Sex chromosomes (gonosomes, allozomes, heterochromosomes, heterosomes or lat. Chromosoma sexuale) contain genes for sex determination and function.
Thomas Morgan discovered them in 1910 while studying the wine fly. Human karyotype contains a pair of chromosomes labeled as sex chromosomes:
the female sex contains two homologs of chromosomes designated as X chromosomes; thus the female sex is homogametic since it produces only one type of egg cells, that is, all eggs have an X chromosome.
The male sex contains a pair of heterologous-different- chromosomes designated as X and Y and is therefor heterogamethane because it produces two types of gametes: X-sperm and Y-sperm.
The X chromosome is a type of sex chromosome, which means that it determines the sex of an individual: women in the case that the organism has two X chromosomes, and men if it has only one. According to its size, X chromosome belongs to group C of the human karyotype and is located between chromosomes number 6 and 7. According to the position of the centromere it falls into the submetacentric category. It is estimated that this chromosome contains around 800 to 900 genes that carry instructions for protein synthesis. These proteins perform various functions in the body, determine the sex of an individual and give it that gender`s characteristics. X chromosome contains a much larger number of genes compared to the Y chromosome, many of which do not determine sexual characteristics. In other words, majority of the genes located on the X chromosome are for bodily traits rather than for sexual ones. The knowledge of the number of these genes was made possible by Human genome mapping.
The Y chromosome is a chromosome that`s characteristic for the male sex. Men contain one Y chromosome, while women do not have any. According to the position of the centromere, it belongs to acrocentric chromosomes, and according to the 1960 Denver Conference it is classified in group G by length. It is built from 59 million DNA base pairs, which represents about 3% of the total amount of DNA in a cell. It is estimated that this chromosome contains around 50 to 60 genes that carry valuable instructions for protein synthesis.
Gender anomalies: XY women and XX men
Today, it is known that the entire Y chromosome is not needed to determine the male sex, but only one part of its short arm called SRY which contains a gene for testicular development as early as in the embryonic stage. It is just the beginning of the development of the male sex, and later once the whole series of genes that control this is activated the male will fully develop. The location of the SRY gene is near the end of the chromosome arm, which carries with it certain risks. One thing that can happen is that a part of the chromosme containing the SRY gene breaks off during spermatogenesis and is lost (deletion) or attaches to e.g. X chromosome (translocation). As a result, the following will occur: Y chromosome that does not have the SRY-gene and an X chromosome that has the SRY gene attached to it. When such sperm cells fertilize eggs, the so-called XY women or XX men occur. Therefore, the male sex is not determined by the Y chromosome itself but by the presence of the SRY gene on it.
XX (SRY gene on one of the X chromosomes) men were first described in 1964. They occur with a frequency of 1:20 000 male newborns. They have a phenotype similar to men with Klinefelter's syndrome: the outside of the male genitalia, is rather small and undeveloped and in the testicles there is no spermatogenesis resulting in infertility. They are also shorter than normal men which is evidence that on Y the chromosome is located the growth gene.
XY (the Y chromosome is missing the SRY gene) women are genetic men with a pronounced female phenotype. They have female external genitalia and Miller's canals. Due to the lack of the SRY gene on the Y chromosome, indifferent gonads develop into ovaries, but since there are no two X chromosomes the ovaries pass into the connective bands.
45, XO men is just one of the plenty possibilities and they are specific because they have only 45 chromosomes rather than 46 which is typical for humans. In such individuals, part of the Y chromosome is attached to one of the autosomes due to reciprocal translocation, not the X chromosome as that in XX men. The phenotype of these men depends on: the size of the Y chromosome segment that is translocated, as well as the specific autosome to which that part is translocated. Thus, for example, a boy with translocation of part Y of chromosome to the short arm (p-arm) of chromosome 5 has the Cri-du-chat syndrome or more commonly known as the cat`s cry syndrome. Infants with this condition often have a high-pitched cry that sounds like that of a cat. The disorder is characterized by intellectual disability and delayed development, small head size (microcephaly), low birth weight, and weak muscle tone (hypotonia) in infancy. Affected individuals may also have distinctive facial features, including widely set eyes (hypertelorism), low-set ears, a small jaw, and a rounded face. In some cases children with the cri-du-chat syndrome are born with a heart defect.
Listen to the cri-du-chat characteristic sound at 0:45 in the video bellow.
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