, while others behaved like the original tall hybrids, giving a
generation consisting of talls and dwarfs in the proportion of three of
{20} the former to one of the latter. Counting showed that the number of
the talls which gave dwarfs was double that of the talls which bred true.

T x D --------------------- P
|
T(D) --------------------- F_1
|
+-----------+-----------+-------------+
T T(D) T(D) D --- F_2
| +---+----+--+ +--+----+---+ |
T T T(D) T(D) D T T(D) T(D) D D --- F_3
| |
T D --- F_4

If we denote a dwarf plant as D, a true breeding tall plant as T, and a
tall which gives both talls and dwarfs in the ratio 3 : 1 as T(D), the
result of these experiments may be briefly summarised in the foregoing
scheme.[2]

Mendel experimented with other pairs of contrasted characters and found
that in every instance they followed the same scheme of inheritance. Thus
coloured flowers were dominant to white, in the ripe seeds yellow was
dominant to green, and round shape was dominant to wrinkled, and so on. In
every case where the inheritance of an alternative pair of characters was
concerned the effect of the cross in successive generations was to produce
three and only three different sorts of individuals, viz. dominants which
bred true, dominants which gave both dominant and recessive offspring in
the ratio 3 : 1, and recessives which always bred true. Having determined a
general scheme of inheritance which experiment showed to hold good for each
of the seven pairs of alternative characters with which he worked, Mendel
set himself to providing a theoretical interpretation of this scheme which,
as he clearly realised, must be in terms of germ cells. He {21} conceived
of the gametes as bearers of something capable of giving rise to the
characters of the plant, but he regarded any individual gamete as being
able to carry one and one only of any alternative pair of characters. A
given gamete could carry tallness _or_ dwarfness, but not both. The two
were mutually exclusive so far as the gamete was concerned. It must be pure
for one or the other of such a pair, and this conception of the purity of
the gametes is the most essential part of Mendel's theory.

[Illustration: FIG. 1.

Scheme of inheritance in the cross of tall wi