Mendel's law \Men"del's law\
A principle governing the inheritance of many characters in
animals and plants, discovered by Gregor J. Mendel (Austrian
Augustinian abbot, 1822-84) in breeding experiments with
peas. He showed that the height, color, and other characters
depend on the presence of determinating factors behaving as
units. In any given germ cell each of these is either present
or absent.
Note: The following example (using letters as symbols of the
determining factors and hence also of the individuals
possessing them) shows the operation of the law:
Tallness being due to a factor T, a tall plant, arising
by the union in fertilization of two germ cells both
bearing this factor, is TT; a dwarf, being without T,
is tt. Crossing these, crossbreeds, Tt, result (called
generation F1). In the formation of the germ cells of
these crossbreeds a process of segregation occurs such
that germ cells, whether male or female, are produced
of two kinds, T and t, in equal numbers. The T cells
bear the factor "tallness," the t cells are devoid of
it. The offspring, generation F2, which arise from the
chance union of these germ cells in pairs, according to
the law of probability, are therefore on an average in
the following proportions: 1 TT : 2 Tt : 1 tt; and thus
plants pure in tallness (TT) and dwarfness (tt), as
well as crossbreeds (Tt), are formed by the
interbreeding of crossbreeds. Frequently, as in this
example, owning to what is called the dominance of a
factor, the operation of Mendel's law may be
complicated by the fact that when a dominant factor (as
T) occurs with its allelomorph (as t), called
recessive, in the crossbreed Tt, the individual Tt is
itself indistinguishable from the pure form TT.
Generation F1, containing only the Tt form, consists
entirely of dominants (tall plants) and generation F2
consists of three dominants (2 Tt, 1 TT) to one dwarf
(tt), which, displaying the feature suppressed in F1,
is called recessive. Such qualitative and numerical
regularity has been proved to exist in regard to very
diverse qualities or characters which compose living
things, both wild and domesticated, such as colors of
flowers, of hair or eyes, patterns, structure, chemical
composition, and power of resisting certain diseases.
The diversity of forms produced in crossbreeding by
horticulturists and fanciers generally results from a
process of analytical variation or recombination of the
factors composing the parental types. Purity of type
consequently acquires a specific meaning. An individual
is pure in respect of a given character when it results
from the union of two sexual cells both bearing that
character, or both without it.
[Webster 1913 Suppl.]
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