Why there is a mismatched pair in chromosomes called XY in human males?

Females produce gametes (ova) with only X chromosomes, and males produce gametes (sperm) with either X or Y chromosome. So, during fertilization, there is an equal chance of getting either XX or XY genotype. The X chromosome has characteristics of the female gender, while the Y chromosome has characteristics of the male gender. Now, in XX, there are only X chromosomes, so the child with genotype XX will be a girl. But, in XY, there is a Y chromosomes, which is "dominant" over the X chromosome. So, the child with genotype XY will be a boy.

Note: The only two possibilities of genotype during fertilization is XX and XY. YY is not possible because females don't produce gametes with Y chromosomes.

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I think tis may be the answer...:

We say mismatched because the Y-chromosome is shorter than its matching X-chromosome. The Y-chromosome has been called the male chromosome and the X-chromosome has been called the female chromosome. These names are not completely accurate descriptions but they have found their way into common usage. Because the Y-chromosome is shorter than the X-chromosome and is found only in males, some genes present on the mismatched X-chromosome are present without any matching alleles. Thus, genetic defects on the Y-chromosome affect only males.

One explanation of why the Y-chromosome is shorter than the X-chromosome involves the deletion of useless genes. Part of the normal genetic function of chromosomes is the exchange of genetic material. As the chromosomes align in pairs within cells, they often swap bits of DNA. This means that genes found on one chromosome find their way into others, usually their matching mate. Occasionally, bits of DNA and the associated genes travel from one chromosome pair to another. For example, a few genes from chromosome number 3 may work their way into chromosome number 20. The Y-chromosome, being the bearer of maleness does not completely combine with its paired X-chromosome. Perhaps many millions of years ago these two were very similar. But as time went by the Y-chromosome started not to recombine with the X-chromosome.

By not recombining the Y-chromosome has isolated its genes. This means that genes which find their way onto the Y-chromosome never get much of a chance to pass their code on to the genome of any offspring. Over the millennia these extraneous genes have been cut from the Y-chromosome, leaving it much shorter than its X-chromosome counterpart.

this is written in an encyclopedia...

so it should be ryt...

:) CHEERS!!!!