EFFECTS OF INBREEDING AND CROSSBREEDING. 35 



simply to the impossibility of such a formulation until more was 

 known of the principles of ordinary heredity. 



This same remark apphes to a number of experiments on inbreeding 

 of animals which were made before the rediscovery of Mendel's law. 

 Crampe (1883) and Ritzema-Bos (1894) inbred rats and obtained 

 marked degeneration in fertility and vitality. In Crampe's strain 

 there was also considerable decline in weight and many abnormalities 

 appeared, effects which Ritzema-Bos did not obtain. Weismann and 

 Von Guaita bred mice brother and sister for many generations and 

 noted a decline in fertility. Similar results were obtained by Fabre- 

 Domengue with pigeons. 



MENDELIAN HEREDITY AND THE PROBLEM OF INBREEDING. 



With the rediscovery of Mendel's law, the explanation of at least 

 one class of effects attributed to inbreeding at once became clear. 

 It had often been noted that in the human race certain rare abnor- 

 malities, of which albinism is a good example, most frequently 

 appeared among the progeny of consanguineous marriages. Good 

 reasons were soon found for believing that albinism in man is a simple 

 Mendelian recessive. A recessive factor can come into bodily expres- 

 sion only if it is received from both parents. A rare recessive trait 

 is transmitted by many more people than actually show it. Never- 

 theless, these transmitters are relatively uncommon in the total popu- 

 lation and the chances of union between them are not great. If, 

 however, a given individual can transmit the trait, the chances are by 

 no means small that a close relative will also have received the factor 

 from the common ancestor and be a transmitter. Thus it is clear 

 why consanguineous matings should frequently bring to light such 

 traits as albinism. We see that inbreeding can not cause abnormali- 

 ties of this kind to appear in a stock from which the genetic basis is 

 absent. It is, however, a system of mating which is likely to reveal 

 any abnormalities carried out of sight in the stock by recessive factors. 

 We can see how inbreeding could lead to the frequent appearance of 

 abnormalities in Crampe's stock of rats and yet fail to do so in the 

 stock of Ritzema-Bos. 



Castle and students (1906) inbred the fruit fly Drosopliila melano- 

 gaster for 59 generations of brother-sister mating. While much steril- 

 ity and low fertility appeared in the early generations, it was found 

 possible to maintain high fertility by selection of lines. The segrega- 

 tion of recessive factors for low fertihty was clearly indicated. These 

 results have been confirmed as far as the main features are concerned 

 in later experiments with the same fly by Moenckhaus, Hyde, and 

 Wentworth. 



Davenport (1908) called attention to the fact that in most known 

 cases the dominant character in a pair of Mendehan allelomorphs was 



