Geneticists exploit the normal life cycle of an organism to test for the dominance or recessivity of alleles. To see how this is done, we need first to review the type of cell division that gives rise to gametes (sperm and egg cells in higher plants and animals). Whereas the body (somatic) cells of most multicellular organisms divide by mitosis, the germ cells that give rise to gametes undergo meiosis. Like somatic cells, premeiotic germ cells are diploid, containing two homologs of each morphologic type of chromosome.
The two homologs constituting each pair of homologous chromosomes are descended from different parents, and thus their genes may exist in different allelic forms. In mitosis DNA replication is always followed by cell division, yielding two diploid daughter cells.
In meiosis one round of DNA replication is followed by two separate cell divisions, yielding four haploid (1n) cells that contain only one chromosome of each homologous pair. The apportionment, or segregation, of the replicated homologous chromosomes to daughter cells during the first meiotic division is random; that is, maternally and paternally derived homologs segregate independently, yielding daughter cells with different mixes of paternal and maternal chromosomes. As a way to avoid unwanted complexity, geneticists usually strive to begin breeding experiments with strains that are homozygous for the genes under examination. In such truebreeding strains, every individual will receive the same allele from each parent and therefore the composition of alleles will not change from one generation to the next.