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Comprarison
of Meiosis and Mitosis
Geneticists
exploit the normal life cycle of an organism to
test for the dominance or recessivity of
alleles. To see how this is done, we need first
to review the type of cell division that gives
rise to gametes (sperm and egg cells in higher
plants and animals). Whereas the body (somatic)
cells of most multicellular organisms divide by
mitosis, the germ cells that give rise to
gametes undergo meiosis. Like somatic cells,
premeiotic germ cells are diploid, containing
two homologs of each morphologic type of
chromosome.
The two
homologs constituting each pair of homologous
chromosomes are descended from different
parents, and thus their genes may exist in
different allelic forms. In mitosis DNA
replication is always followed by cell division,
yielding two diploid daughter cells.
In meiosis one
round of DNA replication is followed by two
separate cell divisions, yielding four haploid
(1n) cells that contain only one chromosome of
each homologous pair. The apportionment, or
segregation, of the replicated homologous
chromosomes to daughter cells during the first
meiotic division is random; that is, maternally
and paternally derived homologs segregate
independently, yielding daughter cells with
different mixes of paternal and maternal
chromosomes. As a way to avoid unwanted
complexity, geneticists usually strive to begin
breeding experiments with strains that are
homozygous for the genes under examination. In
such truebreeding strains, every individual will
receive the same allele from each parent and
therefore the composition of alleles will not
change from one generation to the next.
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