GENETICS IN THE SERVICE OF MAN — GLASS 309 



Hemophilia cannot be eliminated; its lowest limit is set by mutation 

 at a gene frequency of 1 in 50,000. 



For eugenics, the induction of mutations by X-rays or chemicals is 

 too potent a tool, since the vast majority of all mutations produce 

 harmful consequences. Hardly 1 in 500 or a thousand improve its 

 possessor's fitness in the local enviromnent. Although most mutations 

 are recessive, and consequently do not show up for many generations, 

 until in the course of time they become homozygous, nevertheless it is 

 clearly undesirable to fill the human germplasm with an overload of 

 harmful genes. It is likely, according to several methods of estima- 

 tion, that even now everyone possesses at least one lethal gene that in a 

 double dose would destroy us. Other detrimental genes are probably 

 about three times as numerous as the lethal genes. These genes per- 

 sist in the population because they are hidden, each kind being pre- 

 vented from becoming more common because of natural selection. In 

 the elimination of detrimental genes by natural selection a lot of de- 

 sirable genes are also lost each time, for selection can act only on indi- 

 viduals ; that is to say, on entire combinations of genes. It cannot pick 

 out the solitary gene that is chief offender. The reassortment and re- 

 combination of genes that is the chief outcome of sexual reproduction 

 represent salvation here, since they render it unlikely that the same 

 unoffending genes will be lost each time a particular kind of detri- 

 mental gene is eliminated. 



The rapidity with which the frequency of a harmful gene can be 

 reduced by selection depends upon the percentage of carriers who man- 

 ifest the effects of that gene. A full dominant, for example, with a 

 fitness of only half normal, would in 20 generations, or about 550 years, 

 be reduced to one-millionth of its original frequency. It would almost 

 certainly before that time reach the level where it would be maintained 

 by fresh mutation. Consequently, any particular harmful dominant 

 trait is already, because of natural selection, very rare. It is of no 

 eugenic consequence to try to deal with them. Only special circum- 

 stances, like the failure to manifest themselves until late in life, bring 

 a few dominant traits, such as Huntington's chorea, a very unfortunate 

 nervous affliction that is a form of St. Vitus' Dance, within the scope 

 of eugenic measures. 



On the other hand, a recessive with a fitness of one-half normal 

 would in 20 generations be reduced by only 40 percent. What is more, 

 the amount of reduction declines in each successive generation, as the 

 proportion of heterozygous, concealed genes becomes larger and larger. 

 The frequency of a common recessive gene, like those for blue eyes or 

 red hair, might therefore be reduced by special measures very quickly; 

 but the reduction in frequency of a gene that is already rare is almost 

 imperceptible. Since natural selection has already been at work for 



