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The XY sex-determination systemis a sex-determination systempresent in many mammals, including humans, some insects (Drosophila), some snakes, some fish (guppies), and some plants (Ginkgotree). In this system, the sexof an individual usually is determined by a pair of sex chromosomes. Typically, females have two of the same kind of sex ...
Over time, genes that were beneficial for males and harmful to (or had no effect on) females either developed on the Y chromosome or were acquired by the Y chromosome through the process of translocation. [15] Until recently, the X and Y chromosomes in mammals were thought to have diverged around 300 million years ago. [16]
Females therefore have 23 homologous chromosome pairs, while males have 22. The X and Y chromosomes have small regions of homology called pseudoautosomal regions. An X chromosome is always present as the 23rd chromosome in the ovum, while either an X or Y chromosome may be present in an individual sperm. [4]
Nettie Stevens. Nettie Maria Stevens (July 7, 1861 – May 4, 1912) [ 1 ] was an American geneticist who discovered sex chromosomes. In 1905, soon after the rediscovery of Mendel 's paper on genetics in 1900, she observed that male mealworms produced two kinds of sperm, one with a large chromosome and one with a small chromosome. When the sperm ...
While all human chromosomes contain repeats, more than 30 million letters of the Y chromosome — out of 62.5 million — are repetitive sequences, sometimes called satellite DNA or junk DNA.
[4] [3] The human Y chromosome, consisting of 62,460,029 base pairs from a different cell line and found in all males, was sequenced completely in January 2022. [5] The current version of the standard reference genome is called GRCh38.p14 (July 2023). It consists of 22 autosomes plus one copy of the X chromosome and one copy of the Y chromosome.
Among the six species, the Y chromosome exhibited much more variability than the X chromosome. For example, the X chromosomes of humans and chimpanzees are about 98 percent identical in makeup.
The X chromosome is one of the two sex chromosomes in many organisms, including mammals, and is found in both males and females. It is a part of the XY sex-determination system and XO sex-determination system. The X chromosome was named for its unique properties by early researchers, which resulted in the naming of its counterpart Y chromosome ...