Cracking the AP Biology Exam
We already know that humans contain 23 pairs of chromosomes. Twenty-two of the pairs of chromosomes are called autosomes. They code for many different traits. The other pair contains the sex chromosomes. This pair determines the sex of an individual. A female has two X chromosomes. A male has one X and one Y chromosome—an X from his mother and a Y from his father. Some traits, such as color blindness and hemophilia, are carried on sex chromosomes. These are called sex-linked traits. Most sex-linked traits are found on the X chromosome and are more properly referred to as “X-linked.”
Since males have one X and one Y chromosome, what happens if a male has a defective X chromosome? Unfortunately, he’ll express the sex-linked trait. Why? Because his one and only X chromosome is defective. He doesn’t have another X to mask the effect of the bad X. However, if a female has only one defective X chromosome, she won’t express the sex-linked trait. For her to express the trait, she has to inherit two defective X chromosomes. A female with one defective X is called a carrier. Although she does not exhibit the trait, she can still pass it on to her children.
You can also use the Punnett square to figure out the results of sex-linked traits. Here’s a classic example: A male who has normal color vision and a woman who is a carrier for color blindness have children. How many of the children will be color-blind? To figure out the answer, let’s set up a Punnett square:
Notice that we placed a bar above any defective X to indicate the presence of a defective allele. And now for the results. The couple would have one son who is color-blind, a normal son, a daughter who is a carrier, and a normal daughter. The color-blind child is a son.
A look at the cell nucleus of normal females will reveal a dark-staining body known as a Barr body. A Barr body is an X chromosome that is condensed and visible. In every female cell, one X chromosome is activated and the other X chromosome is deactivated during embryonic development. Surprisingly, the X chromosome destined to be inactivated is randomly chosen in each cell. Therefore, in every tissue in the adult female one X chromosome remains condensed and inactive. However, this X chromosome is replicated and passed on to a daughter cell.
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