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 in 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. 
An essentially similar mechanism has been found in a number of 
mammals, including man. The large number of chromosomes in 
these cases, however, make it difficult to establish the facts beyond 
question. 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 
45957°— 23— Bull. 905 4 
