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Heredity and Environment 



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FIG. 66. DIAGRAM SHOWING THE POSSIBLE CHROMOSOMAL MECHANISM 

 BY WHICH "CROSSING OVER" is CAUSED. Pairs of chromosomes, one from 

 the father, the other from the mother, are shown in synapsis. On the left 

 they lie parallel to each other and when they separate in the reduction 

 division they remain as they were before union ; in the second column they 

 are shown crossing each other one or more times and in the remaining 

 figures are shown the results of the chromosomes breaking at the points of 

 crossing and the interchange of sections of the two chromosomes. Let- 

 ters indicate loci of allelomorphic factors in the chromosomes. (After 

 Wilson.) 



at more than one place and are then broken at these points we get 

 double or multiple crossing over and a corresponding re-grouping 

 of genes and of characters (Fig. 66). Unless chromosomes of a 

 pair are very tightly twisted two cross-overs will not occur near 

 together and in general the farther apart points are in a chromo- 

 some the more likely is a cross-over to occur. On this basis Mor- 

 gan has constructed a "map" of each chromosome of Drosophila 

 indicating the positions of those genes which have been deter- 



