PRINCIPLES OP 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 wide- 

 spread. The first case was found by Professors Bateson and Punnett, 

 of Cambridge University, in the sweet pea. Cases are known in 

 corn and oats, in the primrose and snapdragon, in chickens and 

 pigeons, in mice as well as in rats, 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 



