\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 F_{1}). 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 F_{2}, 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 F_{1}, containing only the Tt form, consists
      entirely of dominants (tall plants) and generation
      F_{2} consists of three dominants (2 Tt, 1 TT) to one
      dwarf (tt), which, displaying the feature suppressed in
      F_{1}, 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.