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ABSTRACTS
Inbreeding, as Lush has expressed it in a simple phrase, is the mating of closely
related individuals, but where to draw the line in using the terms ' inbreeding' and 'close
breeding ' is a matter of personal preference. The basic fact about inbreeding is that it
increases homozygosity.
Inbreeding notorioulsy has its risks. In our own species many close matings are
forbidden by one code or another, and marriages between cousins are often viewed with
disfavour. The chief reason for any calamitous results of inbreeding is that undesirable
recessive factors inherited by both the breeding animals from a common ancestor are
apt to be passed on by both to the offspring, and so come to light. Many recessive
factors are less desirable in their effects than their dominant allelomorphs, and with
dominance, either complete or partial, two doses of a factor do not produce twice the
effect of one dose. Inbreeding therefore tends to lower the average merit of a stock.
On the practical side, inbreeding has been exploited more in self-fertilizing plants
than in live-stock. In livestock a beginning has been made in this direction, notably
in pigs in America, where brother-sister matings have been made for some generations.
Some inbred strains have collapsed. Others have come to consist of quite good and very
uniform animals. And a cross between inbred strains has resulted in animals with
greater rates of gain.
Once again we have to think about dominance. When it comes to out-crossing
again after inbreeding, it is convenient if the desirable traits of the inbred stock are
dominant. Factors for high production do appear to be dominant more often than
not. This seems to be so in milk production. In wool characters we should be on the
look-out for some recessive factors determining pleasing features. For one reason, taking
hairiness, there are grounds for regarding the production of medulla as a vigorous thing
on the part of the follicle, and evidence from breeding results partly in this country, partly
elsewhere, suggests that in different pairs of factors a dominant partner may tend to
hairiness or a dominant partner may tend to pure wool. Therefore, if inbreeding for
pure wool be undertaken, this should be done on a number of different foundations.
As experimental material for studying both genetic and non-genetic factors, and for
disentangling one from the other, inbred stock is highly important. Selection is rendered
much less effective if we cannot allow for environmental conditions ; and how shall we
learn the effects of varying feeding if we cannot say what was the genetic endowment ?
Inbreeding is already under way in experimental sheep, and it is likely to play a
prominent part inwool research.
Pedigree.—Selection on pedigree is selection on knowledge of direct ancestors and of
collaterals. A poor-laying hen from a carefully bred strain of high average egg-laying
capacity may be preferable to an excellent layer from a low-laying strain. A high pro-
ducer appearing in mediocre stock is likely to be heterozygous for a number of interacting
genes, and therefore to be inferior in breeding worth to an animal backed on both sides
by superior immediate forbears, who consequently has a better chance to be homozygous
for desirable genes. A further reason for paying respect to pedigree in selection is its
usefulness when an animal has to be bred from before it is old enough to reveal what it
is which is precisely what has to be done with a heifer in a dairy herd.
On the opposite side of the account is this quite serious drawback in pedigree selec-
tion, introducing once more Lush's principle that selection in one direction or by one
criterion automatically reduces intensity of selection in another direction or by another
criterion. Whenever animals are chosen or rejected for their ancestors, so far reduced is
selection against their own defects, or for their own excellencies. On balance, in practical
animal husbandry, pedigree is rarely more important than individuality, while progeny,
going some way towards laying bare the genetic make-up of an animal, are more im-
portant than ancestors.
The progeny test.—The importance of judging parents from their offspring is not
the discovery of a geneticist. It long ante-dated any knowledge of Mendelian factor
but genetics endorses the progeny test, showing how it measures more accurately—save
in certain situations, simple or complicated—the breeding worth of an animal than does
any determination of the ascertainable merits of an individual or its ancestors. A bull,
we may be told, had four outstanding daughters. But what were the records of their
mothers ? And how many less notable daughters had he, and how did they compare
with their dams ?