Mendel concluded that each pea plant had two hereditary factors for each of the seven different characters. He surmised that the two factors for each characters became segregated during gamete formation and passed into separate gametes so that each gamete received only one factor for each character. Mendel, then, postulated that new plants received one factor from the male parent and one factor from the female parent. The appearance of yellow pods in the F2 generation indicated to Mendel that the factors had to be separate, particular entities which did not blend or fuse and which remained distinct and unchanged when gametes were formed. In modern terminology, Mendel's character controlling factors are the genes that make up chromosomes. As each diploid cell contains pairs of homologous chromosomes, each cell contains two genes, one on each of the homologous chromosomes, controlling the same character. When homologous chromosomes segregate during meiosis, each gametic cell receives just one of each chromosome type and, therefore, just one of each type of the gene.
Genes that exist in more than one form are called alleles. The allele that always expresses its version of a character is termed the dominant allele while the other allele is termed the recessive allele. In Mendel's peas, for example, the allele for green pods was the dominant allele while the allele for yellow pods was the recessive allele. Cells that have two of the same allele for a certain characteristic are called homozygous while cells that have one of each of the two alleles for the characteristic are called heterozygous. Thus, there are three different genetic combinations, called
