The analysis of plant and animal cells shows that chromosomes are present in homologous pairs, with each member of the pair carrying very similar or identical genes. In humans, for example, there are forty-six chromosomes, but these can be grouped into twenty-three pairs. This set of twenty-three unique chromosomes is known as the haploid number for humans, while the full complement of forty-six chromosomes (two sets of twenty-three) is known as the diploid number. Virtually every somatic (non-sex) cell in the body contains the diploid number, while gametes (egg and sperm) contain the haploid number. Arabidopsis thaliana (a well-studied model plant) has ten chromosomes in a somatic nucleus, two each of five different types. Like humans, Arabidopsis is diploid, with a diploid number of ten and a haploid number of five.

While some plants show this diploid pattern of chromosome number, many others show a different pattern, called...