248 GENETICS 



Bar gene, while the other is normal, the mean number of 

 facets is about 358. Finally, in either male or female, if no 

 Bar gene is present, the number of facets is about 800. 



Later it was discovered that this Bar gene is sometimes 

 modified in a different way, so as to cause a less marked 

 reduction in the number of facets. This modification is 

 known as infra-Bar. A female containing but one of these 

 infra-Bar genes has about 716 facets; if she contains two 

 she has about 348. 



Again, by an extraordinary process, almost or quite 

 unique in known genetics, in some cases two of the Bar 

 genes get united by crossing-over into one X-chromosome. 

 This condition is known as double Bar. When both of the 

 chromosomes carry this double Bar, the number of facets 

 is but about 25. If one carries double Bar, the other single 

 Bar, the number of facets is about 36. 



Thus the number of facets in the eye depends in many 

 ways on the number and conditions of the genes present. 

 This may be shown in the following diagram, representing 

 the conditions in the two X (or Y) chromosomes of any 

 individual, with the corresponding numbers of eye facets 

 present. In this diagram 



+ = the normal, unmodified X-chromosome 



Y = the Y-chromosome of the male 



B ^ the Bar gene 

 iB = the infra-Bar gene 



BB =. the double Bar condition (two Bar genes in one 

 chromosome) 



Chr. I : + iB B iB B iB B BB BB BB BB 

 Chr. 2 : + + + iB^T B B + iB B BB 

 Facets 800 716 358 348 98 73 65 45 42 36 25 



All these various conditions are inherited as unit-differ- 



