SOME POSSIBLE BEARINGS OF GENETICS ON PATHOLOGY 5 



hybrid flies mate, then a quarter of the offspring will have vestig- 

 ial wings. It might seem then that the character had appeared 

 for the first time in the race, if one did not know its past. In re- 

 ality its gene may have been there for some time. Probably many 

 of the recessive defects and malformations that appear in the 

 human race — at least those due to hereditary factors — have had 

 representative genes in the germ-plasm for several generations be- 

 fore they have appeared on the surface. 



We do not know how widespread recessive genes are in the 

 human germ-plasm. The fact that defective individuals appear 

 in certain communities may be safely interpreted to mean that in- 

 dividuals bearing the same gene have at last come together. On 

 the other hand, the absence of such individuals from the commu- 

 nity, at larg _ e, may only mean that the chance of suitable combi- 

 nations is small, and does not mean necessarily that the gene in 

 question is confined to the community within which the defects 

 have been recorded. 



My illustration may give, however, an entirely erroneous idea 

 as to the chance of a recessive character contaminating the race. 

 If one can control the matings, so that outbreeding takes place 

 each time, the result would undoubtedly be like that in our dia- 

 gram ; but what chance is there for a recessive character, that is 

 neither beneficial nor injurious, if left to itself, to contaminate 

 widely the race with its gene? The answer is that for any one 

 defect there is hardly any chance at all. On the other hand, there 

 is always a possibilitv that a defect may become widespread de- 

 spite the chances against each in turn. If a recessive character is 

 selected against each time it appears on the surface, the chance is 

 extraordinarily small that the gene for such a character could 

 ever become widespread in a race. If the recessive character is 

 advantageous, its chance is somewhat better, but still the chance 

 that it may be lost is very great. 



Let us turn for a moment to the inheritance of a Mendelian 

 dominant character, and to simplify the situation let us first as- 

 sume that the character itself is neither advantageous nor disad- 

 vantageous. 



