Laws of Segregation and Independent Assortment

Gregor Mendel (1822–84) was a scientist and Augustinian monk who lived and worked in what is now the Czech Republic. Through a series of methodical experiments on pea plants—the results of which he published in 1866—Mendel established the foundations of modern genetics.

Mendel carefully bred the pea plants to create plants that were pure breeds, or what we now call homozygous, of one or more of seven traits. He then crossbred the plants, keeping track of how many of the offspring displayed the traits of each parent. If all offspring had the trait of one of the parents, he called that trait "dominant"; Mendel called the traits that did not appear "recessive." Mendel made the observation that a dominant trait will be uniformly observed in the offspring of a parent that is homozygous for that trait, which he called the principle of uniformity.

Mendel's observations of recessive traits led to his second principle of inheritance, the principle of segregation. Segregation is the separation of the genes of one parent from that of the other during gamete formation. Passing genetic material from generation A to generation B occurs via the process of meiosis. A cell duplicates its genetic material (DNA) and divides in meiosis to create four reproductive cells, called gametes. Each gamete carries half the needed chromosomes. When the male and female gametes join in sexual reproduction, each supplies half of the genetic material, producing a diploid zygote. Mendel discovered that traits could be dominant or recessive. Each parent cell contributed an allele, an alternative form of a gene, and passed on specific genetic information. In sweet peas, the allele for purple flowers is dominant over the allele for white flowers.

Mendel did not know about the existence of genes, chromosomes, or the process of meiosis in forming gametes, so he determined this principle through statistical analysis of the traits observed in crossbreeding experiments. The experiment is called a monohybrid cross, since it cross-fertilizes organisms that have one differing trait.

After cross-fertilizing two homozygous parents—called the P generation—a first generation of offspring, called the F₁, first filial, was formed. These offspring all displayed the dominant trait of the homozygous parent, but the offspring were heterozygous. Mendel then self-fertilized these offspring. Plants are convenient for this procedure, since Mendel could just move pollen from the anther to the stigma of the plant by using a small paintbrush. Mendel recorded the occurrence of each trait in the second generation of offspring (F₂). In the F₂ generation, the ratio of plants displaying the dominant trait to those displaying the recessive trait was 3:1. The recessive trait appeared in the F₂ generation after being absent in the F₁ generation. This told Mendel that the F₁ generation did carry the genetic material for that trait and that it could be segregated from the dominant trait in a gamete in order to reappear in F₂ offspring that were homozygous for the recessive trait. With a modern understanding of genetics, the segregation can be understood as the result of the two meiotic divisions. A logical extension of Mendel's monohybrid cross experiments, which examined plants that differed in one trait, was the dihybrid cross, which examined plants that differed in two traits. In this case, Mendel crossed parent plants (P generation) that were homozygous for the traits of having round or wrinkled seed shape and having yellow or green seed color. Roundness and yellow color are dominant traits, so all the offspring in the F₁ generation had round, yellow seeds and were heterozygous for both traits. Because of the principle of segregation, Mendel expected that self-fertilization of the F₁ generation would lead to F₂ offspring that would express both the dominant and the recessive traits. The question was whether there would be a link between the two traits. The results of Mendel's experiments showed that each trait could appear independently. The alleles that are segregated or separated during the formation of gametes are reunited in random fashion during fertilization. In this way, recessive traits carried in the genome of offspring might later reappear. Mendel stated this finding as his principle of independent assortment. Independent assortment is the random distribution of homologous chromosome pairs of genes on different chromosomes to the gametes during meiosis.