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.
Example of a human karyotype: 23 pairs of homologous chromosomes (one pair is the homologous 'sex' chromosomes - in this case X and Y = a boy) III. Meiosis reduces chromosome number from diploid (2n) to haploid (n) 1 diploid cell (2n) cell becomes 4 haploid (n) germ cells (eggs or sperm).