Chromosome: Encyclopedia II - Chromosome - Number of chromosomes in different species

Chromosome - Number of chromosomes in different species

Normal members of a particular species all have the same number of chromosomes (see the table).

Asexually reproducing species have one set of chromosomes, which is the same in all body cells.

Gametes, reproductive cells, are haploid [n] and have one set of chromosomes. Sexually reproducing species have somatic cells, body cells, which are diploid [2n] having two sets of chromosomes, one from the mother and one from the father. Gametes are produced by meiosis of a diploid germ line cell. During meiosis, the matching chromosomes of father and mother can exchange small parts of themselves (crossover), and thus create new chromosomes that are not inherited solely from either parent. When a male and a female gamete merge (fertilization), a new diploid organism is formed.

Some animal and plant species are polyploid [Xn] and have more than two sets of chromosomes. Agriculturally important plants such as tobacco or wheat are often polyploid compared to their ancestral species. Wheat has a haploid number of seven chromosomes, still seen in some cultivars as well as the wild progenitors. The more common pasta and bread wheats are polyploid having 28 (tetraploid) and 42 (hexaploid) chromosomes compared to the 14 (diploid) chromosomes in the wild wheat^[1].

Historical note: In 1921, Theophilus Painter claimed, based on his observations, that human sex cells had 24 chromosomes, giving humans 48 chromosomes total. It wasn't until 1955 that the number was clearly shown to be 23.

Adapted from the Wikipedia article "Number of chromosomes in different species", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki