NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
280 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [VI, III
looms up the Charybdis of the likelihood that the analysis will reveal so many
genetic factors interacting with each other as to make the synthesis of the problem
in its practical application almost an impossibility. An adequate reply to the
opening questions of this paper demands the discussion of this point.
To those who have time to reflect, the practical outcome of the work of the
plant-geneticist is little short of amazing. For many crops he has managed to
make two blades grow where none could grow before. We, animal geneticists,
have good cause for envy, and it is well that we should point out that this cause
lies not merely in the greater rate with which one generation of agricultural plants
succeeds another and the large populations which can be conveniently raised, but
also in the fact that the productivity of a plant is, as a rule, infinitely more easy
to measure and to assess than is the productivity of an animal.
The problem of an increase in the productivity of our live stock—and the
maintenance of that increase—is not so simple as, say, the problem of an increased
yield in maize. Let me refer to the work of Winter [3], which has been recently
the subject of some discussion. To quote 'Student' [4], Winter 'succeeded, by
continuous mass selection, in producing two races of maize one of which has more
than twice, and the other less than one-third, the normal oil-content'. ' The
movements of the means were, respectively, more than twelve and seven times the
" inherent " standard deviation.' ' Student' estimates that at least 100 to 300
factors would be needed and considers that the actual number runs into thousands.
Dealing with this point, Dr. Fisher [5] states that the experiment (which ran for
twenty-eight years) is a direct demonstration that selection has the exact effects
that selectionists have ascribed to it.
Can we do likewise if we breed our cattle upon the same principle of selection ?
There, is no doubt that selection can greatly increase the productivity of scrub
stock, but as the productivity of improved stock rises so does the rate of improve-
ment decrease. The progress becomes so slow relative to the passage of years
that we must now perforce accustom our ears to the astonishing slogan of certain
advisers of our farmers that it is useless to use a bull that is out of a high-producing
cow, a statement based on the fact that certain bulls of such breeding do not fulfil
the expectations of the owner. Nevertheless the occurrence of such animals does
fulfil the expectations of the geneticist.
It is curious that some of those of our practical advisers who most decry the
simple selection embodied in the ancient rule of ' mate the best to the best' do
themselves advise merely an elaboration of simple selection. So keen are these
people on their slogan of the ' progeny test' as the salvation of the British dairy
industry, that they go out of their way to decry past methods of selection—com-
bined or not with pedigree. They quite ignore the point that the progeny test is
merely the logical refinement of existing methods of selection.
THE INHERITANCE OF PRODUCTIVITY IN FARM LIVE STOCK.
283
REFERENCES
1. J. Mackintosh. The Progeny-Testing of Dairy Bulls. J. Brit. Dairy Farmers
Assoc., 1934, 46, 11-31.
2. T. E. Woodward and R. R. Graves. Some Results of Inbreeding Grade
Guernsey and Grade Holstein-Friesian Cattle. U. S. Dept. Agric. Tech.
Bull. No. 339, 1933.
3. F. L. Winter. Continuous Selection for Composition in Corn. J. Agric. Res.,
1929, 39, 451-75.
4. ' Student'. Evolution by Selection. Eugenics Review, 1933, 24, 293-6.
5. R. A. Fisher. Number of Mendelian Factors in Quantitative Inheritance.
Letter to Nature, 1933,131, 400.
THE INHERITANCE OF PRODUCTIVITY IN FARM
LIVE STOCK
III. BREEDING FOR EGG-PRODUCTION
A. W. GREENWOOD
(Institute of Animal Genetics, University of Edinburgh).
Discussing the future possibilities for increasing egg-production in poultry,
Jull [ 1 ] has stated that progress largely depends upon the poultry-breeder's
ability to control heredity. Further, he suggests that " heredity can be con-
trolled and directed best only when the knowledge of poultry-breeders develops
sufficiently to enable them to select breeding stock that will transmit to their
offspring the most desirable qualities. Selection is the keynote in the programme
of future development."
Selection without an exact knowledge of the mode of hereditary transmission
of desirable qualities, but with a belief that they are transmissible from parent
to offspring, has been the basis of all breeding practices in the past. It has been
responsible, together with improvements in husbandry, for the tremendous in-
crease in the production records of our modern domesticated breeds when com-
pared with the ancestral types from which they have sprung. Such advance
bears witness to the importance of heredity.
In attempting to assess the possible value of the science of genetics to the
cause of increased fecundity in poultry, we should begin by considering the results
that are now obtainable when selection along certain definite lines is practised.
