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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.
A small number of mammals also lack a Y chromosome. ... and this homomorphism was maintained for at least 5.4 million years ... the X 1 chromosome shares ...
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.
In humans, the presence of the Y chromosome is responsible for triggering male development; in the absence of the Y chromosome, the fetus will undergo female development. In most species with XY sex determination, an organism must have at least one X chromosome in order to survive. [2] [3]
Most spiders have a variation of the XO system in which males have two different X chromosomes (X 1 X 2 O), while females have a pair of X 1 chromosomes and a pair of X 2 chromosomes (X 1 X 1 X 2 X 2). [1] Some spiders have more complex systems involving as many as 13 different X chromosomes. [1] Some Drosophila species have XO males. [10]
In both the non-animals and the comb jellies, researchers found 14 groups of genes located on separate chromosomes. But in the sponges, researchers found that those 14 groups had been rearranged ...
Areas homologous to the bird Z chromosome are scattered throughout X3 and X5. [4]: fig. 5 Although the sex-determination system is not necessarily linked to that of birds and definitely not to that of therian mammals, the similarity at least allowed for the conclusion that mammals evolved sex chromosomes twice. [5]
At least one genome has been sequenced from every order. [62] [63] These include at least one species in about 90% of extant avian families (218 out of 236 families recognised by the Howard and Moore Checklist). [64]