ECONOMICAL
Since heredity lies at the basis of the breeder's work, it is evident that
any contribution to a more exact knowledge of this subject must prove of
service to him, and there is no doubt that he will be able to profit by
Mendelian knowledge in the conduct of his operations. Indeed, as we shall
see later, these ideas have already led to striking results in the raising
of new and more profitable varieties. In the first place, heredity is a
question of individuals. Identity of appearance is no sure guide to
reproductive qualities. Two individuals similarly bred and
indistinguishable in outward form may nevertheless behave entirely
differently when bred from. Take, for instance, the family of sweet peas
shown on Plate IV. The F_2 generation here consists of seven distinct
types, three sorts of purples, three sorts of reds, and whites. Let us
suppose that our object is to obtain a true breeding strain of the pale
purple picotee form. Now from the proportions in which they come we know
that the dilute colour is due to the absence of the factor which
intensifies the colour. Consequently the picotee cannot throw the {154} two
deeper shades of red or purple. But it may be heterozygous for the purpling
factor when it will throw the dilute red (tinged white), or it may be
heterozygous for either or both of the two colour factors (cf. p. 44), in
which case it will throw whites. Of the picotees which come in such a
family, therefore, some will give picotees, tinged whites, and whites,
others will give picotees and tinged whites only, others will give picotees
and whites only, while others, again, and these the least numerous, will
give nothing but picotees. The new variety is already fixed in a certain
definite proportion of the plants; in this particular instance in 1 out of
every 27. All that remains to be done is to pick out these plants. Since
all the picotees look alike, whatever their breeding capacity, the only way
to do this is to save the seed from a number of such plants _individually_,
and to raise a further generation. Some of them will be found to breed
true. The variety is then established, and may at once be put on the market
with full confidence that it will hereafter throw none of the other forms.
The all-important thing is to save and sow the seed of separate individuals
separately. However alike they look, the seed from different individuals
must on no account be mixed. Provided that due care is taken in this
respect no long and tedious process of selection is required for the
fixation of any given variety. Every possible variety arising from a cross
appears in the F_2 generation if only a sufficient {155} number is raised,
and of all these different varieties a certain proportion of each is
already fixed. Heredity is a question of individuals, and the recognition
of this will save the breeder much labour, and enable him to fix his
varieties in the shortest possible time.
Such cases as these of the sweet pea throw a fresh light upon another of
the breeder's conceptions, that of purity of type. Hitherto the criterion
of a "pure-bred" thing, whether plant or animal, has been its pedigree, and
the individual was regarded as more or less pure bred for a given quality
according as it could show a longer or shorter list of ancestors possessing
this quality. To-day we realise that this is not essential. The pure-bred
picotee appears in our F_2 family though its parent was a purple bicolor,
and its remoter ancestors whites for generations. So also from the cross
between pure strains of black and albino rabbits we may obtain in the F_2
generation animals of the wild agouti colour which breed as true to type as
the pure wild rabbit of irreproachable pedigree. The true test of the pure
breeding thing lies not in its ancestry but in the nature of the gametes
which have gone to its making. Whenever two similarly constituted gametes
unite, whatever the nature of the parents from which they arose, the
resulting individual is homozygous in all respects and must consequently
breed true. In deciding questions of purity it is to the gamete, and not to
ancestry, that our appeal must henceforth be made. {156}
Improvement is after all the keynote to the breeder's operations. He is
aiming at the production of a strain which shall combine the greatest
number of desirable properties with the least number of undesirable ones.
This good quality he must take from one strain, that from another, and that
again from a third, while at the same time avoiding all the poor qualities
that these different strains possess. It is evident that the Mendelian
conception of characters based upon definite factors which are transmitted
on a definite scheme must prove of the greatest service to him. For once
these factors have been determined, their distribution is brought under
control, and they can be associated together or dissociated at the
breeder's will. The chief labour involved is that necessary for the
determination of the factors upon which the various characters depend. For
it often happens that what appears to be a simple character turns out when
analysed to depend upon the simultaneous presence of several distinct
factors. Thus the Malay fowl breeds true to the walnut comb, as does also
the Leghorn to the single comb, and when pure strains are crossed all the
offspring have walnut combs. At first sight it would be not unnatural to
regard the difference as dependent upon the presence or absence of a single
factor. Yet, as we have already seen, two other types of comb, the pea and
the rose, make their appearance in the F_2 generation. Analysis shows that
the difference between the walnut {157} and the single is a difference of
two factors, and it is not until this has been determined that we can
proceed with certainty to transfer the walnut character to a single-combed
breed. Moreover, in his process of analysis the breeder must be prepared to
encounter the various phenomena that we have described under the headings
of interaction of factors, coupling and repulsion, and the recognition of
these phenomena will naturally influence his procedure. Or again, his
experiments may show him that one of the characters he wants, like the blue
of the Andalusian fowl, is dependent upon the heterozygous nature of the
individual which exhibits it, and if such is the case he will be wise to
refrain from any futile attempt at fixing it. If it is essential it must be
built up again in each generation, and he will recognise that the most
economical way of doing this is to cross the two pure strains so that all
the offspring may possess the desired character. The labour of analysis is
often an intricate and tedious business. But once done it is done once for
all. As soon as the various factors are determined, upon which the various
characters of the individual depend, as soon as the material to be made use
of has been properly analysed, the production and fixation of the required
combinations becomes a matter of simple detail.
