Punnett: Eliminating Feeblemindedness 



465 



would be immediately established, and 

 would persist as such after a further 

 generation provided that no fresh dis- 

 turbance arose. If the disturbing cause 

 were small, the new position would 

 be so close to the old that, for practical 

 purposes, the population might be 

 regarded over a few generations as 

 unchanged. 



The seemingly large proportion of 

 heterozygotes entailed in a stable popu- 

 lation by even a small proportion of 

 recessives helps us to understand a 

 feature of certain collections of human 

 pedigrees which at one time appeared 

 puzzling. Albinism, for example, be- 

 haves on the whole as a recessive. 

 Nevertheless, albinos appear among the 

 offspring in an appreciable proportion 

 of matings where either one or both 

 parents are normal, and where no 

 consanguinity can be detected. The 

 same is true of feeblemindedness. This 

 becomes less difficult to understand 

 when we realize that the heterozygotes 

 are bound greatly to outnumber the 

 recessives whenever these form a small 

 proportion of a stable population. 



There is a further point which merits 

 brief consideration. At what rate can 

 we hope to free a population of an 

 undesirable recessive character by iso- 

 lating, or by otherwise sterilizing those 

 individuals which exhibit the character? 

 In a recently published book^ I have 

 been able through the kindness of Mr. 

 Norton, of Trinity College, Cambridge, 

 to give a table showing the rate at 

 which the constitution of a population 

 changes when either the dominants or 

 the recessives are subjected to selective 

 elimination of known value. In this 

 table, however, the most intense form 

 of selection dealt with is the elimination 

 of 50%, at each generation, of the 

 dominants or recessives. Though it is 

 unlikely that legislative measures, unless 

 very stringent, would succeed in getting 

 rid of a higher proportion of feeble- 

 minded without their leaving offspring, 

 it may yet be of interest to enquire how 

 rapid the rate of elimination would be 

 if all feebleminded were henceforth 

 eliminntpd completely, from, the breed- 



ing stock of the population. I am 

 indebted to my friend G. H. Hardy for 

 kindly working out for me the brief 

 table appended. Assuming that all 

 recessives are eliminated, their propor- 

 tion in the population passes 



From 1 in 100 to 1 in 1,000 in 22 generations. 

 From 1 in 1,000 to 1 in 10,000 in 68 generations. 

 From 1 in 10,000 to 1 in 100,000 in 216 



generations. 

 From 1 in 100,000 to 1 in 1,000,000 in 684 



generations. 



If therefore the proportion of feeble- 

 minded in the United States is 3 per 

 1,000 today it would require something 

 over 250 generations, or about 8,000 

 years, before the proportion was re- 

 duced to 1 in 100,000, and nearly four 

 times this length of time before the 

 feebleminded were as few as 1 in a 

 million. The prospect of the success of 

 the segregation method is not hopeful. 

 Though it may be sure it is very, very 

 slow. For the rarer the recessive be- 

 comes the more frequently, relatively, is 

 it produced through the mating of two 

 heterozygotes. Clearly if that most 

 desirable goal of a world rid of the 

 feebleminded is to be reached in a 

 reasonable time some method other than 

 that of the elimination of the feeble- 

 minded themselves must eventually 

 be found. The great strength of this 

 defect in the population lies in its 

 heterozygotic reserves; if the campaign 

 against it is to meet with success it is 

 at these that it must be directed. 

 This is so clearly perceived by Dr. East 

 that we cannot do better in concluding 

 than by quoting the closing sentences 

 of his paper. 



"We have assumed that a normal 

 mentality is completely dominant over 

 a defective one. Is this true? Com- 

 plete dominance is rare among those 

 characters commonly studied by animal 

 and plant geneticists. Is it not likely 

 that the Binet-Simon or other proper 

 tests would show that carriers of mental 

 defects exhibit a' lower mentality than 

 pure normals? Would it not be wise 

 to start some investigations along this 

 line?" 



-Mimicry in Butterflies, Cambridge, 1915, pp. 154-156. 



