Source : Webster's Revised Unabridged Dictionary (1913)

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 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.

Source : WordNet®

Mendel's law
     n : (genetics) one of two principles of heredity formulated by
         Gregor Mendel on the basis of his experiments with
         plants; the principles were limited and modified by
         subsequent genetic research