I40 INTRODUCTION TO SEXUAL PHYSIOLOGY 



Reduced Fertility as a Result of Inbreeding 



The fact that inbreeding may result in a reduced fertility 

 or even in complete sterility has already been discussed in 

 dealing with the fertilisation of the ovum. It was pointed out 

 that as a practical matter inbreeding often results in an apparent 

 loss of vigour, this being manifested in a variety of ways, one 

 of which is an increasing tendency towards sterility. At the 

 same time there is a growing body of evidence to show that 

 different degrees of productivity, down to complete sterility, 

 may be inherited as though they were Mendelian units or genes. 

 In the light of this conception the effects both of inbreeding 

 and cross-breeding would seem to acquire a new significance. 

 It has been shown that in some insects (Drosophila) complete 

 sterility may be a sex-linked character. In certalin inbred strains 

 of cattle and other domestic animals it has been suggested that a 

 factor for sterility was present. Thus in the famous Duchess 

 family of Shorthorn cattle, which eventually became extinct, 

 it is possible that a sterility gene formed part of the factor complex, 

 and that this, as a result of close and continuous inbreeding, 

 kept on reappearing with increased frequency until no more 

 fertile animals were produced. Moreover, it has been shown 

 that with Drosophila, as also with the rat and the guinea-pig, 

 a high degree of fertility can be maintained in successive genera- 

 tions of inbred animals by selecting from the more fertile 

 individuals. Further, there is direct cytological evidence that 

 in Drosophila sterility may be correlated with the absence of a 

 particular chromosome (Morgan). 



Conversely, the increased vigour and fertility brought about 

 by cross-breeding may be due to the establishment of a more 

 excellent factor-complex rather than to any mysterious stimula- 

 tion effect of the heterozygous condition. 



An example taken from the work of Emerson upon maize 

 breeding, in w^hich the same principles hold, will make this 

 clear. A dwarf race of maize which was almost completely sterile 

 was crossed with a tall plant so deficient in chlorophyll production 

 that it was unable to produce seed, although it had some functional 

 pollen. The cross-bred plants, on the other hand, were tall, 

 dark green, and produced well- developed ears. Here, normal 

 stature was contributed by one parent and proper chlorophyll 



