The genetic and the operative evidence relating to secondary sexual charactersMorgan, Thomas Hunt
Science
The genetic and the operative evidence relating to secondary sexual characters
Morgan, Thomas Hunt
Genetics; Poultry; Sex (Biology)
In his later paper (1909) Davenport gives fuller information in regard
to some of the F₁ cases reported in his first paper, as well as the F₂
results. Thus, in the cross of Silky to Minorca, that gives black F₁
birds, the F₂ count gave 210 black, 57 game, and 95 white--approximately
the expectation for two pairs of factors, one of them giving white
(9:3:4). Silky by White Leghorn gave white F₁’s, but the males developed
red on the wing bow and saddle when they became mature, and the female a
faint blush of salmon (“red”) on the breast. In F₂ there were whites,
games, and blacks, approximating to expectation for three pairs of
factors, one being a dominant white (52:9:3). Silky by Buff Cochin gave
a washed-out buff, but with the jungle coloration partly developed in
the tail (black) and hackles and wing bow (redder buff). Davenport
represents the Buff Cochin as having lost the jungle patterns and
coloration, while the Silky retains it. The heterozygous condition of
the genes for the wild-type color in F₁ is made responsible for the part
development of color. The White Silky is represented as carrying the
factor for black (N), hence in F₂ both black and game-colored birds are
expected and they were obtained. When Black Cochin is crossed to Buff
Cochin, the F₁ males are in general like the game (black and red) while
the females are black (except for some red on the hackle). In this case
Davenport represents the Black Cochin as showing a factor for
jungle-fowl pattern, but lacking the color that is assumed in his other
formulæ to go with this pattern. What is meant by this change is not
quite clear to me, unless Davenport supposes there is an independent
factor for the jungle-fowl pattern which may be filled in by other
colors determined by other factors. But were there enough F₁ birds to
exclude the possibility that jungle-fowl birds would not appear in this
cross?
Davenport has reported a cross between a female White Cochin and a male
Tosa (wild type) from which the daughters were Tosa, except that the
shafting was broadened, and the saddle feathers and proximal secondaries
were obscurely barred (black and buff); the sons were also like the
Tosa, but every feather was repeatedly barred (see above). In F₂ there
were 15 white, 25 game, and 16 barred birds. Davenport concludes that
“barring is clearly heterozygous and confined to the male sex,” and in a
footnote he adds that the sex-linked barring factor of the Plymouth Rock
is different from that of this Cochin-Tosa cross, but Goodale informs me
that the barring that appeared in this cross is probably the same as
that in Barred Rocks.
Public-domain text, read in full here on John Shaqi.
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