SCOPE OF GENETICS 17 



have it ; and if either parent has one dose of 

 the factor and the other has none, then on 

 an average half the family will have it, and 

 half be without it. 



To know whether the parent possesses 

 the factor or not may be difficult for reasons 

 which will presently appear, but often it is 

 quite easy and can be told at once, for there 

 are many factors which cannot be present in 

 the individual without manifesting their pre- 

 sence. I may illustrate the descent of such 

 a factor by the case of a family possessing 

 a peculiar form of night-blindness. The 

 affected individuals marrying with those 

 unaffected have a mixture of affected 

 and unaffected children, but their unaffected 

 children not having the responsible ingredi- 

 ent cannot pass it on^ 



• ITie investigation of this remarkable family vins made 

 originally by Cunier. The facts have been reexamined and the 

 pedigiee niuch extended by Nettleship. The numerical results 

 are somewhat irregular, but it is especially interesting as being 



B. 2 



