te
rule, but that the inheritance of each pair was absolutely independent of
the other. Thus, for example, when a tall plant bearing coloured flowers
was crossed with a dwarf plant {25} bearing white flowers the resulting
hybrid was a tall plant with coloured flowers. For coloured flowers are
dominant to white, and tallness is dominant to dwarfness. In the succeeding
generation there are plants with coloured flowers and plants with white
flowers in the proportion of 3 : 1, and at the same time tall plants and
dwarf plants in the same proportion. Hence the chances that a tall plant
will have coloured flowers are three times as great as its chance of having
white flowers. And this is also true for the dwarf plants. As the result of
this cross, therefore, we should expect an F_2 generation consisting of
four classes, viz. coloured talls, white talls, coloured dwarfs, and white
dwarfs, and we should further expect these four forms to appear in the
ratio of 9 coloured talls, 3 white talls, 3 coloured dwarfs, and 1 white
dwarf. For this is the only ratio which satisfies the conditions that the
talls should be to the dwarfs as 3 : 1, and at the same time the coloured
should be to the whites as 3 : 1. And these are the proportions that Mendel
found to obtain actually in his experiments. Put in a more general form, it
may be stated that when two individuals are crossed which differ in two
pairs of differentiating characters the hybrids (F_1) are all of the same
form, exhibiting the dominant character of each of the two pairs, while the
F_2 generation produced by such hybrids consists on the average of 9
showing both dominants, 3 showing one dominant and one recessive, {26} 3
showing the other dominant and the other recessive, and 1 showing both
recessive characters. And, as Mendel pointed out, the principle may be
extended indefinitely. If, for example, the parents differ in three pair of
characters A, B, and C, respectively dominant to a, b, and c, the F_1
individuals will be all of the form ABC, while the F_2 generation will
consists of 27 ABC, 9 ABc, 9 AbC, 9 aBC, 3 Abc, 3 aBc, 3 abC, and 1 abc.
When individuals differing in a number of alternative characters are
crossed together, the hybrid generation, provided that the original parents
were of pure strains, consists of plants of the same form; but when these
are bred from a redistribution of the various characters occurs. That
redistribution follows the same definite rule for