Hello -
Meiosis is cell division which results in each daughter cell having
half of the number of chromosomes of the mother cell - eukaryotic
cells contain two copies of each chromosome, after meiosis only a
single copy of each chromosome is left.
There are two ways in which meiosis increases genetic variation.
1) Random selection of chromosomes - all of the copies of chromosomes
are lined up and divided between the daughter cells, however, in each
cell the division is independent such that all combinations of pairs
of chromosomes are produced. For instance, assuming three chromosomes
(with M for maternal and P for paternal) the following combinations
are available:
Chromosome
1 M M M M P P P P
2 M M P P M M P P
3 M P P M M P M P
2) Crossing over - whilst the chromosomes are aligned in the mother
cell, before division into the chromosomes which will be passed to the
daughter cells, homologous (similar) regions may be exchanged between
the pair of chromosomes; thus the maternal and paternal chromosomes
can exchange genes, resulting in hybrid chromosomes (neither
completely maternal nor paternal)
M P M P
M P P M
M P P M <----- region which has crossed
M P P M
M P ----> M P
M P M P
M P M P
M P M P
M P M P
A brief overview of the genetic variations available:
http://www.radford.edu/~rsheehy/genetics/Meiosis/B245OMeiVar.html
A nice tutorial with much better pictures than I can produce here with
text:
http://www.biology-online.org/2/2_meiosis.htm
http://www.biology-online.org/2/3_crossing_over.htm
synarchy
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