336 The Journal 
Starting from this observed fact, it is 
very easy to frame a hypothetical expla- 
nation of why inbreeding and pure- 
breeding produce prepotency. Most 
geneticists would probably agree on it. 
The explanation which fits the facts is 
this: that prepotency is due to the pres- 
ence of a number of homozygous factors 
for the valuable characters of the breed.?° 
Ordinarily these factors will have to be 
dominant, but it is not difficult to think 
of cases where prepotency, within a 
single herd, say, might be due to the 
possession of homozygous recessive 
factors. 
Inbreeding will make factors homo- 
zygous, if they were not so to start with. 
So will pure breeding, the mating of like 
with like, accompanied by selection, as 
it usually is. It is therefore easy for a 
Mendelist to see why a purebred animal 
is ordinarily prepotent over a scrub. 
Further, although the breeder is se- 
lecting for only a few characters, and 
rendering these homozygous by appro- 
priate mating and selection, he is auto- 
matically, at the same time, making 
other characters homozygous. Making 
a breed homozygous (1.e., prepotent) in 
one dominant character will tend to 
make it homozygous (1.e., prepotent) 
in all dominant characters. Thus the 
appearance-of “individual prepotency”’ 
will be increased. 
of Heredity 
I do not say that this is the explana- 
tion of prepotency, but it is at least an 
explanation which, so far as I can see, 
brings the observed facts in harmony 
with Mendelian results.?! 
Whatever be the origin of prepotency, 
from a genetic point of view, the experi- 
ence of breeders leaves little doubt as to 
the best course to follow, in the light of 
our present knowledge, if one wishes to 
secure that commercially valuable result 
known as prepotency. It may be ex- 
pected from the mating of the best ani- 
mals, selected to the same standard 
(7.e., pure-bred) or better still, related 
by blood; and by selection in each gen- 
eration of the animals that have the 
desired character in the highest degree..- 
The prepotency thus obtained is a 
measurable fact. It can be explained by 
a Mendelian hypothesis which makes it 
a matter of homozygous, dominant 
characters; but such an hypothesis is, in 
respect to the valuable traits of farm 
animals, still unproved and I do not 
think we are in a position to say whether 
or not it covers the whole ground. And 
until the geneticists have supplied the 
necessary proof, practical breeders will 
continue to talk about prepotent indi- 
viduals, when they refer to individuals 
that are prepotent in a certain number of 
commercially valuable characters. 
20 Since we are dealing with hypotheses, I add that this condition should be accompanied by a 
minimum number of necessary complementary factors and a maximum number of duplicate 
factors; and, as Wentworth suggests, linkage may also be involved. ' : . 
21 It is quite possible that the breeders are right in claiming that inherited vigor plays a part in 
prepotency. C ( L : 
some of the geneticists think impossible. 
If so, this would make a very near approach to that ‘‘individual prepotency’’ which 
Until proof is forthcoming, it would be well to keep an 
open mind and not say that there is no such thing as individual or general prepotency, even 
though many supposed cases of it are shown to be merely cases of character prepotency. 