An examination of the production figures at egg-laying contests testified to the
skill of the intelligent breeder in developing highly fecund strains of birds. His
methods of selection, however, when improved flocks are dealt with lead to negli-
gible progress over a fairly long period of time. This has been demonstrated by
Dunn [ 2 ] from an analysis of egg-laying contest figures in America. Over a
period of nine years he found that there was little material change in average
egg-production. This is very significant. Since only a small percentage of a
flock of birds is rigorously selected by certain standards for participation in egg-
laying trials, it follows that the results obtained represent practically the highest
individual performances possible with the methods of selection now used by
breeders. The production of the flock as a whole, however, usually falls far short
of these individual records, and perhaps the most pressing question to be faced
from the economic stand-point is that of devising a means of raising the figure
for average production, even in the most improved flocks.
The problem of improving egg-yield by breeding has been the incentive
to much scientific work, which has taken the form of analytical attempts to
define and measure the desirable qualities that a hen should possess in order
( 284 )
THE INHERITANCE OF PRODUCTIVITY IN FARM LIVE STOCK.
285
that maximum production may be attained. From observation and experiment
Goodale and Sanborn [ 3 ] conclude that there are, at least, five desirable qualities
which, if present in a bird, lead to high annual production. These are :—
1. Early sexual maturity.
2. High intensity of production.
3. No winter pause in production.
4. Non-broodiness.
5. High persistency of production.
Even a casual acquaintance with the hen reveals that for certain periods
in her reproductive life, egg-production ceases for varying lengths of time. The
most definite gap in egg-laying occurs during the time when the old plumage
is being discarded and the new feather-covering is growing in. Since this is a
characteristic annual phenomenon, typical of the vast majority of birds, it will
be seen how important its effect is upon the annual-production record of the pullet
— a figure which is arbitrarily fixed as the number of eggs laid in the 365-day
period from the time when she produces her first egg. Since the moult is very
variable, both in point of time and duration, it is obvious that the annual pro-
duction increases in proportion as the onset of the moult is retarded and approaches
the end of the 365-day period. At the Institute of Animal Genetics, Edinburgh,
we have several hens among our flock that, over a two-year period, have failed
to undergo the moult, and egg-production has not ceased during this time. This
is the ideal persistency.
High intensity means that a bird must be persuaded to lay as often as possible
within the prescribed period. " Winter clutch size ", i.e., the average number of
eggs laid in succession up to March 1, is used as an indicator of intensity of pro-
duction, because it has been found to be significantly correlated with annual
egg-production.
Most pullets are hatched so that they become sexually mature in the autumn.
It is rarely found, however, that continuous production occurs throughout the
winter months without the appearance of at least a few successive days on which
no eggs are laid. A break in the continuity of production of this magnitude
classifies the bird as exhibiting "winter pause".
Another physiological manifestation that affects the ultimate performance
of a hen is that of broodiness. During this phase of maternal expression, egg-
production ceases for a variable length of time, and is recommenced only after
the signs of broodiness have waned. This is a problem which affects particularly
that branch of the industry concerned with the heavy breeds of poultry ; broodi-
ness is a comparatively rare phenomenon in leghorns, for example,
286 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [VI, III
Recorded observations of many workers have shown that quick-maturing
pullets tend to make the highest annual records, and this emphasizes the necessity
for considering early sexual maturity when increased records are desired.
These five qualities are, perhaps, obvious ones to select for, if an increase
in egg-yield is desired ; it will be interesting to see what amount of improvement
may be obtained in a relatively unimproved flock by continuous selection along
these lines. Fortunately, the results of two separate experiments are available,
one carried out by Marble and Hall [ 4 ] and the other begun by Goodale and con-
tinued by Hays and Sanborn [5] (Table I).
TABLE I
Marble and Hall, 1930 . . . .
Leghorns
.
.
.
.
.
.
Average production increased from 118
to 196 eggs
.
.
.
.
.
Percentage increase 66.1 . . .
Goodale, Hays and Sanborn, 1934.
Rhode Island Reds.
Average production increased from 134
to 222 eggs.
Percentage increase 65. 6.
It is not suggested that such a rate of improvement could be obtained in
the best flocks by means of selection based on the five qualities outlined above,
because in all probability the higher averages produced now-a-days have resulted
from an unconscious use of these qualities as a basis of selection over a compara-
tively long period of years. Both experiments started with birds that did not
give a very high average egg-yield, and continuous selection for increased egg-
production was maintained over a period of twenty years. The results obtained
in both sets of experiments showed a remarkable similarity in the amount of
increase in the average yield, and this is the more striking when it is considered
that not only were different breeds used, but that the initial egg-production of the
two flocks was also different. From these experiments it is possible to deduce
that the methods employed are along the right lines but the question has still
to be faced as to what is the final degree of improvement in egg-yield that may
be obtained by these methods.
