CHAPTER FOURTEEN 



ChromosoTnal Mutations 



One of the major achievements of classical genetics was the demonstra- 

 tion, principally by T. H. Morgan and his associates, that each gene can 

 be assigned to a definite locus, or position, in its chromosome by means 

 of cross-over tests. If a wild-type female Drosophila is mated to a yellow- 

 bodied, white-eyed male, the Fi females should all be heterozygous, with 

 the two dominant genes in the X chromosome derived from the mother 

 and the two recessive genes in the X chromosome derived from the father. 

 If no crossing over occurs, and if these Fi females are backcrossed to the 

 recessive type, then all of the backcross generation should be of one pa- 

 rental type or the other. Actually these types ( gray-bodied, red-eyed, and 

 yellow-bodied, white-eyed) make up 98.5 per cent of the offspring, but 

 the remaining 1.5 per cent comprises flies which are gray-bodied but 

 white-eyed, or yellow-bodied but red-eyed. 



LONGITUDINAL DIFFERENTIATION OF THE 

 CHROMOSOMES 



Crossing Over. These exceptional types can be accounted for only by 

 the exchange of genes between the two X chromosomes of the female 

 parent. The frequency of the exchange is typical for any two pairs of 

 genes which may be studied, and is the same in reciprocal crosses. If the 

 cross involves yellow and cut (a wing mutant), the exceptional types will 

 always make up 20 per cent of the backcross generation. If white and cut 

 are tested, then the exceptional types comprise 18.5 per cent of the prog- 

 eny. These data give a basis for mapping the relative positions of the 

 genes in the chromosome, if it be assumed that the frequency of crossing 

 over is a function of the distance between the genes. Thus the white and 

 yellow genes, which show only 1.5 per cent of crossing over, should be 

 fairly close together in the chromosome, while white and cut should be 

 twelve times as far apart. Such crossover experiments between sets of 

 three pairs of genes have made it possible to map the chromosomes of the 

 genetically better known plants and animals. They leave no room for 

 doubt that the chromosomes must be differentiated longitudinally. 



Specificity of Synapsis. Cytological evidence of the longitudinal dif- 

 ferentiation of the chromosomes is also available. The synapsis of homolo- 



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