An excellent example of the practical application of Mendelian principles
is afforded by the experiments which Professor Biffen has recently carried
out in Cambridge. {158} Taken as a whole English wheats compare favourably
with foreign ones in respect of their cropping power. On the other hand,
they have two serious defects. They are liable to suffer from the attacks
of the fungus which causes rust, and they do not bake into a good loaf.
This last property depends upon the amount of gluten present, and it is the
greater proportion of this which gives to the "hard" foreign wheat its
quality of causing the loaf to rise well when baked. For some time it was
held that "hard" wheat with a high glutinous content could not be grown in
the English climate, and undoubtedly most of the hard varieties imported
for trial deteriorated greatly in a very short time. Professor Biffen
managed to obtain a hard wheat which kept its qualities when grown in
England. But in spite of the superior quality of its grain from the baker's
point of view its cropping capacity was too low for it to be grown
profitably in competition with English wheats. Like the latter, it was also
subject to rust. Among the many varieties which Professor Biffen collected
and grew for observation he managed to find one which was completely immune
to the attacks of the rust fungus, though in other respects it had no
desirable quality to recommend it. Now as the result of an elaborate series
of investigations he was able to show that the qualities of heavy cropping
capacity, "hardness" of grain, and immunity to rust can all be expressed in
terms of Mendelian factors. Having once analysed his material {159} the
rest was comparatively simple, and in a few years he has been able to build
up a strain of wheat which combines the cropping capacity of the best
English varieties with the hardness of the foreign kinds, and at the same
time is completely immune to rust. This wheat has already been shown to
keep its qualities unchanged for several years, and there is little doubt
that when it comes to be grown in quantity it will exert an appreciable
influence on wheat-growing in Great Britain.
[Illustration: FIG. 30.
Curves to illustrate the influence of selection.]
It may be objected that it is often with small differences rather than with
the larger and more striking ones that the breeder is mainly concerned. It
does not matter much to him whether the colour of a pea flower is purple or
pink or white. But it does matter whether the plant bears rather larger
seeds than usual, or rather more of them. Even a small difference when
multiplied by the {160} size of the crop will effect a considerable
difference in the profit. It is the general experience of seedsmen and
others that differences of this nature are often capable of being developed
up to a certain point by a process of careful selection each generation. At
first sight this appears to be something very like the gradual accumulation
of minute variations through the continuous application of a selective
process. Some recent experiments by Professor Johannsen of Copenhagen set
the matter in a different light. One of his investigations deals with the
inheritance of the weight of beans, but as an account of these experiments
would involve us in the consideration of a large amount of detail we may
take a simple imaginary case to illustrate the nature of the conclusions at
which he arrived. If we weigh a number of seeds collected from a patch of
plants such as Johannsen's beans we should find that they varied
considerably in size. The majority would probably not diverge very greatly
from the general average, and as we approached the high or low extreme we
should find a constantly decreasing number of individuals with these
weights. Let us suppose that the weight of our seed varied between 4 and 20
grains, that the greatest number of seeds were of the mean weight, viz. 12
grains, and that as we passed to either extreme at 4 and 20 the number
became regularly less. The weight relation of such a collection of seeds
can be expressed by the accompanying curve (Fig. 30). Now if we select for
{161} sowing only that seed which weighs over 12 grains, we shall find that
in the next generation the average weight of the seed is raised and the
curve becomes somewhat shifted to the right as in the dotted line of Fig.
