Why indeed? A woman has XX, men have XY. The Y chromosome is much smaller than the X chromosome, and virtually useless. It carries only around 100 genes. OTOH, The X chromosome contains several important genes, such as the hemophilia gene, the gene that controls the excretion of phosphate by kidneys, and the dystrophin gene, abnormalities in which accounts for Duchenne's Muscular Dystrophy.
If the X chromosome is so important, how then do men manage with only X chromosome, while women need two? The surprising answer is that soon after birth, a woman randomly inactivates one X chromosome in every cell of hers. It might be the paternal X chromosome or the maternal X chromosome that's inactivated, but not the same X chromosome in every cell, as otherwise women would be as prone to X linked disorders as men.
This is done by an extra-genetic mechanism called epigenetics- usually by methylation of cytosine bases in DNA, where they occur next to guanine bases- a pairing known as CpG- p stands for the phospate in the deoxyribose sugar that joins up nucleotide bases like cytosine and gunanine in the DNA. This has the effect of shutting off genes which are downstream of the methylated CpG unit.
Epigenetics is an amazing phenomenon and may explain some of the most amazing events seen in the inheritance of acquired traits. More on that later.
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