CREST.
In my report of 1906 I called attention to the nature of inheritance
of the crest in the first and second generations. The result seemed
simple enough on the assumption of imperfect dominance. However, in
later generations difficulties appeared, one of which was referred
to in a lecture given before the Washington Academy of Sciences in
1907. I stated (1907, p. 182), that "when a crested bird is crossed
with a plain-headed one, and the crested hybrids are then crossed
_inter se_, the extracted recessives of the second hybrid generation
are plain-headed, to be sure, but they show a disturbance of certain
feathers." This was an illustration of the statement that recessives
which are supposed to come from two pure recessive gametes show in
their soma traces of the dominant type. Dr. W. J. Spillman, who
was present, made the suggestion that the crest is composed of two
characters, _T_ and _t_, instead of a simple element, and that
when _t_ alone is present the result will be the roughened _short_
feathers on top of the head.
Further studies demonstrate the validity of this suggestion. There
are in the crest two and probably three or more factors. There is
a factor that determines length of the feathers and a factor that
determines their erectness. There is probably also an extension
factor that controls the area that the crest occupies on the head.
Thus flatness of position dominates over its absence (or erectness).
This is seen even in the first generation. Figs. 5, 6, 8, and 17
of my report of 1906 show this very plainly. They also show that
continued growth of feather is dominant over interrupted growth. Thus
in the second hybrid generation I got birds with short and erect
feathers and one of these is shown in fig. 11 of the 1906 report.
That shortness is recessive is proved by various matings of extracted
short × short crest. Of 29 offspring none have a higher grade than 1,
grade 10 being of full length. On the other hand, two parents with
long feathers in the crest (grades 6 to 8) give 5 offspring of grade
1, 12 of grades 5 to 10, thus approaching the 1:3 ratio expected from
two DR parents. That erectness is recessive is proved by various
matings of extracted erect × erect crest. Of 25 offspring none has
a lower grade than 4 (1 case) or 5 (1 case). On the other hand, two
parents with extracted non-erect feathers give in 46 offspring 13
with feathers whose grade of erectness is 6 or higher and 33 with a
grade of 5 or below--of these half of grade 0--close to the expected
1:3. The evidence is conclusive that there are two factors in crest
that behave in Mendelian fashion--a factor determining the prolonged
growth of the feather and a factor causing the feathers to lie
repent.
The area of the head occupied by the crest is also variable. This
was estimated in tenths for each of the parents and offspring. Two
principal centers of variation appeared, at 3 and at 8, or roughly
one-third and two-thirds the full area. The results, being based
on estimates, are not wholly satisfactory, but so far as they go
they indicate that when both parents have a crest that belongs to
the lower center of variation their offspring belong chiefly if not
exclusively to that center; but when they both belong to the upper
center of variation a minority of the offspring belong to the lower
center. Provisionally it may be concluded that extensive crest is
dominant over the restricted crest or that there is an "extension
factor."