There are almost as many different types of four-wheel-drive systems as there are four-wheel-drive vehicles. It seems that every manufacturer has several different schemes for providing power to all of the wheels.
Most mammals have only one pair of sex chromosomes in each cell. Males have one Y chromosome and one X chromosome, while females have two X chromosomes. In mammals, the Y chromosome contains a gene, SRY, which triggers embryonic development as a male. The Y chromosomes of humans and other mammals also contain other genes needed for normal sperm production.
A normal human cell should contain exactly 46 chromosomes. They are matched into 23 pairs. Pairs one through 22 are numbered by size and appearance, and are the same in males and females. The 23rd pair is referred to as the sex chromosomes. Typically, females have 2 X chromosomes, while males have an X and a Y.