CHAPTER XII 

 VARIATION IN LINKAGE 



Crossing over is not absolutely fixed in amount, but 

 is variable. This statement does not refer to variability 

 in tbe number of crossovers due to random sampling, 

 but to variability due to fluctuation in environmental 

 conditions, or due to internal changes in the mechanism of 

 crossing over itself. For example, it has been shown 

 that the amount of crossing over in Drosophila is different 

 at different temperatures, and it has also been shown that 

 there are factors (genes) carried by the chromosomes 

 themselves that affect the amount of crossing over. These 

 questions, that have already been touched upon in other 

 connections, may be taken up here in more detail. 



The work of Plough on the influence of temperature 

 on crossing over in Drosophila, that has already been 

 utilized, was concerned with the influence of different tem- 

 peratures on the number of crossovers obtained. It may 

 be recalled that he found that when the eggs were sub- 

 jected to a given temperature during a certain stage in 

 their maturation the amount of crossing over that took 

 place, as shown in the kinds of flies produced, was definite 

 in the sense that the average results were predictable for 

 each specific temperature, and that there are values for 

 different temperatures which, when plotted, give the curve 

 drawn in Fig. 56. 



Further details of one of the experiments may serve 

 to make its significance clearer. Three points (or loci) 

 were made use of that involved three mutant genes (and 

 their diagnostic characters, of course). Males, pure for 

 the three mutant characters, black body color, purple eyes, 



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