754 



THE AMERICAN NATURALIST 



[Vol. L 



m equal numbers from matings of Nn X nn included in category 

 B, and neither can produce affected children in turn, unless 

 mated to a heterozygous normal or a recessive, we do not see 

 why the normals should produce affected children in half as 

 many cases as their eataractous sibs. If the chances for obtain- 

 ing such mates were the same the number of matings which pro- 

 duce affected children should be approximately the same. 4 



The second part of the quotation is correct only when both 

 normal and abnormal F 1 individuals have an equal chance to 

 mate with individuals who are either affected or carry the ab- 

 normality in a recessive condition. The chances of the two 

 classes mating with such individuals are probably not equal be- 

 cause individuals affected with cataract would have a harder 

 time to find a mate than their normal brothers or sisters and 

 there would be a greater tendency towards consanguineous mar- 

 riages and consequently a greater chance of mating with eata- 

 ractous individuals. The frequent intermarrying among affected 

 stocks is well known with other abnormalities. Hence normal per- 

 sons carrying the affectation in a heterozygous condition would 

 be more likely to marry into unrelated stocks with a far less pro- 

 portion of heterozygous individuals than would their affected 

 sibs. If this is true then the expectation of the number of mat- 

 ings of affected F x individuals giving affected children in turn 



* In answer to a letter sent to Dr. Danf orth asking about the above point 

 the following was received Avhich shows that we did not understand his 

 meaning correctly: 



In reply to your letter of September 29, I do not say in the paragraph 



many normal as eataractous individuals should produce affected offspring 

 but, on the contrary, that the normals, taken as a group, should produce 

 affected children "in half as many cases" (♦. e., at half as many births) 

 as do the eataractous. The families of numerous individuals in both 

 groups would be expected to contain no eataractous individuals at all, but 

 in those families (equal number for each group) where such children may 

 occur there should be in the long run half as many cases in families with 

 normal parents as in families with one eataractous parent. The remainder 



