This assumption at once allows of applying the law of
probability, and to infer that of each kind of pollen grains one-half
will reach egg-cells with the same quality [297] and the other half
ovules with the opposite character.
Calling P pollen and O ovules, and representing the active mark by P and
O, the latent qualities by P' and O', they would combine as follows:
P + 0 giving uniform pairs with the active mark,
P + 0' giving unequal pairs,
P' + 0 giving unequal pairs,
P' + 0' giving uniform pairs with the latent mark.
In this combination the four groups are obviously of the same size, each
containing one-fourth of the offspring. Manifestly they correspond
exactly to the direct results of the experiments, P + O representing the
individuals which reverted to the specific mark, P' + O' those who
reassumed the varietal quality and P + O' and P + O' those who
hybridized [298] for the second time. These considerations lead us to
the following form of Mendel's,
P + O = 1/4 Active or 1A,
P + O'
> = 1/2 Hybrid or 2 H,
P' + O
P' + O' = 1/4 Latent or 1 L,
Which is evidently the same as Mendel's empirical law given above.
To give the proof of these assumptions Mendel has devised a very simple
crossing experiment, [299] which he has effected with his varieties of
peas.
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