1^2 GENETICS 



This relation becomes of particular significance when It 

 Is contrasted with another, to be taken up next. 



5. In some cases, as set forth on page 136, two diverse 

 defects in different chromosomes do not yield normals when 

 the two chromosomes are brought together in the same 

 individual. Instead they yield a defective condition that is 

 usually intermediate between the defects due to the chro- 

 mosomes taken separately; or they yield a condition that 

 approaches one of these defects more closely than the 

 other. Thus when the chromosome that produces eosin eye 

 is brought together with that which produces coral eye, 

 the two give, not the normal red eye, but an eosin-coral eye. 



This, it was suggested, is because the two recessive de- 

 fects must be at the same point in the chromosome; they 

 must affect corresponding genes of the two chromosomes, 

 two members of the same pair, such as e and e' In figure 35. 

 In consequence, when the two are together (as in figure 

 3S)i both genes of that pair are defective; no correspond- 

 ing normal gene is present, so that a normal individual Is 

 not produced. 



This situation again furnishes a test of the idea that 

 the genes are in linear order. For if the two defective genes 

 are indeed in the same gene pair (e and e', figure 35) then 

 the two must, according to the linear theory, be at the same 

 distance from any third gene, as A or T, and therefore they 

 must give the same exchange ratio with any third gene. 



This can of course be tested experimentally In such an 

 organism as Drosophila; It has been done In many cases. 

 In the example we have mentioned, eosin eye and coral eye 

 both give with yellow body the exchange ratio 1.5 per 

 cent. It Is therefore true that they are located in the same 

 part of the chromosome. The two also give the same ratios 

 when tested with any other gene : for example their ex- 

 change ratio with vermilion eye Is about 33 per cent. There 

 Is a whole series of eye colors in Drosophila that acts In the 



