Crossing-over: Genetic Recombination
National Center for Human Genome Research, National Institutes
of Health. "New Tools for Tomorrow's Health Research." Bethesda, MD:
Department of Health and Human Services, 1992.
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).
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.
Go to Graphics Gallery: Chromosome Crossing-over
and Recombination During Meiosis (Crossing-over #2)
Go to next story: Into the Looking Glass
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