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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 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.
This was previously thought to be the highest chromosome number known for a mammal, [3] but it has since been found that the plains viscacha rat or red viscacha rat (Tympanoctomys barrerae) has 4x = 2n = 102. [4]
Organisms in which a particular chromosome, or chromosome segment, is under- or over-represented are said to be aneuploid (from the Greek words meaning "not", "good", and "fold"). Aneuploidy refers to a numerical change in part of the chromosome set, whereas polyploidy refers to a numerical change in the whole set of chromosomes. [44]
Chromosome numbers are usually determined at the metaphase stage during mitosis. Usually, the diploid chromosome number (2n) is referenced, unless dealing with a polyploid series in which case the base number or number of chromosomes in the genome of the original haploid is quoted.
Euploidy and aneuploidy describe having a number of chromosomes that is an exact multiple of the number of chromosomes in a normal gamete; and having any other number, respectively. For example, a person with Turner syndrome may be missing one sex chromosome (X or Y), resulting in a (45,X) karyotype instead of the usual (46,XX) or (46,XY).
The doubling of its chromosome number was presumably by errors in mitosis or meiosis within the animal's reproductive organs. [15] A comparison of the chromosomes of the plains viscacha rat and the mountain viscacha rat suggested that the chromosomes of the plains viscacha rat increased relatively rapidly (in evolutionary terms) due to a ...
Lemur species diversity increases as the number of tree species in an area increase and is also higher in forests that have been disturbed over undisturbed areas. [12] Evidence from the Subfossil records show that many of the now extinct lemurs actually lived in much drier climates than the currently extant lemurs.