From Wikipedia,
the free encyclopedia.
Chromosomal crossover is
the process by which two
chromosomes, paired up during
Prophase I of
meiosis, exchange some distal
portion of their
DNA. Crossover occurs when two
chromosomes, normally two
homologous instances of the
same chromosome, break and then
reconnect but to the different end
piece. If they break at the same
place or
locus in the sequence of base
pairs, the result is an exchange
of
genes. This outcome is the
normal way for crossover to occur.
If they break at slightly
different loci, the result can be
a duplication of genes on one
chromosome and a deletion of these
on the other. This is known as an
unequal crossover. If
chromosomes break and rejoin on
opposite sides of the
centromere, the result can be
one chromosome being lost during
cell division.
Any pair of homologous
chromosomes may be expected to
cross over three or four times
during meiosis. This reduces the
genetic linkage between genes
on the same chromosome. The
genetic variation of a population
is thereby increased through
chromosomal crossover.
Independent assortment is a
somewhat related process operating
on the complete set of
chromosomes.
The physical basis of crossing
over was first demonstrated by
Harriet Creighton and
Barbara McClintock in
1931.
See also
