In every cell with a double system of factors, there are necessarily
present two representatives from each set of allelomorphs, but these may
or may not be alike--or in technical language the individual may be
homozygous, or heterozygous, as regards the given set of alternative
factors. Looking at it from another angle, there is a single visible
character in the plant or animal, but it is produced by a double factor,
in the germ-plasm.
When the immature germ-cell, with its double system of factors, matures,
it throws out half the factors, retaining only a single system: and the
allelomorphic factors which then segregate into different cells are, as
has been said above, ordinarily uninfluenced by their stay together.
But the allelomorphic factors are not the only ones which are segregated
into different germ-cells, at the maturation of the cell; for the
factors which are not alternative are likewise distributed, more or less
independently of each other, so that it is largely a matter of chance
whether factors which enter a cross in the same germ-cell, segregate
into the same germ-cell or different ones, in the next generation. This
is the next fundamental principle of Mendelism, usually comprehended
under the term "segregation," although, as has been pointed out, it is
really a double process, the segregation of alternative factors being a
different thing from the segregation of non-alternative factors.
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