134 
R. Ruggles Gates. 
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 bloc 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: ywAb‘ "VMS*" ' RFB' theSe factors 3,1 bein S P resent 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, 
ywabj vms r f b ' — 6 
YWABj VMS RFB' — 8 
Single-crossover eggs. 
YWa bj vms r f b' — 2 
YWABj vms r f W — 2 
ywabj VMS RFB' — 2 
YWABj Vms r f b' — 1 
YWAB> VMS rfb' — 1 
y w a bj vms RFB' — 1 
Double-crossover eggs, 
y w a bj VMS RFb' — 1 
But such types as yWaBj 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 
