CONSTITUTION OF THE CHROMOSOMES 151 



nant gene A corresponding to the defective gene a, while 

 chromosome i supplies a dominant gene K corresponding 

 to the defective gene k. 



Note now the relation of these facts to the exchange 

 ratios. If two chromosomes with defective genes (as a and 

 k in figure 35) yield normals, the defective genes are con- 

 ceived to be in different parts of the chromosomes. They 

 would therefore be at different distances from any third 

 gene, as T in figure 2S- -^^^ since the exchange ratios de- 



a e K T 



1 ♦OOO^OCXXXXXDOCOOOOCXXX^O 



2 ooco^ooc<x:>^ooc>ooooooooo 



A e' k T 



Figure 35. Diagram to illustrate certain relati«ns in crossing-over, 

 or the exchange of parts of the chromosomes. The oval bodies repre- 

 sent the genes. See text. 



pend on distances, they should give different exchange ra- 

 tios with that third gene. The gene k would give with T a 

 smaller ratio than would the gene a. In general, when two 

 recessive genes in different chromosomes yield dominants, 

 the two recessive genes should have diverse exchange ratios 

 with any third gene. 



Is this in fact the case? Experimentation shows that it is. 

 For example, in Drosophila, vermilion eye and eosin eye 

 mated together give normals. The yellow-vermilion ratio 

 is 34.5; the yellow-eosln ratio is 1.5. It is clear therefore 

 that vermilion and eosin are In diverse parts of the chro- 

 mosome. 



This turns out to be a general rule. When two recessive 

 genes In different chromosomes give when brought together 

 a dominant or normal individual, these two recessive genes 

 have diverse exchange ratios with any third gene, showing 

 that they are In different parts of the chromosomes. 



