PRINCIPLES OF LIVESTOCK BREEDING. 25 



As an illustration, we will consider the case of one of the grass- 

 hoppers in which, owing to the large size of the cells and the rela- 

 tively small number of chromosomes, the facts are considered to be 

 beyond question. In this case, study of the cells from the digestive 

 tract and muscle fibers of males shows 6 pairs of similar chromosomes 

 and 1 odd chromosome, making 13 in all. The body cells of females 

 contain 7 pairs of chromosomes, 6 pairs of which resemble the pairs 

 in the male, while those in the seventh pair resemble the odd chro- 

 mosome of the male. The same numbers are found m the repro- 

 ductive cells before the final division which results in the functional 

 reproductive cells. The final division is peculiar, as already noted, 

 in that the chromosomes are sorted bodily into two groups. All the 

 egg cells must contain 7 chromosomes, 1 from each pair. The sperm 

 cells, on the other hand, are necessarily of two kinds, half containing 

 6 and half 7. The two kinds must, of course, be formed in exactly 

 equal numbers. The inference is clear that any egg cell which happens 

 to be fertilized by a sperm containing 6 chromosomes will develop 

 into a grasshopper with only 13 chromosomes in the body cells, and 

 hence a male, while fertilization by a sperm containing the odd 

 chromosome, i. e., 7 chromosomes in all, Will result in the number 

 14, and hence a female. From this it appears that sex is determined 

 by chance at the moment of fertilization. 



A similar mechanism has been found in a number of the mammals, 

 including man. In the latter case, according to Von Winiwarter, 

 there are 23 chromosomes in the male-determining sperms and 24 

 in those which determine the female sex. 



It is naturally more difficult to establish the facts beyond question 

 where such large numbers are involved. Fortunately, however, 

 there is a wholly independent line of evidence which leads to the 

 same conclusion. This is the evidence from characteristics linked 

 with sex in inheritance. 



SEX-LINKED INHERITANCE. 



In the human species the mode of inheritance is best understood 

 in the case of abnormalities which keep appearing in particular 

 families. Most of these traits are inherited as if due to a single 

 dominant or recessive factor. There are a number, however, includ- 

 ing color blindness and hemophilia, which have long been known to 

 follow a very peculiar mode of inheritance. These traits usually 

 affect only males, yet are never transmitted from father to son, 

 and do not reappear in the descendants of the sons. The daughters 

 of affected males, on the other hand, though not affected them- 

 selves, are very apt to have affected sons. This rule was discovered 

 as early as 1820 by Nasse in the case of hemophilia, a condition in 



5254°— 20— Bull. 905 4 



