Heredity - 491 



Fig. 26-14. Diploid chromosomes of Drosophila mela- 

 nogaster (female). I, II, III, and IV indicate the chro- 

 mosomes that are mapped in Figure 26-19. 



two pairs of genes that determine the body 

 color and wing length of the flies. 



In crossing one stock of fly, which self- 

 breeds true for black body and vestigial (re- 

 duced) wing, with another true-breeding 

 stock having gray body and large wings, the 

 ¥ 1 individuals are found to have gray bodies 

 and long wings (Fig. 26-15). Accordingly, the 

 gray (B) gene dominates black (b) and the 

 normal long wing gene (V) is dominant to 

 the vestigial (y); also both of the Pj flies are 

 homozygous as to both allelic pairs (Fig. 26- 

 15). Thus when an Fj hybrid male is back- 

 crossed to the double recessive female, in- 

 stead of getting four kinds of offspring such 

 as would be obtained if the genes assorted 

 independently, only two kinds of flies are 

 obtained (Fig. 26-15). Fifty percent of the 

 backcross offspring have black bodies and 

 short wings, and the other 50 percent have 

 gray bodies and long wings. In other words, 

 the black body and short wing, and the gray 

 body and long wing, are linked as to their 

 inheritance, since both of these gene pairs 

 have loci in the same pair of chromosomes. 



Incomplete Linkage: Crossing Over. The 

 foregoing case is called complete linkage be- 

 cause the two gene pairs in the one pair of 

 chromosomes are always inherited together. 

 But complete linkage is a rare phenomenon: 

 it occurs only in the male Drosophila and 

 very few other known organisms. A back- 



cross between a female ¥ r hybrid and a 

 double recessive male gives very different re- 

 sults, in that the offspring are of four types: 

 gray-vestigial, gray-long, black-long, and 

 black-vestigial; but these offspring are not 

 produced in equal numbers (Fig. 26-16). In 

 fact the results of such a cross cannot be 

 stated as a simple ratio, but only on the fol- 

 lowing percentage basis: 



gray-long 41% 



black-vestigial 41% 



gray-vestigial 9% 



black-long 9% 



2% 



18% 



In all cases of incomplete linkage, the 

 original combinations of characters — those 

 possessed by the P 1 parents — are found in a 

 majority of the offspring: but new combina- 

 tions occur in a minority of the progeny. 

 Moreover, the two majority classes are always 

 equal to each other — as is also true of the 

 minority classes. 



To explain these results, attention must be 

 focused upon the T 1 female hybrid. This 



\b and B 

 hybrid received chromosomes \v V 



from her homozygous parents and trans- 

 mitted these chromosomes without change to 

 a large majority of the eggs (Fig. 26-16). But 

 some of the eggs of the hybrid fly, due to a 



b B 



shift in the gene loci, received V and v 

 chromosomes, and these eggs gave rise to the 

 minority classes. 



An equal exchange of genes between the 

 chromosomes of a homologous pair is called 

 crossing over, and crossing over has impor- 

 tant genetic consequences. In fact a careful 

 study of the cross-over phenomenon has 

 yielded a determination of the exact sequence 

 of location of virtually all known genes in 

 the different chromosomes of Drosophila. 



Crossing over takes place during synapsis 

 (p. 479), while the homologous chromosomes 

 are entwined about each other so intimately 

 that in many cases they appear to fuse com- 

 pletely. 



After synapsis it usually is not possible to 



