Law

The relative numbers of offspring of each phenotype that Mendel observed formed the basis for his Laws of Heredity. The first law addresses how alleles of a single gene are passed to offspring.

In particular, the ratio of 1 true-breeding dominant: 2 hybrid dominant: 1 true-breeding recessive offspring that Mendel observed from each F1 self-cross allowed Mendel to conclude that each individual organism has two hereditary elements that specify each trait. In a population, there are different versions, or alleles, of these hereditary elements, which contribute to the diversity of traits observed. For example, for the genetic element that specifies plant height, one allele specifies tall plants, while another specifies short. Each individual organism has two alleles that specify each trait, or, in modern terms, two copies of each gene.

Mendel concluded that, although an individual has two separate alleles controlling each trait, only one allele would be passed to offspring during the production of a gamete. Which of the two alleles inherited was random, so if all of the population from a cross were compared, 50% of gametes inherit one allele, and 50% the other. He summed this up in his first law of heredity, called The Law of Equal Segregation.

The Law of Equal Segregation: During the production of gametes, the two alleles of a gene are divided (segregated) among gametes, so that each gamete receives only one allele. This results in equal numbers of gametes with each allele.

What this means in simpler terms is that an individual has two copies of each gene. When that individual reproduces, half of its offspring get one allele, and half get the other allele.