Such inactivation stops transcription from occurring, hence making sure a potentially toxic double dose of X-linked genes does not occur. An inactivated X chromosome gets condensed into a small, dense structure in the nucleus, and is called a Barr body.
Barr bodies are commonly used to determine sex. Changes in the structure or number of X chromosomes can lead to a number of diseases.
For example, trisomy X syndrome is caused by the presence of three X chromosomes instead of two. Turner syndrome occurs when women inherit only one copy of the X chromosome. Some women have a rare super color vision trait called tetrachromacy , which is linked to the X chromosome.
These women can see up to million shades of color because they have four types of cone cells in their eye instead of the usual three. Two types of genetic mutations mostly make the difference; these were previously referred to as Morris syndrome and Swyer syndrome but are now collectively referred to as disorders of sex development DSD.
These people have an extremely high level of testosterone and other male sex hormones, but the testosterone does not affect the foetal cells that usually develop into male sexual organs because of a mutation in the androgen receptor gene. These people therefore have male chromosomes but are women socially and in external appearance.
They do not have internal female sexual organs, and they form testicles that remain concealed in the abdominal cavity. Typically, most of the girls with androgen insensitivity syndrome discover by puberty that they differ from other girls. They do not menstruate, and most will never be able to give birth. Apart from the discovery that more women have XY chromosomes than previously assumed, the researchers were also surprised about the variation in when these girls and women discover that something is different.
The girls with androgen insensitivity syndrome were diagnosed at an average age of 7—8 years old but some year-old women with the syndrome had not yet been diagnosed. They just do not know why. Even more surprising, however, is the fact that the average age of girls being diagnosed with gonadal dysgenesis, previously known as Swyer syndrome, is 17 years.
The reason for this high age at diagnosis is presumably that these women actually develop sexual organs that are almost normal. Women with gonadal dysgenesis have a mutation in the SRY gene of the Y chromosome that encodes for a protein known as the testicular determining factor that normally results in the testicles developing in the early weeks of foetal development.
While the chromosomes for other parts of the body are the same size and shape — forming an identical pairing — the X and Y chromosomes have different structures. The X chromosome is significantly longer than the Y chromosome and contains hundreds more genes. Because the additional genes in the X chromosome have no counterpart in the Y chromosome, the X genes are dominant. This means that almost any gene on the X, even if it is recessive in the female, will be expressed in males.
These are referred to as X-linked genes. Genes found only on the Y chromosome are referred to as Y-linked genes, and expressed only in males. Genes on either sex chromosome can be called sex-linked genes. There are approximately 1, X-linked genes, though most of them are not for female anatomical characteristics. In fact, many are linked to disorders such as hemophilia, Duchenne muscular dystrophy, fragile-X syndrome and several others.
They are responsible for red-green color blindness, considered the most common genetic disorder and found most often in males. The non-sex feature X-linked genes are also responsible for male pattern baldness. In contrast to the large X chromosome, the Y chromosome contains only 26 genes. Sixteen of these genes are responsible for cell maintenance.
Nine are involved in sperm production, and if some are missing or defective, low sperm counts or infertility may occur. One gene, called the SRY gene, is responsible for male sexual traits. The SRY gene triggers the activation and regulation of another gene, found on a non-sex chromosome, called the Sox9. The Sox9 triggers the development of non-sexed gonads into testes instead of ovaries.
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