Non-Disjunction in Drosophila. 37 



exchange of genes between a pair of chromosomes, is shown by 

 the fact that when a number of factors are involved whose position 

 at intervals along the chromosome is known from the crossing over 

 percentages, a group of them go over en blcc and the rest remain in 

 the chromosome (i.e., group) where they were before. Thus (Morgan 

 etal. 1915, p. 66) if a hybrid female is made up, having received from 

 her mother the mutant factors yellow, white, abnormal, bifid, 

 vermilion, miniature, sable, rudimentary, and forked, and from her 

 father the normal allelomorphs of these together with the dominant 

 mutant factor for bar eye, her formula may be written as 



follows : ' VMS S RFB' these factors a11 being present in 



the two X chromosomes. When such females were bred, they 

 were found to produce the following kinds of eggs, with the 

 frequency indicated : 



Non-crossover eggs. 



ywabi vms rfb' 6 



YWABi VMS RFB' 8 

 Single-crossover eggs. 



YWabi vms rfb' - 2 



YWABi vms r f b' 2 



y w a bj VMS RFB' 2 



YWABi Vms r f b' 1 



YWABi VMS r f b' 1 



y w a bj vms RFB' 1 

 Double-crossover eggs. 



y w a bi VMS RFb' 1 



But such types as yWaB; VmS rfB' were not found, showing 

 that indiscriminate reassortment of the factors between a pair of 

 chromosomes does not occur. 



Double crossing over, in which the ends of a pair of 

 chromosomes exchange while the central portion remains as before, 

 lessens the effect of single crossovers and so is called interference. 

 The result is as though the percentage of crossing over between 

 certain points was reduced. 



Now if a female with vermilion eyes is crossed with a wild 

 red-eyed male, the vermilion being recessive the daughters will 

 have red eyes like their father because they received his 

 X chromosome with the red factor which is dominant to vermilion. 

 But the sons will have vermilion eyes like their mother, since they 

 received her X chromosome and only the inactive Y from the 

 father. This criss-cross inheritance will always happen when the 



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