From an analysis of their flock records for the years 1928 to 1932, Hays and
Sanborn [ 5 ] show that whereas, over 50 per cent of their birds exhibited three
or four of the desirable characteristics, and 16 per cent all five of them (Table II),
this improvement was only about one-sixth of that theoretically anticipated,
for there is no known reason why all the birds cannot carry all five desirable charac-
teristics.
THE INHERITANCE OF PRODUCTIVITY IN FARM LIVE STOCK.
287
TABLE II
Relation of Number of Desirable Characters to Annual Production
Characters Number
0
1
2
3
4
5
Birds
Number
31
158
375
717
648
371
Birds
Per cent
of
total
1.3
6.9
16.3
31.2
28.2
16.1
Egg-
production.
Average
per bird
149
157
174
201
227
252
That these particular desirable characters are inherited has now been
established and a reasonable idea of their mode of inheritance has been gained.
Early sexual maturity apparently depends on two independent dominant
genes, either of which can produce, for example, the onset of egg-laying in the
Rhode Island Red breed before the hens are 215 days old. One of these genes
is sex-linked, the other autosomal (Eo and E'E').
For high intensity of production, also, two dominant genes are necessary
(II and I'I'). Hens carrying gene 1 have alone an average winter-clutch size
greater than 2, but less than 2.6. Gene 1' gives a clutch size of about 2.6. Both
genes together increase the clutch size up to 3 or more eggs throughout the winter
season. Both genes are autosomal and cumulative in effect.
For winter pause there seems to be only one dominant gene, M. Desirable
birds therefore carry the recessive form, mm.
Broodiness has been found to be inherited on a two-factor basis. Two
dominant complementary genes, A and C, are necessary to produce it. There
is no evidence of sex-linkage and non-broody birds are of three general classes :
(1) those lacking both A and C ; (2) those carrying A and lacking C ; and (3) those
carrying C and lacking A. The latter two classes make it evident why broodiness
is so difficult to eliminate completely from a flock, since females of these con-
stitutions, though not exhibiting broodiness themselves, will produce broody
offspring if mated to males carrying the complementary gene they lack.
The data for persistency indicate that it is inherited on the basis of a single
dominant autosomal gene, P.
L
288 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [VI, III
So far as experimental work has shown, the ideal hen should have a genetic
constitution with regard to the desirable characters leading to high egg-produc-
tion, as follows : Eo, E'E', II, I'I', mm aacc, PP.
It will readily be seen why progress by methods of selection is apt to be
extremely slow, for there are possible in a flock, 65,496 * genetically different
classes of birds with regard to the five characters involved, and only one of these
would give fixity of type. Again, since five of the eight genes, from which these
characters originate, are dominants, distinctions between their heterozygous
and homozygous forms are small, and at present apparently undetected, so that
if, as must occasionally occur, a hen homozygous for all the desirable qualities
should arise, the chances of its being recognized as such are extremely small. In
the case of the male, the difficulties are further increased by the fact that, though
for true-breeding stock he must carry the desired characters in the homozygous
form, yet he cannot in himself exhibit them.
It can well be imagined then that the chances of producing a strain with
constant genetical constitution is extremely remote when selection methods alone
are practised; and it is indeed doubtful whether much further progress would
be made in the best flocks, although considerable progress is possible in un-
improved stock.
If we refer back to the original statements of Jull that ' the possibility of
increasing egg-production depends on the poultry breeder's ability to control
heredity ', and ' that selection is the keynote in the programme of future develop-
ment ', a certain amount of confusion arises. Even with the knowledge of the
mode of inheritance of the necessary qualities leading to high annual egg-produc-
tion, it cannot be said that a policy of selection along these lines owes anything
to the fact that a genetical analysis has been made. In fact, it may be said that
ignorance of the genetical work should produce about the same amount of im-
provement in a flock, because the qualities selected for are just those which, if
inherited, would tend to increase production. So far, the service of genetics
has been to prove what has hitherto been a belief, viz., that certain desirable
qualities are inherited in a definite and orderly manner.
Unless we are to agree with Babcock and Clauson [ 6 ] that the real service
of genetics to animal breeding lies in promoting clarity of thought, the geneticist
must show how the knowledge gained by experiment can best be used to improve
existing breeding practices. In this particular field he must show the breeder
how he can control heredity.
* This figure is taken from Hays' publication but is obviously incorrect. The
number of genetical classes possible when dealing with eight genes, only one of which
is sex-linked, would be 4,374 when females alone are considered, and 6,561 if males
are considered: a total of 10,935.
THE INHERITANCE OF PRODUCTIVITY IN FARM LIVE STOCK.
