314 GENETICS 



are heterozygous is greater, the process of producing perma- 

 nent homozygous races having certain characteristics is 

 slower and more uncertain. It may require many genera- 

 tions of selective inbreeding before a permanent stock is 

 established. 



It is in this way that have been produced the various 

 "pure" or constant races or breeds of dogs, rabbits, fowls, 

 and cattle. In many such races there still remain a few gene 

 pairs that are heterozygotic. Thus, in a race in which most 

 individuals are AABBCCDD, etc., there might be a few 

 Individuals AABBCcDD. The recessive gene c will have no 

 effect, until it happens that two such individuals mate, 

 whereupon some of the offspring have the constitution 

 AABBccDD. The recessive character due to cc will then 

 unexpectedly appear, giving individuals that fail to show 

 some of the usual characteristics of the breed. Such indi- 

 viduals are not allowed to breed further. 



Thus It Is clear that by the production of new gene com- 

 binations In biparental reproduction, and by making these 

 lasting through vegetative reproduction or by inbreeding, 

 many new types or breeds, perhaps differing greatly from 

 the original parents, are produced. This is a very large 

 factor in bringing about the great variety observed In or- 

 ganisms. 



NOTES AND REFERENCES ON CHAPTER XII 



1. Page 302. E. M. East and H. K. Hayes (1914),^ Genetic Anal- 

 ysis of the Changes Produced by Selection in Experiments with 

 Tobacco. Amer. Naturalist, vol. 48, pp. 5—48. 



2. Page 304. Wm. Bateson (1909), Mendel's Principles of Hered- 

 ity. Plate VI, p. 294. 



3. Page 304. E. Baur (1924), JJntersuchungen iiber das PFesen, die 

 Entstehung und die Vererbung von Rassenunterschieden bei Antir- 

 rhinum ma jus. Bibliotheca Genetica, Bd. 4, 170 pages, 5 plates. 



4. Page 305. H. Nachtsheim (1922), Die Entstehung der Kanin- 

 chenrassen im Lichte ihrer Genetik. Zeitschrift fiir Tierziichtung und 

 Ziichtungsbiologie, Bd. 14, pp. 53-109. 



