200 
Journal of Agricultural Research voi. xxviii, No. 3 
EXPERIMENTAL STUDY OF THE SEX RATIO IN THE DOMESTIC 
FOWL 
MATERIALS 
During the first year of this experimental study reciprocal matings were made 
between Barred Plymouth Rocks and single-comb White Leghorns. Two 
pens of birds were used, each pen consisting of five Barred Plymouth Rock and 
five White Leghorn pullets, the first pen being mated to a Barred Plymouth Rock 
cockerel and the second to a White Leghorn cockerel. In the matings between 
Barred Plymouth Rocks and White Leghorns all the chicks were either entirely 
or largely white, the white of the White Leghorn being dominant to the sex- 
linked barring factor of the Barred Plymouth Rock. 
During the last two years matings were made between Barred Plymouth Rock 
pullets and single-comb Brown Leghorn cockerels. The mating for the year 
1920-21 consisted of 10 pullets and 1 cockerel and for the year 1921-22 two pens 
were mated, each consisting of 10 pullets and 1 cockerel. In these matings the 
sex-linked barring factor of the Barred Plymouth Rock is dominant to the 
Brown Leghorn plumage coloration. The male progeny is always barred, with 
shanks of the normal yellow color of both parental breeds. The females are 
always black or black tinged with brown, with black-pigmented shanks. At 
hatching time it is very easy to distinguish the sexes, since the males invariably 
have a small patch of white down on the back of the head and the shanks are 
always light in color, while the females have no white patch on the back of the 
head and the shanks are always black or very dark in color. The identification 
of the sex at hatching time is quite reliable, and during the two years out of 1,370 
chicks only six errors were made in identifying the sex prior to dissection or 
observation of the secondary sexual characters of the growing birds. 
As far as the observations in this study are concerned, it can be said that 
external appearances of the chicks at hatching time give no indication of reversi¬ 
bility of sex. The characteristic secondary sexual features of the male were 
always associated with a male as shown by dissection, and so with all females. 
Birds which were allowed to reach the adult stage showed no signs of an inter¬ 
mediate sex condition or of changing from one sex to another. 
Of the 50 females used in this study, the records of only 45 are analyzed, since 
either through early death or very poor production the records of 2 birds for 
each of the first two years have been discarded and in the third year female 
No. 1508 produced not 1 fertile egg out of 63 laid. 2 
METHODS 
All of the birds were trap-nested throughout each year. Complete records 
were kept of production, and during the last two years the eggs were weighed 
daily. Throughout the three years the eggs were placed in the incubators on 
an average of about every 10 days. From the time of gathering the eggs to the 
conclusion of the work each season practically every egg has been accounted for. 
All cracked eggs have been recorded, whether they were cracked by the hen in 
the nest, by the attendant in gathering the eggs, or by the operator during the 
period of incubation. Detailed records were kept of all eggs which were infertile, 
embryos which died before the twelfth day of incubation and on which sex 
could not readily be obtained, embryos which died on and after the twelfth day 
2 The first and last year’s work was done in the department of poultry husbandry of Macdonald College, 
while the second year’s work was done in the department of genetics of the University of Wisconsin, the 
birds being very kindly supplied by Prof. J. G. Halpin, of the department of poultry husbandry of that 
institution. ' 
