290 EVOLUTION, GENETICS, AND EUGENICS 



has already been shown to be sex linked. The result in F r is that all 

 females are red long and all males are white miniature. Inbreed the 

 individuals of F t and we get in F 2 : 33.5 per cent white miniatures, 

 33.5 per cent red longs, 16.5 per cent white longs, and 16.5 per cent red 

 miniatures. In other words, the crossing-over percentage is 33. If 

 the crossing-over percentage were to equal or exceed 50 per cent, it 

 would mean that there is no linkage at all, for if the two allelomorphic 

 genes were in different pairs of chromosomes we should have even 

 chances of two independent characters coming together or staying 

 apart. Thus we may say that in the first experiment the linkage (99 

 per cent) is very high and the crossover percentage is very low (1 

 per cent), while in the second experiment the linkage is relatively weak 

 (67 per cent, or only 17 per cent stronger than no linkage at all) and 

 the crossover percentage is relatively high (^^ per cent). 



The mechanism of crossing-over. — "If it be admitted," say 

 Morgan, Sturtevant, Muller, and Bridges, in their volume The Mech- 

 anism of Mendelian Heredity, "that Mendelian factors are carried by 

 chromosomes it can not be denied that interchange between homolo- 

 gous chromosomes must occur, for sex-linked factors cross over from 

 each other, and yet are known to be in the same pair of chromosomes, 

 since they all follow the X-chromosome in its distribution. The evi- 

 dence allows no other interpretation. But why should crossing-over 

 take place so rarely between certain factors and so often between 

 others? We can make use here of certain information in regard to 

 the chromosomes that gives a very simple answer to the question. In 

 the early germ cells, before the maturation period begins, the chromo- 

 somes appear to be scattered in the nuclei, and the homologous chro- 

 mosomes in many cases show no tendency to lie together, although in 

 some animals, e.g., in many flies, the members of a pair are often found 

 side by side. In this early period the germ cells divide as do other cells 

 and thereby increase in numbers. But at the termination of this 

 period, the homologous chromosomes unite in pairs. There has. been 

 much controversy as to how this union takes place, but in some cases 

 at least, the uniting chromosomes twist around each other as they come 

 together. This is illustrated to the left in Figure 73. As a consequence, 

 parts of one chromosome will come to lie now on one, now on the other 

 side of the mate. If when the twisted chromosomes separate, the parts 

 on the same side go to the same pole the end result will be that shown 

 to the right of Figure 73. Each chromosome has interchanged a 

 part with its mate. This process has been called crossing-over. It is, 



