Inbreeding 373 



Apparently the effect of inbreeding in human beings is the 

 same as in other organisms. If in a human family there are 

 several recessive genes for serious defects and if they are nor- 

 mally not expressed because of dominant genes, brother-and- 

 sister or even first-cousin marriages will tend to produce chil- 

 dren that are homozygous for some of these serious recessive de- 

 fects. In those families, inbreeding will be highly undesirable 

 if not tragic. If, however, the recessives in a certain family 

 produce a more desirable result than their dominant alleles, in- 

 breeding will not only not be harmful but will also produce a 

 more beneficial result than outcrossing. Unfortunately, most 

 families probably possess some very harmful rather than only 

 desirable recessive genes so that in general inbreeding should 

 not be encouraged. 



One of the most extensive studies of the effect of inbreeding 

 in animals was carried on over twenty years ago by Wright. 

 About 34,000 animals were studied over a period of 15 years. 

 They include 25,000 animals in 23 separate lines, each of which 

 was descended from an original pair of guinea pigs by brother- 

 and-sister matings. They also include about 4000 animals in a 

 control stock in which inbreeding was very carefully avoided 

 and about 5000 animals in crosses among the various inbred lines. 

 The results of this inbreeding showed no obvious degeneration 

 but did indicate an average decline in vigor in all characteristics. 

 This decline was most marked in the frequency and size of litter. 

 The decline, also, was greater in the gains made after birth than 

 in the weight at birth and was also greater in the percentage of 

 animals raised than in the percentage born alive. There was a 

 decline in fertility and in resistance to tuberculosis, but there 

 was no effect on the sex ratio. Dr. Wright concluded that loss in 

 vigor, especially in fertility, is a more or less direct consequence 

 of close inbreeding. 



Other interesting results also came to light in this study. The 

 segregation of new color types and patterns appeared, and dif- 

 ferent families and subfamilies became true-breeding for dif- 

 ferent colors and patterns. Some subfamilies bred true for a 

 tendency toward a reappearance of an ancestral fourth toe on 

 the hind feet, but relatively few monstrosities appeared. In 

 some families, eyeless individuals or those with rudimentary legs 

 were found, but there was no connection between the tendency 



