paratively large closed populations. This is perhaps of more value in
poultry populations, in which the number of females is limited, than it
is in populations of the larger domestic animals, but populations with up
to 150 females can now be dealt with.

We are, however, interested today not so much in the total amount
of inbreeding which has occurred but rather in the rate at which it has
increased. This is usually expressed in the terms of the increase in the
inbreeding coefficient in each generation. In the days before artificial
insemination, a reasonable average for the rates of inbreeding of closed
populations of domestic animals was about 0.4% per generation. But
there are some very large populations which have been inbred very
rapidly at periods in their history. | have already mentioned Shorthorns.
Rather more recently studies have been made on Red Danish cattle, which
have shown that over the period of thirty years from 1915 - 1945, the
rate of inbreeding had been as high as 1.6% per generation (2). This
had been due mostly to heavy concentration on the blood of two only
slightly related sires, both of whom in 1947 contributed 18% of the
genes in the whole breed. | might mention here that it has been possible
by following individual genes affecting blood groups and other poly-

m orphisms to see that the concept of the inbreeding coefficient really
w orks in practice.

Why should we be concerned about the rate of inbreeding? Is in-
breeding necessarily harmful? In the past too close a concentration on
the blood of individual animals has produced difficulties for two related
reasons. There are many cases in the literature in which a fashionable
bull has proved to be carrying a harmful recessive gene which has not
becomeobvious until his blood has become very widely spread throughout
the whole breed. For instance, one of the two influential sires in the
Red Danish breed of cattle mentioned above carried a gene causing para-
lysis in the hindquarters (3). In Great Britain the spread of a recessive
causing dropsy in cows in Ayrshires was similarly due to the spread of a
previously rare gene by concentration on one particular bull (4). But,
in addition to the emergence of obvious recessive defects, there is
strong evidence that inbreeding also has an effect on various aspects of
vigour and reproductive ability not clearly attributable to individual
genes though of course having a genetic background. In the classic ex-
periments on inbreeding in dairy cattle carried out at Beltsville for in-
stance, it proved impossible to carry the lines above a coefficient of in-
breeding of about 50% because the fertility was so much reduced. From
the sporadic occurrences of inbreeding in ordinary commercial cattle as
w ell as from deliberate inbreeding experiments, the evidence is conclu-
sive that animals which are the product of a father-daughter mating yield
about 8% less milk than do their non-inbred herd mates (5). This is a
reduction of 0.3% for each 1% increase in the inbreeding coefficient.

Geneticists often describe the rate of inbreeding as that which would

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