152 Linkage and Crossing Over 



If crossing over took place between whole chromosomes, as 

 was once thought, and not between chromatids, a break in the 

 chromosomes between the two loci in 30 per cent of the primary 

 oocytes would produce 30 per cent of crossover gametes and 70 

 per cent of gametes of the parental types. As crossing over takes 

 place only between two out of four chromatids, however, each 

 nonparental t^^pe must have resulted from an oocyte that pro- 

 duced two nonparental and also two parental gametes. Crossing 

 over must therefore have taken place between chromatids in 60 

 per cent of the oocytes. 



Let us assume for purposes of illustration that there were 

 originally 50 primary oocytes and that crossing over occurred 

 in 30 of them. Each oocyte formed four eggs. To simplify the 

 problem, it is assumed here that the four cells of each primary 

 oocyte are functional. The 20 oocytes that did not have a 

 crossover between the c and sp loci would form 80 parental-type 

 gametes. The 30 oocytes that had a crossover would produce 

 60 crossover and 60 noncrossover gametes, because only two 

 chromatids from every such oocyte crossed over. Out of the 

 200 gametes that were produced from the original 50 oocytes, 

 only 60 or 30 per cent would be crossover types. A break of 

 two chromatids followed by a fusion of the broken ends produces 

 a chiasma. The 20 oocytes that had no break between c and 

 sp would have no chiasmata in that region, but each of the other 

 30 oocytes would have one chiasma between those two loci. 

 Therefore, 60 per cent of the oocytes would have one chiasma in 

 that region but only 30 per cent of the gametes would be of the 

 nonparental or crossover type (Fig. 50). For every 2 per cent 

 of the oocytes that have a chiasma between two given genes, 

 only 1 per cent of the eggs would show genetic crossing over 

 between these genes. 



Crossing Over in Male Drosophila. When a cross between a 

 c sp / c sp female and an Fi male {C Sp / c sp) is made, a rather 

 unique phenomenon is encountered. Half the offspring will be 

 C Sp and the other half will be c sp, and there will be no 

 crossover types (Fig. 496). This is the reciprocal of the pre- 

 vious cross. All the gametes of this Fi male are parental types 

 because no breaking and realignment of chromatids at pachy- 

 tene and no chiasmata are formed in the male Drosophila. Un- 

 less such breaks and chiasmata occur, no crossover types of 



