170 



GENETICS 



extremely convenient for determining what characteristics 

 are linked in Drosophila. There are many other organisms 

 in which crossing-over is not limited to one sex. 



By breeding for successive generations and by the study 

 of exchanges between the chromosomes, the following 



[amnetes 



GaYY\et 



aYTaetes 



Figure 39. Diagram of the results of linkage, with relation to the 

 gametes. The two gametes above carry respectively the linked genes 

 AB and ab. The zygote produced by their union carries both AB 

 (in one chromosome) and ab (in the other). Then the majority of 

 the gametes formed by this zygote carry again the linked genes AB 

 or ab; only a minority carry Ab or aB. 



groups of linked characters have been found in the fruit- 

 fly.2 



Group I. A large group of sex-linked characters, more 

 than 100 in number. These all follow in heredity the distri- 

 bution of X-chromosomes, as described in earlier pages. Any 

 two or more of these characters may be linked together and 

 connected with the same X-chromosome. 



Group II. A large group of typical Mendellan characters, 

 50 to 100 or more in number. These follow the method of 



