36 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 
In technical language, prepotency depends primarily on two things : 
The factors back of the characteristic must be dominant and each 
pair of factors must be homozygous. Other considerations, such as 
the number of factors involved and their linkage relations as well as 
the system of mating, play a part in determining whether the pre- 
potency of an individual dies with him or is handed on to his de- 
scendants. Most of these elements of prepotency are beyond con- 
trol and can simply be accepted thankfully when they appear. It is 
possible, however, to bring out such prepotency as is in a stock and 
preserve prepotency when it has appeared by breeding so as to fix the 
desired characteristics. Fixation means simply to make all the 
hereditary factors involved homozygous. 
VARIATION. 
Before discussing the methods of fixing characters it will be well 
to go briefly into the causes of variation. In the first place it must be 
recognized that a great deal of variation is not hereditary. Different 
characteristics are affected in very different degrees by outside con- 
ditions. Hereford cattle produce only white-faced, red calves, 
whether raised under the best of conditions or under the worst. 
These same conditions, on the other hand, may make all the difference 
between well-finished animals which win in the show ring and animals 
which would appear discreditable even to a scrub herd. The way to 
eliminate this kind of variation, of course, is to give all the stock uni- 
formly favorable conditions. 
Unfortunately there is in many cases variation which is neither 
hereditary nor due to controllable outside conditions. As already 
pointed out, there are hereditary differences in the average size of 
litter produced by different breeds of swine. There are also heredi- 
tary differences within the breeds, but their influence is so slight that 
Poland-China sows born in litters of 13 or more have been found to 
farrow less than one pig more on the average than sows born in litters 
of one, two, or three. Outside conditions undoubtedly play a part, 
but to a very large extent the size of litter produced by a sow seems 
to be beyond control. 
Even variations in coat color, at least with respect to pattern, are 
not always due to heredity. Spotted guinea pigs vary all the way 
from nearly solid black to solid white. In a mixed stock it is easy to 
show that the whiter parents have on the average the whiter offspring, 
and vice versa, but analysis of the figures in a stock raised by the 
Bureau of Animal Industry indicated that most of the variation was 
due to chance irregularities in the course of development, and was 
thus beyond control. 
The importance of such irregularities in development can be meas- 
ured roughly by the degree of asymmetry found. Thus the patterns 
