Sex Determination and Linkage
In reality, there are two different types of chromosomes. The first type are known as autosomes, and they are the same in all organisms. Except in cases of chromosomal aberration, they all have alike homologous chromosome. The second kind are know as the sex chromosomes, and they are the ones that determine the sex of the organism.
In humans, there are 22 pairs of autosomes, and one pair (the 23^rd) of sex chromosomes. For the human female, their 23^rd pair consists of two X chromosomes. For the human male, their 23^rd pair consists of an X and a Y. Thus when gametogenesis occurs (the act of creating gametes through meiosis), a female will always produce and egg with an X chromosome. Since a male will produce sperm with both X’s and Y’s, it is the male that determines the sex of the child.
Female
		X			X
Male	X	XX (female)			XX (female)
	Y	XY (male)			XY (male)
Since the X chromosome has certain genes that are not on the Y, and are not sexually related. These trait are more commonly known as sex-linked traits. For instance, Hemophilia (the inability of blood to clot) and Color blindness (The inability to distinguish between colors) are recessive sex linked traits. This means that the gene that causes these two trait are on the X chromosome. Thus a woman can have one X for colorblindness, and still see fine because the other X dominates over it. On the other hand, if a man has the gene for colorblindness, there is no other X chromosome to dominate over it (because of the Y), and so the man is colorblind. This is why hemophilia, colorblindness and other sex linked traits are always more common in men versus women.
Carrier Female for hemophilia
Normal Male				X		X*
			X	XX Normal Female		XX* Female Carrier
			Y	XY Normal Male		X*Y Male w/ Hemop.
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