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The list of organisms by chromosome count describes ploidy or numbers of chromosomes in the cells of various plants, animals, protists, and other living organisms.This number, along with the visual appearance of the chromosome, is known as the karyotype, [1] [2] [3] and can be found by looking at the chromosomes through a microscope.
The ZW sex-determination system is a chromosomal system that determines the sex of offspring in birds, some fish and crustaceans such as the giant river prawn, some insects (including butterflies and moths), the schistosome family of flatworms, and some reptiles, e.g. majority of snakes, lacertid lizards and monitors, including Komodo dragons.
The basic number of chromosomes in the somatic cells of an individual or a species is called the somatic number and is designated 2n. In the germ-line (the sex cells) the chromosome number is n (humans: n = 23). [4] [5] p28 Thus, in humans 2n = 46. So, in normal diploid organisms, autosomal chromosomes are present in two copies.
The ends of the XY chromosomes in a human cell in metaphase, highlighted here in green, are all that is left of the original autosomes that can still cross over with each other. Sex determination systems may have evolved from mating type, which is a feature of microorganisms.
The first step of the Human Genome Project took place when Tjio and Levan, in 1956, reported the accurate diploid number of human chromosomes as 2n = 46. [ 6 ] During this phase, data on the karyotypes of hundreds of mammalian species (including information on diploid numbers, relative length and morphology of chromosomes, presence of B ...
Polyploidy occurs in some tissues of animals that are otherwise diploid, such as human muscle tissues. [45] This is known as endopolyploidy. Species whose cells do not have nuclei, that is, prokaryotes, may be polyploid, as seen in the large bacterium Epulopiscium fishelsoni. [46] Hence ploidy is defined with respect to a cell.
Male birds have two Z chromosomes (ZZ), and female birds have a W chromosome and a Z chromosome (WZ). [78] A complex system of disassortative mating with two morphs is involved in the white-throated sparrow Zonotrichia albicollis , where white- and tan-browed morphs of opposite sex pair, making it appear as if four sexes were involved since any ...
Parthenogenetic offspring in species that use either the XY or the X0 sex-determination system have two X chromosomes and are female. In species that use the ZW sex-determination system, they have either two Z chromosomes (male) or two W chromosomes (mostly non-viable but rarely a female), or they could have one Z and one W chromosome (female).