738 



INHERITANCE 



To determine which of these alternatives is correct, Moewus made 

 1,400 crosses between the large and the small types (D and d) . Of these, 

 1,357 yielded as usual zygotes which gave 50 percent large cells with 

 long flagella, 50 percent small cells with short flagella. The remaining 

 73, or 5.2 percent of all, yielded zygotes that gave 50 percent large cells 

 with short flagella, and 50 percent small cells with long flagella (Fig. 

 171). In other words, these 73 zygotes gave only cells carrying the new 

 combinations. It appeared, therefore, that the factors for size of body 

 and length of flagella are merely linked, through the fact that the two 



Figure 171. New combinations resulting from crossing over, a and c, large cells 

 with short flagella, b, small cell with long flagella. (After Moewus.) 



are close together in the same chromosome. Their distance apart is such 

 as to yield crossing over in about 5.2 percent of the zygotes. This was 

 confirmed by making 600 crosses between the new combinations, one 

 parent having large body cells with short flagella, the other small body 

 cells with long flagella (Fig. 171 ) . Of the zygotes, 570 gave equal num- 

 bers that were like the parents, while 30 (or 5 percent) gave equal 

 numbers of the original combinations — large body with long flagella, 

 and small body with short flagella. These 30 therefore gave 100 percent 

 crossovers. 



In these cases an extraordinary situation appears, seemingly unique 

 in crossing over. In all these cases, any zygote that yields any crossover 

 combination yields exclusively crossovers. The 73 zygotes of the first 

 1,400 mentioned in the preceding paragraph yielded 50 percent of one 



