ABSTRACT
175
The essential fact to be stressed is this : Genetics puts livestock breeding in truer
perspective. Do not imagine that light has shone down from Heaven, or radiated from
an aloof human brain. Like all scientific advance, an understanding of the principles of
genetics has been won by detailed work. This has been carried out not on pigs and cows—
or only to a comparatively minute extent on such animals—but on more favourable
material, on peas and beans, flies, locusts, rabbits, but there is ample evidence that the
mechanism of heredity is the same in large domestic animals.
The geneticist has chosen organisms that breed quickly and in profusion. He has
picked characters for their convenience of study, like coat colour, rather than for their
economic importance, like milking-capacity. He has conducted planned breeding ex-
periments. That means he has made matings through which he addressed questions to
Nature, as far as possible one at a time, and so thought out as to call forth an answer of
Yes or No. These methods of work are very different from those which the breeder, with
a living to make from his stock, is compelled to follow. We should know precious little
about genetics if geneticists had eschewed mice as vermin and elected to work with useful
creatures like cattle.
We want to know what is genetically sound in livestock breeding. We have to
inquire how inborn differences come about between individuals of the same species, in-
cluding close relatives. We have to recognize that the genetic makeup of our animals
consists of separate paired units, and to understand how these units are sorted and passed
on from parents to offspring. Then we shall have a criterion for judging breeding schemes,
and be in a position to evaluate the methods that are used in selection for weighing breed-
ing worth.
' The newer knowledge of genetics ', says Gove Hambridge, ' is already reflected in
animal-breeding practices, and it will undoubtedly make for more certain and more rapid
progress, and fewer costly errors, on the part of those who will take the trouble to under-
stand it.' Let us face realities and give credit where credit is due. If we find ourselves
relieved from imagining that magic merit resides in pedigree, it is to Mendel that we are
indebted for enlightenment.
Mendelian inheritance.—The aim in animal genetics is the progress of the group, and
this building up, depending on finding out something which the practical breeder is not
well placed to discover for himself, may well be claimed to be harder than obliterating
insect pests, elusive though such creatures assuredly are.
Most characters of economic importance in livestock do not have a simple Mendelian
basis. The best known factor-pairs in livestock are those affecting colour. 'Single
factor ' is a more convenient term than ' single factor-pair ', but it will be understood
that an opposite number, a contrasted allelomorph, is taken for granted.
Lethal factors.—One member of a pair of factors may be very much what we do want,
and the other very much what we do not want. Thus are we prepared to encounter posi-
tively catastrophic factors, though these are not going to be held aloft in any scaremonger-
ing spirit. Geneticists have perhaps a certain propensity for reminding people of these
fatal factors, as is only natural, for the threat of a few clear-cut losses impresses the ima-
gination more than the hope of small percentage gains from large numbers of animals.
A disastrous condition in sheep found by the Edinburgh workers to be inherited in
simple Mendelian fashion is absence of mammæ. This feature is a simple recessive. A
man may be pardoned for taking for granted that his ewe hoggests possess teats, but if
it happen that this condition, in consequence probably of its introduction into the flock
through two successive rams, has become widespread, the lambs may be very awkwardly
placed. Wriedt found absence of ears in sheep to be recessive, with the ears of the hetero-
zygote short.
In the thoroughbred horse Bleeding is a simple recessive character, and has cut short
distinguished careers by death or by making it safer to retire horses from the turf. This
factor was found by Robertson to be widespread in aristocratic racing stock.
Worse still in its result to the individual is atresia coli, the complete absence of a sec-
tion of the intestine of the horse. Foals so deficient survive but a day or two of post-
natal life. This defect is another inherited as a simple recessive. The fatal gene was
introduced into Japan in one particular Percheron stallion, whose descendants in the
course of a few generations became very numerous.