118 



FIGURE 16-2. Derivation of the pheiiotypic 

 ratio obtained in Drosophila from mating 

 together two dihybrids for linked genes. 



EGGS 



.4 vg + 



.4 + b 



.1 + + 



.1 vg b 



SPERM 

 .5 vg + 



.5 + b 



.5 vg + 





5 

 5 



.5 + b 

 .5 vg + 

 .5 + b 



CHAPTER 16 



+ bpK.2+b/ + b A 



vg + Ly/ .05 + + / vg + Ly 



.05 + + / + b 

 .05 vg b / vg + 

 .05 vg b / + b 



.2 black 

 .05 normal 

 .05 normal 

 .05 vestigial 

 .05 black 



ture wings (/?z), and forked bristles (/). For 

 example, the female dihybrid for y and cv 

 produces about 13 eggs of each 100 which 

 carry crossovers {+-^ovycv). What does 

 this value, and what do the other still different 

 linkage values, mean in terms of meiosis? 



Recall from what was discussed in the pres- 

 ent Chapter and the one preceding that no 

 commitment was made as to where or when 

 the genetic event of crossing over takes place. 

 Since we have been concerned with complete 

 and incomplete linkage as studied in succes- 

 sive generations of individuals, let us consider 

 only crossing over that occurs in the cell line 



2 Normal 

 1 Vestigial 

 1 Black 

 Vestigial 

 black 



giving rise to the gametes (the germ line), 

 ignoring the possibiHty that crossing over 

 occurs in somatic (nongerminal, p. 23) cells. 

 Although the possibilities still remain that 

 crossing over is premeiotic, meiotic, and 

 postmeiotic in occurrence, we shall presume 

 that all crossovers are produced during meio- 

 sis. We have already discussed (pp. 44^6) 

 the theoretical genetic consequences of a 

 chiasma between two pairs of genes during 

 meiosis, and we shall suppose that a chiasma 

 represents physical cytological evidence that 

 a genetic crossing over has occurred. 



The genetic events theoretically associated 



