6. Nature of inflorescence, whether the flowers are arranged along the
axis of the plant; or are terminal and form a kind of umbel.
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[9].
[9] 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 1_D_ : 2_DR_ : 1_R_.
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
9_AB_ : 3_Ab_ : 3_aB_ : 1_ab_.
But on cultivating these plants and allowing them to fertilise
themselves it was found that the members of the