Crossing Over 149 



genes to become separated, and the realignment of segments from 

 different chromatids would allow C and sp and c and Sp to be 

 together in some gametes. Tliis breakage and realignment of 

 the chromatids would have no significance in regard to the be- 

 havior of two given linked genes unless they happened to occur 

 in a region of the chromatids between them. Usually such a 

 break may occur at any place in a chromatid, except possibly 

 near the centromere, and therefore may occur between the two 

 linked genes in a certain percentage of the cases. If we can 

 assume, with reservations, that it can occur at random at any 

 place on the chromosome, it must naturally occur more fre- 

 quently between genes whose loci are far apart on a chromosome 

 than between loci that are close together. If a break with sub- 

 sequent chiasma occurs between two chromatids in the region 

 between two known genes, of the four gametes resulting from 

 the mother cell in which the break occurs, two will have un- 

 broken chromatids and the other two will have new chromatids 

 and will therefore show a new combination of genes. If an Fi 

 fly has the genes (C Sp) from one parent and {c sp) from the 

 other, and if one break occurs between the C and Sp loci in one 

 primary oocyte, of the four eggs and polar bodies that result, 

 two will be {C Sp) and (csp), in other words parental types, 

 and the other two will be (C sp) and (c Sp) and will be non- 

 parental types or recombinations (Fig. 48). However, in 100 

 primary oocytes, a break between those genes would not be 

 expected in every case unless the genes were far apart. The 

 genes C and Sp are actually not so far apart. Every oocyte that 

 did not have a break would produce one egg and three polar 

 bodies, all of which would be of the parental types, whereas each 

 oocyte that had one break would produce two parental and 

 two nonparental types of eggs and polar bodies. The polar bod- 

 ies are not functional, of course, but if 100 oocytes had no 

 break between these loci, they would produce 50 eggs of the 

 paternal type and 50 of the maternal. Of the 100 eggs from 

 the 100 oocytes in which a break occurred, 25 would be paternal 

 and 25 maternal; the other 50 eggs would be nonparental types. 

 Since there are some oocytes in which a break did not occur be- 

 tween C and Sp, there would be more parental than nonparental 

 types of gametes. 



