PRINCIPLES OF LIVESTOCK BREEDING. 23 
stick together in the formation of the reproductive cells, according 
to the way in which they were combined in the production of the 
individual himself, is known as linkage. 
A third color variation in rats, viz, albinism, has been found to be 
linked with both of the yellow variations. On the other hand, the 
other known color variations, white spotting and black, are not only 
inherited wholly independently of each other, but also of albinism 
and the two kinds of yellows. 
This phenomenon of linkage has been found to be very widespread. 
The first cases were found by Professors Bateson and Punnett, of 
Cambridge University, in the sweet pea. Cases have also been worked 
out in garden peas, in corn, oats, and wheat, in the tomato and in 
cotton, in the primrose, snapdragon, stocks, and evening primrose, 
in rabbits and mice, as well as in rats, in chickens and pigeons, in 
grasshoppers, silkworms, and flies. By far the most thoroughly 
analyzed case is that of the fruit fly, Drosophila, in which Prof. T. H. 
Morgan and his coworkers, of Columbia University, have studied 
hundreds of Mendelian variations. They find that these variations 
fall into four groups, such that within each group every factor is 
linked more or less with every other factor, while there is never any 
linkage between factors in different groups. It is not merely 
a coincidence that in this fruit fly there are just four pairs of 
chromosomes. 
This statement suggests the accepted explanation of linkage. 
Factors which are carried by the same chromosome tend to stick 
together. The chromosomes appear to maintain their identity 
through all the ordinary cell divisions. Just before the formation of 
the reproductive cells, the homologous chromosomes come together 
and twist around each other, giving a chance for an interchange of 
pieces. The degree of linkage between two factors is believed to 
measure their distance apart within the chromosome. On this basis 
Prof. Morgan and his coworkers have actually been able to make 
maps showing the location of a great number of unit factors in the 
different chromosomes of the fruit fly, which explain the results of 
crosses in a very convincing way. 
The most remarkable corroboration of the chromosome theory of 
heredity has been the bringing of the genetic phenomenon of linkage 
and the visible behavior of the chromosomes into relation with the 
solution of the ancient problem of sex determination. 
THE DETERMINATION OF SEX. 
THE NORMAL METHOD. 
There are few questions connected with animal breeding which 
have aroused so much interest from the earliest times as the deter- 
mination of sex. Hundreds of theories have been advanced, and, 
though repeatedly disproved, keep reappearing. It is only within 
