Smaller of the two sex chromosomes. In male mammals it occurs paired with the other type of sex chromosome (X), which carries far more genes. The Y chromosome is the smallest of all the mammalian chromosomes and is considered to be largely inert (that is, without direct effect on the physical body), apart from containing the genes that control the development of the testes. There are only 20 genes discovered so far on the human Y chromosome, far fewer than on all other human chromosomes. In humans, whether a person is male or female is determined by the presence or absence of a sex-determining gene on the Y chromosome known as SRY (sex determination). Thus, embryos with two X chromosomes (XX) will normally develop as females, while those with one X and one Y chromosome (XY) will normally develop as males.
As a result of meiosis gametes from a female each contain one X chromosome. However, gametes from a male are of two kinds. Half of the gametes contain an X chromosome and half contain a Y chromosome. If an X carrying gamete from a male fertilizes a female gamete the result will be a female. If a Y carrying gamete from a male fertilizes a female gamete, the result will be a male.
In rare cases, people may carry just one or more than two sex chromosomes. Given the small number of genes contained in the Y chromosome and its absence in females, the effects of having a Y chromosome too many (for example, XYY) are relatively benign, compared to abnormalities involving other chromosomes, such as Down's syndrome. The condition of having 47 chromosomes including two X and one Y is called Klinefelter's syndrome, and affects around 1/1000 males.
Sequencing the Y chromosome In the course of the Human Genome Project (see human genome), sequencing the Y chromosome turned out to be particularly challenging because it is in large parts a situation of multiple repeats and inverted copies. As routine sequencing involves the read-out of short DNA fragments that are later pieced together by computers, the repetitiveness of the Y chromosome required special procedures for the assembly of the sequence, which was published in 2003.
Evolution From the sequencing results, researchers have concluded that the unusual number of repeats in the Y chromosome is related to its evolution in the absence of a matching partner. In the beginning of mammalian evolution, X and Y chromosomes must have been a normal pair of chromosomes, except that one contained the active gender determining factor and the other did not. Gradually, the two sex chromosomes grew more and more different, with the Y chromosome shrinking and becoming more and more specialised on male-specific tasks (it is the only mammalian chromosome that specializes on one group of tasks).
In normal chromosome pairs, the presence of a second, largely identical copy serves as a quality control that can keep the content of the chromosome stable in spite of the frequent mutations and the variability contained within populations. Chromosome pairs can exchange equivalent stretches of DNA in a process known as crossing over. For the Y chromosome, which was increasingly lacking common ground with the X chromosome and thus losing the ability to engage in crossing-over with the partner chromosome, the multiple repeats within the same chromosome took over the function of the partner chromosome.
The X/Y system of sex determination is specific to placental mammals. Birds, reptiles, and even the duck-billed platypus (representing a very early branch of mammalian evolution) have different kinds of sex chromosomes.
Effect of chromosomes and genes on health
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