PRINCIPLES OF LIVESTOCK BREEDING. 27 



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. Prof. Raymond Pearl, of the Maine experi- 

 ment station, found indications that the difference in fecundity 

 between Plymouth Rocks and Cornish Indian Games was in part 

 inherited in this way. 



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 Toyoma 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. 



