294 



INTRODUCTION TO CYTOLOGY 



more frequently than others. Such linkage of genes and of characters is 

 well illustrated in the following case in Drosophila (Fig. 169). 



Two well-known characters in Drosophila cultures are black body and 

 vestigial wings. Each of these is a recessive character, appearing in a 

 fly only when the gene has been received from both parents in the reces- 

 sive condition h or v. In the dominant conditions B and V these genes 

 produce, respectively, normal gray body and normal long wings. When a 

 fly, homozygous for both dominant factors, is mated to one with all of 

 the corresponding factors recessive, the offspring all have normal body and 



.^/ ^ 



B^' V 



>*r 

 M 



b 



V N^ 



B b 



83 

 % 



V 



B b 



Back- cross 



b b 



\!^. 



Fig. 169. — Linkage in Drosophila. One pair of somatic chromosomes in each fly is 

 represented by parallel lines; chromosomes in gametes represented by diagonal lines. For 

 explanation, see text. {Adapted from Morgan et at., 1922.) 



wings, because of the dominance of B and V over h and v, respectively. 

 If the females of this Fi generation are back-crossed to the homozygous 

 recessive, flies of four types appear in the next generation: gray-long, 

 black-vestigial, gray- vestigial, and black-long. Those flies with the 

 original combinations (gray-long and black-vestigial) together comprise 

 83 per cent of the total number; only 17 per cent are of the new types 

 (gray- vestigial and black-long). It thus appears that if the two charac- 

 ters, gray body and long wings, are "contributed" to the offspring by the 

 same parent, they tend to appear together in the majority of the individ- 

 uals resulting from the back-cross; in other words, they are "linked." 

 This is explained by the fact that the differential genes concerned are 

 located in the same chromosome. The same is obviously true of the 

 allelomofphic characters, black body and vestigial wings: their genes are 



