ure single
| |
+----------+ |
Single Single Single
| | |
| +-------+ +---------+
Single Single Double Single Double
| | |
| +-------+ +---------+
Single Single Double Single Double

From all this it is clear enough that there is much to be done before the
problem of sex is solved even so far as the biologist can ever expect to
solve it. The possibilities are many, and many a fresh set of facts is
needed before we can hope to decide among them. Yet the occasional glimpses
of clear-cut and orderly phenomena, which Mendelian spectacles have already
enabled us to catch, offer a fair hope that some day they may all be
brought into focus, and assigned their proper places in a general scheme
which shall embrace them all. Then, though not till then, will the problem
of the nature of sex pass from the hands of the biologist into those of the
physicist and the chemist.

* * * * *

{125}

CHAPTER XII

INTERMEDIATES

So far as we have gone we have found it possible to express the various
characters of animals and plants in terms of definite factors which are
carried by the gametes, and are distributed according to a definite scheme.
Whatever may be the nature of these factors it is possible for purposes of
analysis to treat them as indivisible entities which may or may not be
present in any given gamete. When the factor is present it is present as a
whole. The visible properties developed by a zygote in the course of its
growth depend upon the nature and variety of the factors carried in by the
two gametes which went to its making, and to a less degree upon whether
each factor was brought in by both gametes or by one only. If the given
factor is brought in by one gamete only, the resulting heterozygote may be
more or less intermediate between the homozygous form with a double dose of
the factor and the homozygous form which is entirely destitute of the
factor. Cases in point are those of the primula flowers and the Andalusian
fowls. Nevertheless these intermediates produce only pure gametes, as is
{126} shown by the fact that the pure parental types appear in a certain
proportion of their offspring. In such cases as these there is but a single
type of intermediate, and the simple ratio in which this