Part 5

7. Length of stem, whether about 6 or 7 ft. long, or about 3/4 to 1-1/2 ft.

Large numbers of crosses were made between Peas differing in respect of one of each of these pairs of characters. It was found that in each case the offspring of the cross exhibited the character of one of the parents in almost undiminished intensity, and intermediates which could not be at once referred to one or other of the parental forms were not found.

In the case of each pair of characters there is thus one which in the first cross prevails to the exclusion of the other. This prevailing character Mendel calls the dominant character, the other being the recessive character.

Note that by these novel terms the complications involved by use of the expression “prepotent” are avoided.

That the existence of such “dominant” and “recessive” characters is a frequent phenomenon in cross-breeding, is well known to all who have attended to these subjects.

By letting the cross-breds fertilise themselves Mendel next raised another generation. In this generation were individuals which showed the dominant character, but also individuals which presented the recessive character. Such a fact also was known in a good many instances. But Mendel discovered that in this generation the numerical proportion of dominants to recessives is on an average of cases approximately constant, being in fact as three to one. With very considerable regularity these numbers were approached in the case of each of his pairs of characters.

There are thus in the first generation raised from the cross-breds 75 per cent. dominants and 25 per cent. recessives.

These plants were again self-fertilised, and the offspring of each plant separately sown. It next appeared that the offspring of the recessives remained pure recessive, and in subsequent generations never produced the dominant again.

But when the seeds obtained by self-fertilising the dominants were examined and sown it was found that the dominants were not all alike, but consisted of two classes, (1) those which gave rise to pure dominants, and (2) others which gave a mixed offspring, composed partly of recessives, partly of dominants. Here also it was found that the average numerical proportions were constant, those with pure dominant offspring being to those with mixed offspring as one to two. Hence it is seen that the 75 per cent. dominants are not really of similar constitution, but consist of twenty-five which are pure dominants and fifty which are really cross-breds, though, like the cross-breds raised by crossing the two original varieties, they only exhibit the dominant character.

To resume, then, it was found that by self-fertilising the original cross-breds the same proportion was always approached, namely--

25 dominants, 50 cross-breds, 25 recessives, or 1D : 2DR : 1R.

Like the pure recessives, the pure dominants are thenceforth pure, and only give rise to dominants in all succeeding generations studied.

On the contrary the fifty cross-breds, as stated above, have mixed offspring. But these offspring, again, in their numerical proportions, follow the same law, namely, that there are three dominants to one recessive. The recessives are pure like those of the last generation, but the dominants can, by further self-fertilisation, and examination or cultivation of the seeds produced, be again shown to be made up of pure dominants and cross-breds in the same proportion of one dominant to two cross-breds.

The process of breaking up into the parent forms is thus continued in each successive generation, the same 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 both dominant characters. Ab " dominant A and recessive b. aB " recessive a and dominant B. ab " 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, {2 or both aB’s and ab’s.

RATIOS

3 { 1 Ab class may produce either all Ab’s, { 2 or both Ab’s and ab’s.

{ 1 AB class may produce either all AB’s, { 2 or both AB’s and Ab’s, 9 { 2 or both AB’s and aB’s, { 4 or all four possible classes again, namely, { AB’s, Ab’s, aB’s, and ab’s,