ABSTRACT
177
Mention of that very stimulating work by Landauer will itself show that in contem-
plating the genetics of morbidity we are doing our best to look on the bright side. With
lethal factors may conveniently be associated susceptibility to disease. Not too cheerful
a topic, you may indeed say, but if we speak of resistance to disease, that sounds more
optimistic. In breeding for resistance to rust and smut in cereals the plant geneticists
began gaining fame a long time ago. Such resistance may have a monofactorial basis, or
may depend upon a larger number of factors. As is well known, plant breeders have
been able to combine disease resistance of one variety with the more profitable produc-
tive traits of another variety.
In animals the arithmatical restrictions under which the breeder labours are alone
enough to make breeding for disease resistance difficult, but increasing attention is likely
to be paid to the genetic aspect of proneness to disease. The strongest grounds for think-
ing this are afforded by long sustained work on laboratory rodents, guinea-pigs, and mice.
Sewall Wright, by inbreeding guinea-pigs, brother to sister, for many generations ob-
tained lines which showed differences in susceptibility to tuberculosis. In mice, Little
and Dr Maud Slye have shown that genetic factors are highly potent in disposing to can-
cer or defending against it. To take extremes, in one strain of mice every animal would
be safe from cancer, in another strain cancer is stated to be the sole cause of death. The
site in the body of attack by cancer has also been shown to be determined by inherit-
ance.
In a recent survey of animal disease Gowen finds examples of inborn resistance to the
whole range of harmful agents, bacteria, fungi, protozoa, worm parasites, and chemical
poisons. ' So far as I am aware,' he says, ' no investigator who has adequately sought
inherited host differences in disease response has failed to find them.' In fowls he cites
bacillary white diarrhœa and fowl typhoid.
Hutt has made the arresting statement that 87 per cent of the mortality amongst
fowls in Cornell University egglaying tests is due to causes, like tumours and impaction
of the oviduct, not known to be preventable by any satisfactory measures, apart from
breeding. He adds that fortunately evidence is accumulating that by proper breeding
methods there can be established not only strains of fowls resistant to specific diseases,
but also others with marked resistance to disease in general.
In hot countries indigenous cattle stand up to diseases lurking there better than immi-
gres. A missionary farmer in the Tropics once told the author that on this account he
was sorry he had not been content with native cattle instead of grading up. Some endea-
vour has been made to combine native resistance to disease with the qualities of imported
strains, but in this respect the times are yet early. In New Zealand A. H. Ward is likely
to make a contribution to the genetics of liability to mastitis.
The genetics of susceptibility to tuberculosis and cancer in rodents calls for fearless
thinking—or, if you like, fearful—about our own species. Reference will be made to
this matter later under ' Nature and Nurture ', when evidence from human twins will be
quoted that gives specific support to the belief long held by medical men that a genetic
factor (or factors) is important in determining success or failure against T. B. germs.
Linkage.—We now come to a phenomenon of first-rate status in genetics which,
all the same, does not concern the livestock breeder very intimately, though it happens
that the subject of linkage can be introduced through the medium of our local sheep.
Several years ago a sheep breeder made a present to the Massey College of a ram with
enormous numbers of halo-hairs (kemp) all over his body, and he was horned. This
present has resulted in a distinct advance in our understanding of the inheritance of hairi-
ness, but he was no joy to the gentleman in whose flock he was born. Now, the breeder
told me that a kempy birthcoat and horns are apt to be found in association, and this has
proved true in the descendants of this ram.
When this horned kempy ram or his sons like himself were mated with ewes of less
kempy birthcoat, some nine-tenths of the very kempy ram lambs which resulted were
horned, and of the less kempy ram lambs, whose numbers were just about the same as
those of the kempy lambs, only about one-tenth were horned. The rams used in these
breeding experiments clearly were heterozygous for the dominant Mendelian factor for
very much kemp and for the dominant factor for horns, and these two factors have stayed
together in inheritance nine times out of ten, and parted company once in ten times.