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.. BBJih 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 
