194. 



Heredity and Environment 



same way. If a pair of chromosomes are twisted round each other 

 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. 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. If one per cent of 

 "crossing over" occurs the genes are assumed to be one unit of 

 distance apart; if ten per cent, ten units, etc. On this basis Mor- 

 gan and his associates have constructed a "map" of each chromo- 

 some of Drosophila indicating the positions of those genes which 



d 



FIG. 66. DIAGRAM SHOWING THE PROBABLE CHROMOSOMAL MECHANISM 

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

 the father, the other from the mother, are shown in synapsis (a, b, c) and 

 in the reduction division (rf). Homologous chromomeres (or allelomorphic 

 genes) are represented by the black and white circles at the same level. 

 In a and b the chromosomes are shown "crossing over"; in c they have 

 broken at the point of crossing and half of each chromosome is joined to 

 half of the other one; in d the "crossed over" chromosomes are separating 

 in the reduction division (from Morgan). 



