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



In technical language, prepotency depends primarily on two th 

 The factors back of the characteristic must be dominant and 

 pair of factors must be homozygous. Other considerations, su 

 the number of factors involved and their linkage relations as w 

 the system of mating, play a part in determining whether the 

 potency of an individual dies with him or is handed on to hi 

 scendants. Most of these elements of prepotency are beyond 

 trol and can simply be accepted thankfully when they appear, 

 possible, however, to bring out such prepotency as is in a stocl 

 preserve prepotency when it has appeared by breeding so as to fi 

 desired characteristics. Fixation means simply to make al 

 hereditary factors involved homozygous. 



VARIATION. 



Before discussing the methods of fixing characters it will be 

 to go briefly into the causes of variation. In the first place it mi 

 recognized that a great deal of variation is not hereditary. Difi 

 characteristics are affected in very different degrees by outside 

 ditions. Hereford cattle produce only white-faced, red a 

 whether raised under the best of conditions or under the v 

 These same conditions, on the other hand, may make all the diffe 

 between well-finished animals which win in the show ring and an 

 which would appear discreditable even to a scrub herd. The w 

 eliminate this kind of variation, of course, is to give all the stocl 

 formly favorable conditions. 



Unfortunately there is in many cases variation which is ne 

 hereditary nor due to ' controllable outside conditions. As ah 

 pointed out, there are hereditary differences in the average si 

 litter produced by different breeds of swine. There are also h( 

 tary differences within the breeds, but their influence is so slight 

 Poland-China sows born in litters of 13 or more have been fou: 

 farrow less than one pig more on the average than sows born in 1 

 of one, two, or three. Outside conditions undoubtedly play a 

 but to a very large extent the size of litter produced by a sow s 

 to be beyond control. 



Even variations in coat color, at least with respect to patten 

 not always due to heredity. Spotted guinea pigs vary all the 

 from nearly solid black to solid white. In a mixed stock it is ea 

 show that the whiter parents have on the average the whiter offsj 

 and vice versa, but analysis of the figures in a stock raised b 

 Bureau of Animal Industry indicated that most of the variatior 

 due to chance irregularities in the course of development, anc 

 thus beyond control. 



The importance of such irregularities in development can be i 

 ured roughly by the degree of asymmetry found. Thus the pat 



