G. Mendel has succeeded in the correct interpretation of the results
obtained by him in the peas crosses differing in distinct features and he could
very explicitly express the idea of the existence of hereditary factors. The
Danish researcher W. Johannsen (1903, 1909) has termed these factors
genes and has introduced the notions of genotype, phenotype and pure
line. In 1905 W. Bateson has introduced the term genetics to denote the
science dealing with heredity of organisms (Greek: génesis — origin).
The classical experiments of Mendel have found place in almost all
books and manuals on genetics and biology. They are very popular but in
view of their great importance for the development of chromosome theory,
which will be the object of our further consideration, it is exigent to review
some of his principal formulations also known as the laws of Mendel.
Monohybrid Crossing
G. Mendel has crossed two varieties of peas differing in only one feature —
seeds of green and yellow colouring. In offspring of the first generation (F1)
there were registered only yellow seeds. The feature that has emerged was
called by him dominant and the suppressed one — recessive. The
meaning of these results has become clear when plants of the F1
generation have been crossed among themselves. It proved that in the
second generation (F2) 75% of the offspring is with the dominant feature
and 25% of the receive one, i.e. the ratio was 3:1. When crosses were
carried out within the F2 generation with the recessive feature, the entire
offspring has displayed the recessive one. When the F2 crosses have been
performed within the dominant line then 1/3 of the offspring has displayed
the dominant feature, and 2/3 were mixed thus preserving the 3:1 ratio
between the dominant and recessive features.
Mendel has interpreted the obtained results brilliantly. Schematically this
is shown in Figure 2–8. He has assumed that each hereditary feature is
determined by pair of factors. The hereditary factor for the dominant feature
he has designated by the capital letter (A) and the recessive — by the small
one (a). The obtained hybrids possessing both factors were designated as
(Aa). The pure dominants of a given feature have been received the symbol
(Aa) and the recessive ones — (aa).
For purposes of better explanation of the biological essence of these
symbols let us consider an example using terminology that has been
introduced later. The vegetative parental cells of the pure variety of peas with
yellow seeds contain each one a pair of genes (allelic couple) determining
their yellow colour (AA). The pure variety with green seeds has also got two
genes determining its green colour (aa). The one gene comes from the male
parent, while the other one — from the female. The parental cells of the hybrid
plants in F1 bear both genes (Aa). Their gametes (the sexual cells with a
reduced chromosome set obtained in the process of meiosis) contain only one
of the two genes (A) or (a). In the crosses of both varieties of peas which is