ABSTRACTS
Genetics and livestock production. F. W. DRY (1938). Department of
Scientific & Industrial Research, New Zealand, Bulletin No. 64—(contd)
S ELECTION.—The breeder decides which animals shall mate with which. He selects
parents and arranges matings. He does this subject to certain restrictions. On the
male side there -is the difficulty of finding and buying his ideal of a sire. On the female
side he is compelled by the sheer arithmetic of breeding to keep a considerable proportion
of all females born in his flock and herd merely in order to maintain numbers. Selec-
tion, let us be aware, is a sorting, not a creative process. In the light of the concept of
hereditary units we find, says Crew, that selection is not a creative process, but is merely
one of sorting out from a mixture of hereditable characters—or factors—already present
in a stock. To quote Wantworth, breeders have tended to believe that selection is omni-
potent—that is, that any given type of trait can be modified in any direction. Closer
study by geneticists has revealed that selection has certain limitations. The breeder
has assumed that selection in a given direction can stimulate variation in that direction.
The geneticist has shown, instead, that selection simply sorts out variations that happen
to be present or to occur occasionally.
We look at a magnificient bull, but we cannot foretell his achievements as a sire.
We do not know how his daughters will milk, and for all we can see he may carry a catas-
trophic lethal factor.
Factors may be present in an animal, but for some reason be unable to come to
expression, or to readily discernible expression. In the cow a three-teated condition
found by Heizer in America appears to be a simple recessive. He records a bull in an
affected herd showing defective rudimentary teats on one side, so that by looking just a
little more closely than is necessary in the female bovine you can probably accomplish
something by selecting in bulls against this undesirable milk feature. Believe it or not,
I once knew a townsman staying on a farm to ask this innocent question : ' Do you
get any milk from the bull ?'
The same visible characterization, as we have seen, may be based upon different
genetic factors, and so animals that appear when mated to be evenly yoked together
may prove genetically very ill-matched. The aim in selection all the time must be to
choose not merely for visible type (phonotype), but for genetic type (genotype).
Nature and nurture.—The effects of the inborn and the external are so intertwined as
to offer first-class sport to a mind with a taste for sorting tangled skeins.
In dairy cattle, as breeders are fully aware, feeding and frequent milking regardless
of offsetting disadvantages may produce a yield flattering to genetic make-up. Shulgin,
in Russia, acquired extensive figures on the effect of age upon milk yield, which showed
just what breeders would be well prepared to be told. Heifers bred for the first time
before one year and four months gave a lower average lactation yield than heifers bred at
one year and four months or at one year and ten months. At the fifth lactation, however,
the differences were equalized and all groups gave similar values. The same worker
paid attention also to size, and concluded that in order to obtain the best results heifers
should not be bred before attaining the age. of physiological maturity and not before
reaching four-fifths of adult size. No attempt is being made here to discuss at all com-
prehensively non-genetic factors affecting milk and butterfat yield. The above findings
are cited simply by way of illustration. The difficulty of allowing for non-genetic factors
severely limits the effectiveness of selection.
In Romney sheep the degree of non-kemp hairiness is easily affected by non-genetic
factors in some animals, especially it would appear, in sheep with an inborn propensity
for slight hairiness. To cite only one experiment of a series giving similar results, in
which the details of the experiments differed, several animals hairy-tipped on the side
were shorn to the skin on one side while on the other side a covering of a couple of inches
was left. The wool grown during the next two months or so was less hairy on the side
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