Sex-linked traits are characteristics other than sex that are carried by the sex chromosomes (coiled structures in a cell's nucleus that carries the cell's genetic information). Sex chromosomes in humans do more than determine whether a person is male or female, and they can carry such traits as the condition color blindness and the disease muscular dystrophy. Most sex-linked traits occur only on the X chromosome since it is larger than the Y chromosome.
In humans and many other species, females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY). At fertilization (the union of sperm and egg), the new embryo that is created receives half (twenty-three) its chromosomes from the female parent and half (twenty-three) from the male parent. The twenty-three chromosomes in one cell join together as matched pairs with the twenty-three chromosomes in the other cell, so that genes for the same trait (such as height) are situated together on the same chromosome. Once this occurs, whichever gene is dominant (such as tallness) usually gets expressed in the offspring. However, since the male and female sex chromosomes are so different from the other twenty-two sets of chromosomes, which are called autosomes, different rules apply to them. This is because the X chromosome is much larger than the Y chromosome. Since it is larger, it naturally has room for more genes, which carry specific traits. Therefore, when the sex chromosomes pair off and an X chromosome matches up with a Y chromosome (to make a male), many of the genes on the X chromosome do not have matching partners on the Y chromosome. Since the X chromosome is larger and has space for genes that cannot fit on the smaller Y chromosome, males carry slightly less genetic information than females.
The fact that a male's sex chromosomes (XY) are different from a woman's (XX) and are able to carry fewer genes has certain implications that are sometimes very important. It definitely leads to different inheritance patterns between the sexes. Since females have two X chromosomes, if one of these contains a recessive mutation (some type of change in the genetic code), it is likely to be overridden or offset by a dominant, normal gene on the other X chromosome. However, when a male inherits an X chromosome with a recessive mutation, it will appear in him if his Y chromosome does not have a matching gene on it (which it usually does not). When a male has only one gene for a certain trait, it will always be expressed or appear in him.
A well-known sex-linked trait is color blindness. The typical form of this condition is an inability to tell the color green apart from the color red. This condition is caused by a recessive gene carried on the X chromosome, but the trait only shows up when there is no dominant gene to offset it. That is why color blindness shows up most often in males. If a female has an X chromosome with the defective gene carrying color blindness, chances are her other X chromosome has a gene with normal vision, and she will not be color blind. Males have no "other" X chromosome, and so whatever recessive gene they inherit on their one X chromosome is expressed. That is why females are considered "carriers" of certain conditions. One of the more serious conditions for which a female can act as a carrier is for the disease hemophilia, which prevents the blood from clotting. Another is muscular dystrophy, a disease in which the muscles waste away. For a woman to get either hemophilia or muscular dystrophy, both her X chromosomes would have to have the defective recessive gene.