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.
The human male gamete, also known as a sperm cell, contains 23 chromosomes in total. Humans have 22 chromosome pairs as well as two sex chromosomes, for a total of 46 chromosomes. The gamete produced by each sex contains half, or 23, of these chromosomes. Continue Reading.