CHAPTER III 

 LINKAGE 



If two factors lie in the same member of a chromo- 

 some pair we should expect them always to be found 

 together in successive generations of a cross unless an 

 interchange can take place between such a chromo- 

 some and the homologous chromosome derived from 

 the other parent. 



Whenever the two factors remain together in the 

 same chromosome there will be formed equal numbers 

 of gametes containing the two factors and of gametes 

 containing the normal allelomorphs of the two 

 factors. But if pieces of homologous chromosomes 

 are interchanged, then some of the gametes w^ill con- 

 tain one of the factors in question, and an equal 

 number will contain the other factor. The process 

 of interchange between chromosomes is called cross- 

 ing over; the tendency of factors to stay together is 

 called linkage. 



An example may make clearer this process of cross- 

 ing over. The factor for black body color and that 

 for vestigial wings both lie in the second pair of chro- 

 mosomes. If a black vestigial fly is crossed to a 

 wild fly (gray, long wings) (Fig. 21) the offspring are 

 gray with long wings. These Fi flies have one chro- 

 mosome containing both the factor for black and the 

 factor for vestigial, and a homologous chromosome 



48 



