generation, 3/4 of the
   plants  exhibit  the same (dominant) trait as the F[1] generation, and
   1/4  exhibit  the  contrasting  trait  that  disappeared  in  the F[1]
   generation.

   To explain these results, Mendel proposed the existence of particulate
   unit factors (genes) for each characteristic (phenotype).

   The  two  contrasting  traits  of the same gene are called alleles. An
   individual  possessing two copies of the same allele is homozygous. An
   individual possessing different copies of an allele is heterozygous.

   The F[2] genotype ratio is 1:2:1; while the phenotype ratio is 3:1.