Taken from Genentech's Access Excellence
Ever wonder why, except for identical twins, no two people look exactly alike? While genes determine most of our physical characteristics, the exact combination of genes we inherit, and thus our physical traits, is in part due to a process our chromosomes undergo, known as genetic recombination.
Genetic recombination happens during meiosis, a special type of cell division that occurs during formation of sperm and egg cells and gives them the correct number of chromosomes. Since a sperm and egg unite during fertilization, each must have only half the number of chromosomes other body cells have. Otherwise, the fertilized cell would have too many.
Inside the cells that produce sperm and eggs, chromosomes become paired. While they are pressed together, the chromosomes may break, and each may swap a portion of its genetic material for the matching portion from its mate. This form of recombination is called crossing-over. When the chromosomes glue themselves back together and separate, each has picked up new genetic material from the other. The constellation of physical characteristics it determines is now different than before crossing-over.
Tracking the movement of genes during crossing-over helps geneticists determine roughly how far apart two genes are on a chromosome. Since there are more chances for a break to occur between two genes that lie far apart, it is more likely that one gene will stay on the original chromosome, while the other crosses over. So, genes that lie far apart are likely to end up on two different chromosomes. On the other hand, genes that lie very close together are less likely to be separated by a break and crossing-over.
Genes that tend to stay together during recombination are said to be linked. Sometimes, one gene in a linked pair serves as a "marker" that can be used by geneticists to infer the presence of the other (often, a disease-causing gene). there will be much more about this in the next section, on linkage.
After the chromosomes separate, they are parceled out into individual sex cells. Each chromosome moves independently of all the others - a phenomenon called independent assortment. So, for example, the copy of chromosome 1 that an egg cell receives in no way influences which of the two possible copies of chromosome 5 it gets.
Assortment takes place for each of the 23 pairs of human chromosomes. So, any single human egg receives one of two possible chromosomes 23 times, and the total number of different possible chromosome combinations is over 8 million (2 raised to the 23rd power). And that's just for the eggs. The same random assortment goes on as each sperm cell is made. Thus, when a sperm fertilizes an egg, the resulting zygote contains a combination of genes arranged in an order that has never occurred before and will never occur again.