numerical law being followed so far as has yet been observed.
Mendel made further experiments with Pisum sativum, crossing pairs of varieties which differed from each other in two characters, and the results, though necessarily much more complex, showed that the law exhibited in the simpler case of pairs differing in respect of one character operated here also.
In the case of the union of varieties AB and ab differing in two distinct pairs of characters, A and a, B and b, of which A and B are dominant, a and b recessive, Mendel found that in the first cross-bred generation there was only one class of offspring, really AaBb.
But by reason of the dominance of one character of each pair these first crosses were hardly if at all distinguishable from AB.
By letting these AaBb's fertilise themselves, only four classes of offspring seemed to be produced, namely,
Ab showing" dominant A and recessive b.
aB showing" recessive a and dominant B.
ab showing" both recessive characters a and b.
The numerical ratio in which these classes appeared were also regular and approached the ratio
9AB: 3Ab: 3aB: 1ab.
But on cultivating these plants and allowing them to fertilise themselves it was found that the members of the
| Ratios | |||
| 1 | ab class produce only ab's. | ||
| 3 | 1 | aB class may produce either all aB's, or both aB's and ab's.
| |
| 2 |