289
The real limitation of selection lies in the fact that, owing to the apparent
similarity of production of the homozygous and heterozygous gene groupings,
it is impossible to forecast with accuracy the productive capacity of all offspring
from any mating. To illustrate this with regard to one particular quality, say
persistency, making use of genetical knowledge, let us suppose that only females
showing persistency were selected as breeders ; they would have either the genetic
constitution PP or Pp. There could be no evidence from the male himself as
to his genetic constitution, and so, from matings with persistent females, there
are three groups, each of two classes, into which his resultant progeny might
fall (Table III).
TABLE III
Parents Offspring
Group 1—
PP♂×PP♀—PP . . .
PP×Pp—1 PP : 1 Pp . . .
Group 2—
Pp♂ × PP♀—1 PP: 1 Pp . .
Pp × Pp—1 PP : 2 P P : 1 pp . .
Group 3—
pp♂ × PP♀—Pp . . . .
pp × Pp—1 Pp : 1 pp . . .
all persistent and true-breeding.
all persistent, but only half true-breeding.
all persistent, half true-breeding.
three persistent to one non-persistent, but
two in three of persistent not true-breeding.
all persistent, but not true-breeding.
one-half persistent, but not true-breeding.
The fact that females exist in the stock which exhibit persistency, and yet
cannot transmit this quality to all their offspring, makes it extremely unlikely
that, using this as a basis of selection, a strain homozygous for the character would
ever be developed, even though care was taken to breed only from birds all of
whose progeny were persistent. Such a procedure could only tend to limit the
selection of breeders to the first three classes, and since only the first is of fixed
constitution, a certain number of undesirable, i.e., recessive individuals would
almost inevitably occur among the succeeding progenies, which might belong
to any of the first four classes.
Genetical work has given us at least one clear conception, namely, that a
bird may exhibit a desirable character, but only be able to transmit it to part
of its offspring. The immediate application of this knowledge could only be
made if further experiment were carried out in sufficient detail to demonstrate
whether measurable differences between the hetero- and homo-zygous forms
exist. If this could be done, then the development of homozygous strains carrying
all the desirable characters, could be rapidly undertaken. Only by this means
can the control of heredity by breeders be readily visualized.
290 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [VI, III
On the other hand, should this information be unattainable, that is, supposing
measurable differences in egg-production cannot be found between hetero- and
homo-zygous forms, the geneticist still has a method of attack that would even-
tually lead to the analysis of the genetical constitution of any bird. In the first
place, it would be necessary to build up a stock containing all the homozygous
recessive forms of the characters to be investigated. This could be done by selec-
tion much more easily than by the reverse process. The value of selecting for
recessive characters is well illustrated by the results of selection against broodi¬
ness ; this has been almost completely eliminated from many breeds of fowl, and
even in breeds like the Rhode Island Red, where broodiness is of common occur-
rence, the value of selection against it is shown in the figures of Hays and Sanborn,
where the percentage of birds exhibiting this characteristic phenomenon dropped
from 86 in 1916 to 12 in 1932. Theoretically then, the building up of a strain
of birds homozygous for the recessive characters should not offer great difficulty.
Such birds would have the following genetical constitution with regard to the
characters selected against: eo, e'e', ii, i'i', pp, aa cc, (late maturity, low intensity,
low persistency of production and non-broodiness). The only character of the
group that would offer any difficulty would be winter pause, which depends on a
single dominant gene. The recessive homozygous form could not be expressed
in such a strain of birds because of the activity of the other genes selected to give
minimum production ; this hindrance could be overcome, however, by first of all
fixing this character in the stock from which the homozygous recessive strain
would subsequently be developed.
Having once obtained such a strain of birds, it would be possible to determine
accurately, and well within the productive life of the bird, the genetical constitu-
tion of any male or female, by means of matings with the recessive stock. The
desirable qualities, which combine to produce high egg-yield, are most probably
dependent on the same genes in the different varieties of the domestic fowl, so
that, it would not be necessary to obtain testing strains for all the different breeds
of fowl.
Before closing this discussion of the inheritance of egg-production, it is neces-
sary to emphasize the fact that the survey of the problem has been made from the
simplest possible genetical angle. We have dealt only with the behaviour of certain
inherited characters without suggesting factors that might intervene to complicate
breeding practice, and so we have omitted to stress the important rôle played
by environment and its effect on the expression of the gene. It is obvious, however,
that this type of reaction also falls within the province of the geneticist—to
determine how the expression of the genotype may be modified under varying
environmental agencies. It goes without saying, for instance, that nutrition plays
a most important part in deciding, whether or not, the full genetic capabilities