30. By continually selecting we can shift our curve a little more to the
right, _i.e._ we can increase the average weight of the seeds until at last
we come to a limit beyond which further selection has no effect. This
phenomenon has been long known, and it was customary to regard these
variations as of a continuous nature, _i.e._ as all chance fluctuations in
a homogeneous mass, and the effect of selection was supposed to afford
evidence that small continuous variations could be increased by this
process. But Johannsen's results point to another interpretation. Instead
of our material being homogeneous it is probably a mixture of several
strains each with its own average weight about {162} which the varying
conditions of the environment cause it to fluctuate. Each of these strains
is termed a PURE LINE. If we imagine that there are three such pure lines
in our imaginary case, with average weights 10, 12, 14 grains respectively,
and if the range of fluctuation of each of these pure lines is 12 grains,
then our curve must be represented as made up of the three components
A fluctuating between 4 and 16 with a mean of 10
B " " 6 " 18 " " 12
C " " 8 " 20 " " 14
[Illustration: FIG. 31.
Curves to illustrate the conception of pure lines in a population.]
as is shown in Fig. 31. A seed that weighs 12 grains may belong to any of
these three strains. It may be an average seed of B, or a rather large seed
of A, or a rather small seed of C. If it belongs to B its offspring will
average 12 grains, if to A they will average 10 grains, and if to C they
will average 14 grains. Seeds of similar weight may give a different result
because they happen to be fluctuations of different pure lines. But within
the pure line any seed, large or small, produces the average result for
that line. Thus a seed of line C which weighs 20 grains will give
practically the same result as one that weighs 10 grains.
On this view we can understand why selection of the largest seed raises the
average weight in the next generation. We are picking out more of C and
less of A and B, and as this process is repeated the proportion of C
gradually increases and we get the appearance of selection {163} acting on
a continuously varying homogeneous material and producing a permanent
effect. This is because the interval between the average weight of the
different pure lines is small compared with the environmental fluctuations.
None the less it is there, and the secret of separating and fixing any of
these pure lines is again to breed from the individual separately. As soon
as the pure line is separated further selection becomes superfluous.
Since the publication of Darwin's famous work upon the effects of cross and
self fertilisation, it has been generally accepted that the effect of a
cross is commonly, though not always, to introduce fresh vigour into the
offspring, though why this should be so we are quite at a loss to explain.
Continued close inbreeding, on the contrary, eventually leads to
deterioration, though, as in many self-fertilised plants, a considerable
number of generations may elapse before it shows itself in any marked
degree. The fine quality of many of the seedsman's choice varieties of
vegetables probably depends upon the fact that they had resulted from a
cross but a few generations back, and it is possible that they often oust
the older kinds not because they started as something intrinsically better,
but because the latter had gradually deteriorated through continuous
self-fertilisation. Most breeders are fully alive to the beneficial results
of a cross so far as vigour is concerned, but they often hesitate to embark
upon it owing to what was held {164} to be the inevitably lengthy and
laborious business of recovering the original variety and refixing it, even
if in the process it was not altogether lost. That danger Mendelism has
removed, and we now know that by working on these lines it is possible in
three or four generations to recover the original variety in a fixed state
with all the superadded vigour that follows from a cross.
Nor is the problem one that concerns self-fertilised plants only. Plants
that are reproduced asexually often appear to deteriorate after a few
generations unless a sexual generation is introduced. New varieties of
potato, for example, are frequently put upon the market, and their
excellent qualities give them a considerable vogue. Much is expected of
them, but time after time they deteriorate in a disappointing way and are
lost to sight. It is not improbable that we are here concerned with a case
in which the plants lose their vigour after a few asexual generations of
reproduction from tubers, and can only recover it with the stimulus that
results from the interpolation of a sexual generation. Unfortunately this
generally means that the variety is lost, for owing to the haphazard way in
which new kinds of potatoes are reproduced it is probable that most
cultivated varieties are complex heterozygotes. Were the potato plant
subjected to careful analysis and the various factors determined upon which
its variations depend, we should be in a position to remake continually any
good potato without {165} running the risk of losing it altogether, as is
now so often the case.
The application of Mendelian principles is likely to prove of more
immediate service for plants than animals, for owing to the large numbers
which can be rapidly raised from a single individual and the prevalence of
self-fertilisation, the process of analysis is greatly simplified. Even
apart from the circumstance that the two sexes may sometimes differ in
their powers of transmission, the mere fact of their separation renders the
analysis of their properties more difficult. And as the constitution of the
individual is determined by the nature and quality of its offspring, it is
not easy to obtain this knowledge where the offspring, as in most animals,
are relatively few. Still, as has been abundantly shown, the same
principles hold good here also, and there is no reason why the process of
analysis, though more troublesome, should not be effectively carried out.
