PRINCIPLES OF LIVESTOCK BREEDING. 27 
fall into four linkage groups, corresponding to the four pairs of 
chromosomes of this insect. Microscopical study shows that sex 
is determined by one of these chromosomes in essentially the same 
way as that found in the grasshoppers and mammals in that the 
male produces two kinds of sperm. Parallel to this, they find that 
all the factors in one of the four linkage groups follow the sex- 
linked mode of inheritance of hemophilia and color blindness in man 
and the yellow color in cats. 
A number of characteristics have been found in chickens, pigeons, 
and canaries which are linked with sex, but curiously enough the 
relation to sex is exactly the reverse of that described above. The 
barred pattern of Plymouth Rock fowls is a familiar example. When 
a barred male is mated with a black female all the chicks are barred. 
With a black male and a barred female, only the male chicks are 
barred, the females being black. Further tests show that the barred 
females in the first case have no more tendency to transmit black 
than pure-blood Plymouth Rocks, while in the second case the black 
females have no tendency to transmit barring. There is, in other 
words, no inheritance of either of the alternative characters from 
mother to daughter. The conclusion drawn from such experiments 
is that in birds the females produce two kinds of eggs — determining 
the male and female sexes, respectively — while the males produce 
only one kind of sperm. 
This method of sex determination is not limited to birds. It was 
found in a moth by Prof. Doncaster, of Cambridge University, in 
the first case of sex-linked inheritance to be analyzed, and has been 
demonstrated in the silkworm by Toyama and Tanaka, two Japanese 
scientists. 
THE SEX RATIO. 
According to the method of determination outlined above, the 
sexes should be produced in equal numbers in the long run. This, in 
fact, is very nearly true in all the higher animals. Nevertheless, it is 
undoubtedly true that there is generally not exact equality. An 
extensive investigation in Germany by M. Wilckens gave the follow- 
ing numbers of males born to every 100 females: Cattle, 107.3; 
horses, 97.3; sheep, 97.4; swine, 111.8. The sex ratio in man varies 
in different countries, but always shows an excess of males, the 
average ratio being about 105 males to 100 females. These devia- 
tions of the sex ratio from equality are not necessarily out of harmony 
with the present theory of sex determination. The two kinds of 
sperm cells, for example, may differ in their activity or vitality. 
