Genetics 317 



Inbreeding. — It has long been known among plant and animal 

 breeders that the constant crossing of individuals of similar parentage 

 often results in lowered vigor of the progeny. At first it was con- 

 sidered that these results were due to the inbreeding itself; from our 

 studies, however, it is clear that the actual cause of this lowered vigor 

 is the increasing homozygosity of the individuals. Thus many recessive 

 characters become homozygous and appear; previously they were 

 masked by the dominant factors. 



With inbreeding, it is possible to isolate certain characters, pro- 

 duce homozygous lines and then utilize these as breeding stock. By 

 this method of inbreeding, various strains of corn homozygous for such 

 characters as seed size, arrangement of kernels, etc., have become fixed 

 • and constant. Inbreeding, while in general deleterious, has important 

 values as a technique in the development of new and superior strains 

 of plants and animals. 



Outbreeding. — The exact opposite of inbreeding is outbreeding in 

 which two strains of dissimilar parentage are crossed. It has long 

 been known that the immediate offspring of such a cross are usually 

 more vigorous and more desirable than either of the parents. This 

 phenomenon is known as hybrid vigor. 



Again this particular phenomenon is explained by our knowledge 

 of the gene. In any such cross, dominants will mask most of the 

 weaker recessives found in either line. By a combination of inbreeding 

 and outbreeding, many of our better breeds of plants and animals have 

 been developed. 



In recent years, the most spectacular success has been achieved with 

 corn. By careful inbreeding, pure lines with desirable characters have 

 been developed. These lines are then crossed and the resulting progeny 

 are superior in the desired traits over either of the parental stocks. 

 These heterozygous individuals, of course, are not true breeding. Pure 

 breeding lines must be produced continuously to develop the seeds for 

 the hybrid corn. These hybrids are remarkably uniform as to physical 

 traits, no doubt due to the homozygosity of the parental strains. 



Human Genetics. — The principles of genetics apply equally well 

 to human beings; however, our knowledge of human genetics is much 

 more limited than it is of other organisms. This is due to the smaller 

 number of offspring, lack of control over matings, and the large number 

 of chromosomes. These facts make nearly insurmountable difficulties 

 in making any scientific observations. As a result, the only characters 



