The Collaborative International Dictionary of English v.0.48:

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.]