At the same time, it affords the breeder a rational basis for some familiar
but puzzling phenomena. The fact, for instance, that certain characters
often "skip a generation" is simply the effect of dominance in F_1 and the
reappearance of the recessive character in the following generation.
"Reversion" and "atavism," again, are phenomena which are no longer
mysterious, but can be simply expressed in Mendelian terms as we have
already suggested in Chap. VI. The occasional appearance of a sport in a
supposedly pure strain is {166} often due to the reappearance of a
recessive character. Thus even in the most highly pedigreed strains of
polled cattle such as the Aberdeen Angus, occasional individuals with horns
appear. The polled character is dominant to the horned, and the occasional
reappearance of the horned animal is due to the fact that some of the
polled herd are heterozygous in this character. When two such individuals
are mated, the chances are 1 in 4 that the offspring will be horned. Though
the heterozygous individuals may be indistinguishable in appearance from
the pure dominant, they can be readily separated by the breeding test. For
when crossed by the recessive, in this case horned animals, the pure
dominant gives only polled beasts, while the heterozygous individual gives
equal numbers of polled and horned ones. In this particular instance it
would probably be impracticable to test all the cows by crossing with a
horned bull. For in each case it would be necessary to have several polled
calves from each before they could with reasonable certainty be regarded as
pure dominants. But to ensure that no horned calves should come, it is
enough to use a bull which is pure for that character. This can easily be
tested by crossing him with a dozen or so horned cows. If he gets no horned
calves out of these he may be regarded as a pure dominant and thenceforward
put to his own cows, whether horned or polled, with the certainty that all
his calves will be polled. {167}
Or, again, suppose that a breeder has a chestnut mare and wishes to make
certain of a bay foal from her. We know that bay is dominant to chestnut,
and that if a homozygous bay stallion is used a bay foal must result. In
his choice of a sire, therefore, the breeder must be guided by the previous
record of the animal, and select one that has never given anything but bays
when put to either bay or chestnut mares. In this way he will assure
himself of a bay foal from his chestnut mare, whereas if the record of the
sire shows that he has given chestnuts he will be heterozygous, and the
chances of his getting a bay or a chestnut out of a chestnut mare are
equal.
It is not impossible that the breeder may be unwilling to test his animals
by crossing them with a different breed through fear that their purity may
be thereby impaired, and that the influence of the previous cross may show
itself in succeeding generations. He might hesitate, for instance, to test
his polled cows by crossing them with a horned bull for fear of getting
horned calves when the cows were afterwards put to a polled bull of their
own breed. The belief in the power of a sire to influence subsequent
generations, or telegony as it is sometimes called, is not uncommon even
to-day. Nevertheless, carefully conducted experiments by more than one
competent observer have failed to elicit a single shred of unequivocal
evidence in favour of the view. Until we have evidence based upon
experiments which are capable of {168} repetition, we may safely ignore
telegony as a factor in heredity.
Heterozygous forms play a greater part in the breeding of animals than of
plants, for many of the qualities sought after by the breeder are of this
nature. Such is the blue of the Andalusian fowl, and, according to
Professor Wilson, the roan of the Shorthorn is similar, being the
heterozygous form produced by mating red with white. The characters of
certain breeds of canaries and pigeons again appear to depend upon their
heterozygous nature. Such forms cannot, of course, ever be bred true, and
where several factors are concerned they may when bred together produce but
a small proportion of offspring like themselves. As soon, however, as their
constitution has been analysed and expressed in terms of Mendelian factors,
pure strains can be built up which when crossed will give nothing but
offspring of the desired heterozygous form.
The points with which the breeder is concerned are often fine ones, not
very evident except to the practised eye. Between an ordinary Dutch rabbit
and a winner, or between the comb of a Hamburgh that is fit to show and one
that is not, the differences are not very apparent to the uninitiated.
Whether Mendelism will assist the breeder in the production of these finer
points is at present doubtful. It may be that these small differences are
heritable, such as those that form the basis of Johannsen's pure lines. In
this case the breeder's outlook is {169} hopeful. But it may be that the
variations which he seeks to perpetuate are of the nature of fluctuations,
dependent upon the earlier life conditions of the individual, and not upon
the constitution of the gametes by which it was formed. If such is the
case, he will get no help from the science of heredity, for we know of no
evidence which might lead us to suppose that variations of this sort can
ever become fixed and heritable.
* * * * *
{170}