|The Structure of Human Chromosome|
Every living organisms is built by body cell (somatic) and sex cell (gamete). Each of the cells consists of two types of chromosome. Cells in a body are composed of non sex chromosome (autosome), while sex chromosome is composed of gamete (gonosome). Gonosome is chromosome type that determines sex types of an organism. The cell of human body has 46 chromosomes consisting of 22 pairs of body chromosome and 1 pair of sex chromosome.
One pair of sex chromosome is symbolized by XX or XY. The combination of that pair of chromosome occurs when egg cell having sex chromosome X is fertilized by sperm cell having sex chromosome X or Y. If an egg cell having sex chromosome X is fertilized by a sperm cell having sex chromosome X, the result is one pair of sex chromosome XX. An organism having sex chromosome XX is female.
Otherwise, if the one fertilizing the egg cell is a sperm having sex chromosome Y, then it results a pair of sex chromosome XY. An organism having sex chromosome XY is male.
Each of chromosome pairs in autosome is homologous pair. It means both of chromosome pairs have similarity in form, size, and gene type. And so does the pair of sex chromosome X and Y in female. On the other hand, it does not happen to the pair of chromosome X and Y in male.
In male, the pair of sex chromosome X and Y is not exactly homologous chromosomes. It is possible because there are parts of chromosome's arm that do not carry homologous genes. Another part of chromosome's arm carries homologous genes so that the chromosome becomes the only one considered to be homologous. Genes that are only found in sex chromosome are known as genes that link to sex chromosome X (Y). So, in gonosome, there are two types of gene, gene link to sex X or gene link to sex Y. The example of gene link to sex X is gene that causes color blind and hemophilia, whereas gene link to sex Y is genes thats cause hypertrichosis and,hystrix gravior.
source:Essential Biology, 2006.