In regard to the two latter experiments, in which two and three
characters respectively were used, it is interesting to point out that
the form of the hybrid more nearly approaches “to that one of the
parental plants which possesses the greatest number of dominant
characters.” If, for instance, the seed plant has short stem, terminal
white flowers, and simply inflated pods; the pollen plant, on the other
hand, a long stem, violet-red flowers distributed along the stem, and
constricted pods,—then the hybrid resembles the seed parent only in the
form of the pod; in its other characters it agrees with the pollen
plant. From this we may conclude that, if two varieties differing in a
large number of characters are crossed, the hybrid might get some of its
dominant characters from one parent, and other dominant characters from
the other parent, so that, unless the individual characters themselves
were studied, it might appear that the hybrids are intermediate between
the two parents, while in reality they are only combinations of the
dominant characters of the two forms. But even this is not the whole
question.
Mendel points out that, from knowing the characters of the two parent
forms (or varieties), one could not prophesy what the hybrid would be
like without making the actual trial. Which of the characters of the two
parent forms will be the dominant ones, and which recessive, can only be
determined by experiment. Moreover, the hybrid characters are something
peculiar to the hybrid itself, and to itself alone, and not simply the
combination of the characters of the two forms. Thus in one case a
hybrid from a tall and a short variety of pea was even taller than the
taller parent variety. Bateson lays much emphasis on this point,
believing it to be an important consideration in all questions relating
to hybridization and inheritance.
The theoretical interpretation that Mendel has put upon his results is
so extremely simple that there can be little doubt that he has hit on
the real explanation. The results can be accounted for if we suppose
that the hybrid produces egg-cells and pollen-cells, each of which is
the bearer of only one of the alternative characters, dominant or
recessive as the case may be. If this is the case, and if on an average
there are the same number of egg-cells and pollen-cells, having one or
the other of these kinds of characters, then on a random assortment
meeting of egg-cells and pollen-cells, Mendel’s law would follow. For,
25 per cent of dominant pollen grains would meet with 25 per cent
dominant egg-cells; 25 per cent recessive pollen grains would meet with
25 per cent recessive egg-cells; while the remaining 50 per cent of each
kind would meet each other. Or, as Mendel showed by the following
scheme:—
A A a a
| / |
| × |
| / |
A A a a
Or more simply by this scheme:—
A a
| /|
| × |
|/
A a
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