Factors Influencing Crossing Over 167 



centage differs for different pairs of genes because the amount 

 of crossing over depends on the frequency with which breaks 

 in the chromatids occur between the two genes; this frequency 

 is further dependent chiefly upon the distance these genes are 

 apart on the chromatids. Assuming for the present that breaks 

 can occur equally easily at all points on a chromatid, we 

 may say that the farther two genes are apart, the more likely, 

 as the result of chance, breaks will occur between them. As two 

 genes are normally always the same distance apart on a chro- 

 mosome, the chance that a break can occur between them is 

 always the same, and, therefore, the percentage of recombina- 

 tions is always t?ie same. The percentage of crossovers between 

 any two pairs of genes can never be stated until proper experi- 

 ments have been carried out. Once the experiments have been 

 made, the amount of crossing over between those genes can be 

 predicted, but there is no way of telling what that percentage 

 will be before it is tested experimentally. 



Factors Influencing Crossing Over 



The above discussion is based upon the consideration that the 

 experiments are carried out under the same conditions, for the 

 ease with which breaks can occur in the chromatids is influenced 

 by the environment. Heat, X-radiation, and some chemicals 

 may produce striking differences in the percentage of recombina- 

 tions, usually having the effect of increasing the percentage. 

 Plough demonstrated that when Drosophila females are raised 

 at high temperatures, the percentage of crossovers is increased 

 over that obtained when the flies are reared at ordinary room 

 temperature. Strangely enough, however, low temperatures also 

 produce an increased percentage of crossover types as com- 

 pared with ordinary, medium temperatures. 



Crossing over can be affected by internal as well as external 

 factors. Differences in the percentage of recombinations in the 

 different sexes have been mentioned in connection with the male 

 Drosophila and the female silkworm, but they are frequently 

 observed also in other organisms, although in them the sex dif- 

 ferences are not so great. The age of the individual may also 

 affect the frequency of crossing over, for Bridges has demon- 

 strated that, in Drosophila, as the female gets older the percent- 



