CHAPTER III 
LINKAGE 
If two factors lie in the same member of a chromo- 
some pair we should expect them always to be found 
together in successive generations of a cross unless an 
interchange can take place between such a chromo- 
some and the homologous chromosome derived from 
the other parent. 
Whenever the two factors remain together in the 
same chromosome there will be formed equal numbers 
of gametes containing the two factors and of gametes 
containing the normal allelomorphs of the two 
factors. But if pieces of homologous chromosomes 
are interchanged, then some of the gametes will con- 
tain one of the factors in question, and an equal 
number will contain the other factor. The process 
of interchange between chromosomes is called cross- 
ing over; the tendency of factors to stay together is 
called linkage. 
An example may make clearer this process of cross- 
ing over. The factor for black body color and that 
for vestigial wings both lie in the second pair of chro- 
mosomes. If a black vestigial fly is crossed to a 
wild fly (gray, long wings) (Fig. 21) the offspring are 
gray with long wings. These F, flies have one chro- 
mosome containing both the factor for black and the 
factor for vestigial, and a homologous chromosome 
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