hat which has been
termed the "Presence and Absence" theory. On this theory the dominant
character of an alternative pair owes its dominance to the presence of a
factor which is absent in the recessive. The tall pea is tall owing to the
presence in it of the factor for tallness, but in the absence of this
factor the pea remains a dwarf. All peas are dwarf, but the tall is a dwarf
plus a factor which turns it into a tall. Instead of the characters of an
alternative pair being due to two separate factors, we now regard them as
the expression of the only two possible states of a single factor, viz. its
presence or its absence. The conception will probably become clearer if we
follow its application in detail to the case of the fowl's combs. In this
case we are concerned with the transmission of the two factors, rose (R)
and pea (P), the presence of each of which is alternative to its absence.
The rose-combed bird contains the factor for rose but not that for pea, and
the pea-combed bird contains the factor for pea but not that for rose. When
both factors are present in a bird, as in the hybrid made by crossing rose
with pea, the result is a walnut. For convenience of argument we may denote
the presence of a given factor by a capital letter and its absence by the
corresponding small letter. The use of the small letter is merely a
symbolic way of intimating {36} that a particular factor is absent in a
gamete or zygote. Represented thus the zygotic constitution of a pure
rose-combed bird is RRpp; for it has been formed by the union of two
gametes both of which contained R but not P. Similarly we may denote the
pure pea-combed bird as rrPP. On crossing the rose with the pea union
occurs between a gamete Rp and a gamete rP, resulting in the formation of a
heterozygote of the constitution RrPp. The use of the small letters here
informs us that such a zygote contains only a single dose of each of the
factors R and P, although, of course, it is possible for a zygote, if made
in a suitable way, to have a double dose of any factor. Now when such a
bird comes to form gametes a separation takes place between the part of the
zygotic cell containing R and the part which does not contain it (r). Half
of its gametes, therefore, will contain R and the other half will be
without it (r). Similarly half of its gametes will contain P and the other
half will be without it (p). It is obvious that the chances of R being
distributed to a gamete with