20 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 



Without going into more detail it may be said that six independent 

 sets of allelomorphs are known in guinea pigs which cooperate to 

 determine color. The combinations of these factors determine over 

 a hundred distinguishable colors. 



THE CHROMOSOMES AND HEREDITY. 



The present theory of heredity was devised to explain the results 

 of experiments such as those given above. Recent studies of cells 

 under the microscope have apparently brought the mechanism under 

 our eyes. It has already been mentioned that proper methods of 

 staining bring out a certain definite number of rod-shaped bodies, 

 the chromosomes, in the cells of each kind of animal or plant. The 

 reproductive cells are found to contain just half as many as the fer- 

 tilized egg and the body cells. At each ordinary cell division the 

 chromosomes arrange themselves in a ring, each splits lengthwise, 

 and half goes to one daughter cell, half to the other. Thus all the 

 body cells have a double set. In the formation of the reproductive 

 cells, on the other hand, the chromosomes do not split, but the homol- 

 ogous ones, derived from the sperm and egg, pair with each other and 

 then separate, one going to each daughter cell. The reproductive 

 cells thus get only a single set of chromosomes. It will easily be 

 seen that if the hereditary units were located in the chromosomes the 

 observed behavior of the latter would fully account for the laws of 

 heredity illustrated above. 



Summing up, genetic experiments prove the double nature of 

 individuals and the single nature of their reproductive cells in regard 

 to each set of alternative hereditary factors, while the microscope 

 actually shows us the chromosomes in pairs in the body cells in place 

 of the single set to be observed in the reproductive cells. 



LINKAGE. 



As the study of heredity has advanced, a number of complications 

 have been found. These complications, however, have only made 

 closer the parallelism between the facts of heredity and the observed 

 behavior of the chromosomes. The most important of these com- 

 plications is the phenomenon known as linkage. A case studied by 

 Prof. Castle and the writer will serve as an example. 



A few years ago a freak wild rat with yellow fur and red eyes was 

 trapped on a wharf in England. Another wild rat of the same color, 

 but with pink eyes, was trapped in another place. Two strains of 

 yellow rats were developed which could be distinguished only by the 

 color of the eyes. Each strain bred true. Crosses with normal wild 

 rats showed that only one recessive unit factor was involved in each 

 case. It may appear surprising that on crossing the two yellow 



