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.
1 In humans, females have 23 homomorphic pairs and males have 22 homomorphic pairs plus a heteromorphic pair. 2  During meiosis, females produce only one kind of gamete all having one X chromosome. 3  Males produce two kinds of gametes, one with an X and the other with a Y chromosome. XX-XO system in which females have 2 X chromosomes. 2  Males have only 1 X and no additional sex chromosomes. 3  Females have an even number of chromosomes and males have an odd number of chromosomes. 4  This occurs in many species of insects.