150 



Linkage and Crossing Over 



Testcross. If an Fi female whose genetic constitution is 

 C Sp / csp is testcrossed with a c sp / c sp male, will all the 

 offspring be like the parental generation, will they segregate 

 into a 1:1:1:1 ratio as in independent assortment, or will 

 they give some other result? Such a question can best be 

 answered by carrying out an experiment. Fortunately, we do 

 not have to do so as such experiments have been carried out a 



c- 



c — 



Sp- 



sp- 



FiG. 48. The results of the formation of a chiasma between the loci 

 of c and sp. One chromatid of each chromosome has broken and exchanged 

 partners. Four kinds of chromatids have formed, two of which are un- 

 broken, parental types {C Sp and c sp) and two are recombinations or 

 crossover types (Csp and cSp). 



number of times. Data from such a cross involving genes c 

 and sp have been summarized by Bridges and Morgan. They 

 show that out of 10,042 flies, from the testcross, 3037 were 

 curved and nonspeck or noncurved and speck. In other words, 

 69.8 per cent of the offspring were like the original parents 

 whereas 30.2 per cent were nonparental types. 



When two genes on the same chromosome separate from one 

 another because of a break and recombination of chromatids in 

 the region between the loci of those genes, they are said to 

 cross over and the phenomenon is called crossing over. Similarly 

 the nonparental gametes may be called crossover gametes and 

 the parental gametes may be called noncrossover gametes. Since 

 the ratio of the offspring from the testcross was approximately 



