View source for The Sex Chromosomes and Sex Determination

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== The Sex Chromosomes ==

The sex chromosomes include the X and Y chromosomes, which have a crucial role in sex determination. Males are defined by XY chromosomes and females by XX.

==='''X chromosome'''===
* Has more than 153 million base pairs
* Represents about 5% of the total DNA in women's cells, 2.5% in men's
* Contains about 2000 genes compared to the Y chromosome containing 78 genes, out of the estimated 20,000 to 25,000 total genes in the human genome
* Genetic disorders that are due to mutations in genes on the X chromosome are described as X linked.
* X chromosome inactivation is when most genes on one of the two X chromosomes in females are inactivated and do not produce any product. In somatic cells in normal females (but not in normal males), one X chromosome is inactivated early in development, thus equalizing the expression of X-linked genes in the 2 sexes. In normal female cells, the choice of which X chromosome is to be inactivated is a random one. Thus females are mosaic with respect to X-linked gene expression. In patients with extra X chromosomes, any X chromosome in excess of one is inactivated, but not all genes on that chromosome are inactivated.

==='''Y chromosome'''===
* Much smaller than X
* Carries only a few genes of functional importance, e.g. the TESTIS-DETERMINING FACTOR (also SRY protein – Sex-determining Region Y): certain genes that cause the male sex organs to develop. It is located on the short arm of the Y chromosome close to the pseudoautosomal region (see below). 


In male meiosis, the X and Y chromosomes normally pair by segments at the ends of their short arms and undergo recombination in that region. The pairing segment includes the PSEUDOAUTOSOMAL region of the X and Y chroms so called because the X and Y-linked copies of this region are homologue to one another like autosomes.


== Sex Determination ==

In females each ovum carries an X chromosome, whereas in males each sperm carries either X or Y. As there is roughly equal chance of either X-bearing sperm or Y-bearing sperm fertilizing the ovum, the numbers of male and female conceptions are approximately equal.

{|border="1"
|      ||   X chromosome  || Y chromosome
|-
|   X chromosome   ||   XX   || XY
|-
|   X chromosome   ||   XX   || XY 
|}

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== Links ==
=== Related Articles ===
* [[Disorders of the Sex Chromosomes]]
* [[Sexual Differentiation]]
=== Bibliography ===
=== References ===
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