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.