INHERITANCE 



309 



distributed together. Thus during synapsis homologous genes 

 (allelomorphs) often reciprocally cross-over from one synaptic 

 mate to the other, and so become separated from their former gene 

 associates in the same chromosome. When such occurs the ex- 

 changed genes are segregated independently of their former asso- 

 ciates in the same chromosome, and accordingly a greatly in- 

 creased flexibility is afforded the genetic mechanism. (Fig. 195.) 



Wbm 



Fig. 196. — Cy to-genetic map. Terminal portion of the X chromosome 

 from the salivary gland of Drosophila, with the locations of some of the 

 genes indicated, ac, achaete; br, broad; cv, cross- veinless; ec, echinus; fa, facet; 

 N, notch; pn, prune; rb, ruby; sc, scute; w, white; y, yellow. (After Painter.) 



In addition to its great importance in bringing about genetic 

 change, crossing-over affords the geneticist an opportunity to 

 determine the relative positions of different genes in a chromosome. 

 It has been found that the distance between two genes in a chromo- 

 some is, in general, proportional to the percentage of crossing-over 

 which occurs between these genes at synapsis — the longer the 

 distance, the more likely is crossing-over to occur. Accordingly, 

 in very extensive breeding experiments it is possible to construct 

 so-called chromosome maps by plotting the relative positions of 

 the various genes in the chromosomes. In the case of Drosophila 

 the genes for more than six hundred characters have already been 

 mapped. (Fig. 196.) 



4. Mutations 



But the possibilities of genetic change are not necessarily limited 

 by the typical chromosome groups or to crossing-over that usually 



