Crossing Over 



151 



0.7 C Sp : 0.3 C sp : 0.3 c Sp : 0.7 c sp, the gametes of the Fi 

 must have been in the same ratio (Fig. 49a). Obviously, then, 

 a crossover did not occur between the loci of c and sp in every 

 primary oocyte. We may well ask the question, "In how many 

 did it occur?" 



Fi Female Recessive Male 



C 



Sp ■ 



c 

 sp 



c 

 sp 



- c 

 ■ sp 



Gametes; 0.35 



C ■ 



Sp- 



0.35 



sp- 



0.15, 



C- 

 sp- 



0.15 



c ■ 

 'Sp- 



all 



c ■■ 

 sp- 



Offspring: 0.35 Sp- -sp ^-^^ sp- - 



C ■ -c 



c ■ - c 



sp 



0.15 



Parental or 

 Noncrossover Types 



C-- -c 



sp - sp 



0.15 



c - -c 

 Sp ■■ -sp 



Nonparental or 

 Crossover Types 



Parents: 



Recessive 

 Female 



c 

 sp 



c 

 fsp 



Fi Male 



C 



Sp 



c 

 ■sp 



Gametes: ,, c - 

 ^^^sp- 



Offspring: C 

 Spi 



- c 



■sp 



C 



Sp 



c 

 sp 



-c 

 sp 



- c 

 -sp 



Fig. 49. Crosses involving linked genes in Drosophila. (a) A cross of 

 a heterozygous female (CSp/csp) with a recessive male (csp/csp). 

 Crossing over in the female produces four types of gametes in the ratio of 

 7C Sp : SC sp : ScSp : 7 c sp. Four types of offspring result : 0.35 C Sp : 

 0.15 Csp : 0.15 cSp : 0.35 csp. (6) A cross between a recessive female and 

 a heterozygous male. Because no crossing over occurs in the male, only two 

 kinds of gametes are produced and the offspring are in the ratio 0.5 C Sp : 

 0.5 c sp. 



