38 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 



or female birds). The closer the inbreeding, the more rapid will be 

 this fixation of hereditary characteristics. 



The reason why inbreeding fixes characteristics is easy to under- 

 stand. As an illustration, consider a stock of horses in which blacks, 

 bays, and chestnuts are being produced. Recalling that the domi- 

 nant factor H is necessary in order that any black be present, that is, 

 that the horse be other than a chestnut, and that the dominant 

 factor B in the presence of H determines the bay pattern we see that 

 the three colors may be determined by the following combinations 

 of factors : 



Bays BBHH BbHH BBHh BbEh 



Blacks bbHH bbHh 



Chestnuts.. BBhh Bbhh bbhh 



Only the first types of bay and black breed true. Suppose, now, 

 that the horses are mated brother with sister. From time to time, 

 simply by chance, two animals will be mated which are homozygous 

 in one or both of the factors. For example, two bays of formula 

 BbHH may thus be mated. Neither of them transmits the factor 

 for chestnut (h) and it is evident that their descendants will never 

 produce chestnuts so long as they are bred only with each other. 

 Blacks, however, will frequently appear. These blacks, being homo- 

 zygous in both factors (bbHH), will breed true. If brother-sister 

 matings are made among the bays, occasionally matings of the type 

 BBHH by BBHH will be made by chance, in which the factor for 

 black is eliminated as well as that for chestnut. The descendants 

 of this mating will be a true-breeding race of bays. 



Since the unfixed matings are continually giving rise to fixed 

 matings, while the reverse can not occur, continued inbreeding is 

 enough to insure an automatic tendency toward fixation. This 

 fixation of type will tend to occur even if matings are simply made 

 at random, but will, of course, be much more rapid if accompanied 

 by selection, that is, if bays, for example, are mated with bays only 

 in the case above. In this simple case, selection alone will have 

 some tendency toward fixing the bay color, but the process will be 

 exceedingly slow. A single unfortunate mating may undo years of 

 work. It is hardly necessary to add that the case above is so simple 

 that any of the colors can be fixed by much more direct methods, 

 not necessarily involving close breeding. The case is merely used 

 to illustrate the point that inbreeding has an automatic tendency 

 toward making all the hereditary units homozygous, or, in other 

 words, resulting in animals which produce only one kind of repro- 

 ductive cell. 



A type is fixed most rapidly when matings are made between 

 selected brothers and sisters, but the tendency toward fixation is 

 present in almost any system of continued close breeding. There is 



