<P> In some cases there is evidence that the n chromosomes in a haploid set have resulted from duplications of an originally smaller set of chromosomes . This "base" number--the number of apparently originally unique chromosomes in a haploid set--is called the monoploid number, also known as basic or cardinal number, or fundamental number . As an example, the chromosomes of common wheat are believed to be derived from three different ancestral species, each of which had 7 chromosomes in its haploid gametes . The monoploid number is thus 7 and the haploid number is 3 × 7 = 21 . In general n is a multiple of x . The somatic cells in a wheat plant have six sets of 7 chromosomes: three sets from the egg and three sets from the sperm which fused to form the plant, giving a total of 42 chromosomes . As a formula, for wheat 2n = 6x = 42, so that the haploid number n is 21 and the monoploid number x is 7 . The gametes of common wheat are considered to be haploid, since they contain half the genetic information of somatic cells, but they are not monoploid, as they still contain three complete sets of chromosomes (n = 3x). </P> <P> In the case of wheat, the origin of its haploid number of 21 chromosomes from three sets of 7 chromosomes can be demonstrated . In many other organisms, although the number of chromosomes may have originated in this way, this is no longer clear, and the monoploid number is regarded as the same as the haploid number . Thus in humans, x = n = 23 . </P> <P> Diploid cells have two homologous copies of each chromosome, usually one from the mother and one from the father . All or nearly all mammals are diploid organisms . The suspected tetraploid (possessing four chromosome sets) plains viscacha rat (Tympanoctomys barrerae) and golden vizcacha rat (Pipanacoctomys aureus) have been regarded as the only known exceptions (as of 2004). However, some genetic studies have rejected any polyploidism in mammals as unlikely, and suggest that amplification and dispersion of repetitive sequences best explain the large genome size of these two rodents . All normal diploid individuals have some small fraction of cells that display polyploidy . Human diploid cells have 46 chromosomes (the somatic number, 2n) and human haploid gametes (egg and sperm) have 23 chromosomes (n). Retroviruses that contain two copies of their RNA genome in each viral particle are also said to be diploid . Examples include human foamy virus, human T - lymphotropic virus, and HIV . </P> <P> "Homoploid" means "at the same ploidy level", i.e. having the same number of homologous chromosomes . For example, homoploid hybridization is hybridization where the offspring have the same ploidy level as the two parental species . This contrasts with a common situation in plants where chromosome doubling accompanies, or happens soon after hybridization . Similarly, homoploid speciation contrasts with polyploid speciation . </P>

Female gamete with the n number of chromosomes