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ABSTRACT
In cattle, in other parts of the world, a dozen fatal factors, sublethal or lethal, are
known.* In Scandinavian Friesians Wriedt and Mohr found two recessive factors, for
Hairless (extreme reduction of the hairy coat) and Amputated (gross deficiency of feet
and muzzle), to be widespread. The affected calves die about the time of birth. The
spread of these factors in the breed could be traced to the wide use of bulls descended, in
each of the two cases, from a single very famous sire, highly prized, who was demonstrated
from breeding records to have carried the calamitous gene, without in his own person
showing any sign of its presence in his hereditary make-up.
Several examples have thus been given of how a breed can become riddled with a
fatal recessive factor. Wriedt and Mohr show that if the pedigree of any Friesian in
Sweden be traced there is ' an overwhelming probability ' of meeting either a certain very
famous sire who carried the factor for Hairless or another very famous sire who carried the
factor for Amputated, and often both these bulls are encountered in the pedigree. There-
fore beware recessive lethals ! It is in the breed interest, and in the national interest,
that monstrosities should not be silently buried. Bacteria and non-genetic accidents of
development are not for one moment forgotten, but a lookout should be kept for lethal
factors ; and especially if more than one child of the same sire possess the same defect
suspicion is aroused. Lethals are nasty things not only because they destroy animals
valuable in themselves, but in shaking confidence in a breed or herd and in limiting the
number of individuals from which breeding animals can be chosen, therefore checking
progress by increasing the difficulty of maintaining the status of a herd. In slow-breed-
ing animals the proportion of all females born which must be kept in the breeding-flock
merely to maintain numbers is quite high, probably 1 in 2 for sheep in this country, and 2
in 5 for cows.
Wriedt and Mohr very justly take the lethal situation in their Friesian cattle very
seriously. They suggest that a bull should not be allowed to become a bull-father—that
is to say, that his sons should not be accepted as sires unless that bull has been tested by
mating to cows which had proved themselves to carry the lethal factor by producing a
monstrous calf, or by mating to a substantial number of his own daughters. The situa-
tion is complicated, obviously enough, because our stand-by, the back-cross, is out of the
question, for a recessive that passes out the day she is born fails to reach productive age.
The breeding tests proposed are manageable with mice in an endowed institution, but in
breeding cattle for a living they are well-nigh prohibitive.
Again, therefore, is emphasized how cumbersome is the progeny test, which we should
ever be on the alert to escape not by neglect, but by rising above it. In work on another
hereditary monstrosity in cattle Landauer has reported a most suggestive result. The
bull-dog calf in the Dexter is aborted some way short of full term. The condition is here-
ditary, but according to Crew subsidiary factors are involved in addition to what may be
regarded as a primary factor, which is recessive. Landauer thinks it likely that a sub-
sidiary factor which must be present in double dose to give a bull-dog calf can be detected
when present in single dose, in that it produces an increase in a certain kind of white blood
corpuscle.† Two Dexter bulls with no bull-dog calves to their discredit were found to
have normal numbers of the white corpuscles in their blood. If Landauer's suspicions be
confirmed, a quite rapid examination of blood smears erases the progeny test.
So far in New Zealand little has been heard of pathological and lethal factors, and we
will hope this is because we are favoured by their scarcity and not because they have
been kept out of sight, but Mr John M. Ranstead and Dr H. E. Annett have discovered
what are evidently two such simple recessive factors. One, in Shorthorn cattle, causes
absence of the lower jaw, so that the calves cannot feed. The other, in Tamworth pigs,
produces split ears, accompanied sometimes by crippled hind legs, and those piglings with
split ears whose mouths were examined had cleft palates. Affected animals may or may
not be dead when born.
* The distinction is rather a fine one, a factor which allows an animal to be born alive
and to survive for a distinctly abbreviated career, no longer than enough to utter one cry,
being sub-lethal. A factor causing death before birth is lethal.
† The eosinophilia. As these are produced in the marrow of the long bones, and as
the long bones of the skeleton are affected in the bull-dog calf, it is easy to believe that
Landauer may prove correct.