Monday 10 October 2016

Applied Eugenics CHAPTER V! ! !THE LAWS OF HEREDITY!



CHAPTER V!



!



!THE LAWS OF HEREDITY!

!
We have now established the bases for a practicable eugenics program.
Men differ; these differences are inherited; therefore the make-up of the race can be changed by any method which will alter the relative proportions of the contributions which different classes of men make to


!the following generation.

!For applied eugenics, it is sufficient to know that mental and physical differences are inherited; the exact manner of inheritance it would be important to know, but even without a knowledge of the details of the mechanism of heredity, a program of eugenics is yet wholly feasible.

!It is no part of the plan of this book to enter into the details of the mechanism of heredity, a complicated subject for which the reader can refer to one of the treatises mentioned in the bibliography at the close of this volume. It may be worth while, however, to outline in a very summary way the present status of the question.

One of the best attested single characters in human heredity is brachydactyly, "short-fingerness," which results in a reduction in the length of the fingers by the dropping out of one joint. If one lumps together all the cases where any effect of this sort is found, it is evident that normals never transmit it to their posterity, that affected
persons always do, and that in a mating between a normal and an affected person, all the offspring will show the abnormality. It is a good
!example of a unit character.

But its effect is by no means confined to the fingers. It tends to
!affect the entire skeleton, and in a family where one child is markedly brachydactylous, that child is generally shorter than the others. The factor for brachydactyly evidently produces its primary effect on the bones of the hand, but it also produces a secondary effect on all the bones of the body.

Moreover, it will be found, if a number of brachydactylous persons are examined, that no two of them are affected to exactly the same degree. In some cases only one finger will be abnormal; in other cases there will be a slight effect in all the fingers; in other cases all the
fingers will be highly affected. Why is there such variation in the results produced by a unit character? Because, presumably, in each individual there is a different set of modifying factors or else a variation in the factor. It has been found that an abnormality  quite  like brachydactyly is produced by abnormality in the pituitary gland. It is then fair to suppose that the factor which produces brachydactyly does so by affecting the pituitary gland in some way. But there must be many other factors which also affect the pituitary and in some cases probably favor its development, rather than hindering it. Then if the factor for brachydactyly is depressing the pituitary, but if some other factors are at the same time stimulating that gland, the effect shown in the subject's fingers will be much less marked than if a group of
!modifying factors were present which acted in the same direction as the brachydactyly factor,--to perturb the action of the pituitary gland.

This illustration is largely hypothetical; but there is no room for doubt that every factor produces more than a single effect. A white blaze in the hair, for example, is a well-proved unit factor in man; the factor not only produces a white streak in the hair, but affects the


!pigmentation of the skin as well, usually resulting in one or more white spots on some part of the body. It is really a factor for "piebaldism."

For the sake of clear thinking, then, the idea of a unit character due
to some unit determiner or factor in the germ-plasm must be given up, and it must be recognised that every visible character of an individual is the result of numerous factors, or differences in the germ-plasm.
Ordinarily one of these produces a more notable contribution to the
!end-product than do the others; but there are cases where this statement does not appear to hold good. This leads to the conception of multiple factors.

In crossing a wheat with brown chaff and one with white chaff, H. Nilsson-Ehle (1909) expected in the second hybrid generation to secure a ratio of 3 brown to 1 white. As a fact, he got 1410 brown and 94 white,
a ratio of 15:1. He interpreted this as meaning that the brown colour in this particular variety was due not to one factor, but to two, which
were equivalent to each other, and either one of which would produce the same result alone as would the two acting together. In further crossing red wheat with white, he secured ratios which led him to believe that
the red was produced by three independent factors, any one of which would produce red either alone or with the other two. A. and G. Howard later corroborated this work,[48] but showed that the three factors were not identical: they are qualitatively slightly different, although so closely similar that the three reds look alike at first sight. E. M.
!East has obtained evidence from maize and G. H. Shull from shepherd's-purse, which bears out the multiple factor hypothesis.

Apart from multiple factors as properly defined (that is, factors which produce the same result, either alone or together), extensive analysis usually reveals that apparently simple characters are in reality
complex. The purple aleurone colour of maize seeds is attributed by R. A. Emerson to five distinct factors, while E. Baur found four factors responsible for the red colour of snapdragon blossoms. There are, as G.
N. Collins says,[49] "still many gross characters that stand as simple Mendelian units, but few, if any, of these occur in plants or
!animals that have been subjected to extensive investigation. There is now such a large number of characters which at first behaved as units, but which have since been broken up by crossing with suitable selected material, that it seems not unreasonable to believe that the remaining cases await only the discovery of the right strains with which to hybridize them to bring about corresponding results."

Knowing that all the characters of an individual are due to the interaction of numerous factors, one must be particularly slow in assuming that such complex characters as man's mental traits are units, in any proper genetic sense of the word. It will, for instance, require very strong evidence to establish feeble-mindedness as a unit character. No one who examines the collected pedigrees of families marked by feeble-mindedness, can deny that it does appear at first sight to behave as a unit character, inherited in the typical Mendelian fashion. The psychologist H. H. Goddard, who started out with a strong bias against


believing that such a complex trait could even behave as a unit character, thought himself forced by the tabulation of his cases to adopt the conclusion that it does behave as a unit character. And other eugenists have not hesitated to affirm, mainly on the strength of Dr.
!Goddard's researches, that this unit character is due to a single determiner in the germ-plasm, which either is or is not present,--no halfway business about it.

How were these cases of feeble-mindedness defined? The definition is purely arbitrary. Ordinarily, any adult who tests much below 12 years by the Binet-Simon scale is held to be feeble-minded; and the results of this test vary a little with the skill of the person applying it and
with the edition of the scale used. Furthermore, most of the
!feeble-minded cases in institutions, where the Mendelian studies have usually been made, come from families which are themselves of a low grade of mentality. If the whole lot of those examined were measured, it would be difficult to draw the line between the normals and the affected; there is not nearly so much difference between the two classes, as one would suppose who only looks at a Mendelian chart.

!THE DISTRIBUTION OF INTELLIGENCE


It would be well to extend our view by measuring a whole population with one of the standard tests. If the intelligence of a thousand children
picked at random from the population be measured, it will prove (as outlined in Chapter III) that some of them are feeble-minded, some are precocious or highly intelligent; and that there is every possible
degree of intelligence between the two extremes. If a great number of children, all 10 years old, were tested for intelligence, it would
reveal a few absolute idiots whose intelligence was no more than that of the ordinary infant, a few more who were as bright as the ordinary kindergarten child, and so up to the great bulk of normal 10-year-olds, and farther to a few prize eugenic specimens who had as much intelligence as the average college freshman. In other words, this trait of general intelligence would be found distributed through the population in accordance with that same curve of chance, which was discussed when we were talking about the differences
!between individuals.

!Now what has become of the unit character, feeble-mindedness? How can one speak of a unit character, when the "unit" has an infinite number of values? Is a continuous quantity a unit?

If intelligence is due to the inheritance of a vast, but indeterminate, number of factors of various kinds, each of which is independent, knowledge of heredity would lead one to expect that some children would get more of these factors than others and that, broadly speaking, no two would get the same number. All degrees of intelligence between the idiot and the genius would thus exist; and yet we can not doubt that a few of these factors are more important than the others, and the presence of even one or two of them may markedly affect the level of intelligence.


It may make the matter clearer if we return for a moment to the physical. Height, bodily stature, offers a very good analogy for the case we have just been discussing, because it is obvious that it must
depend on a large number of different factors, a man's size being due to the sum total of the sizes of a great number of bones, ligaments, tissues, etc. It is obvious that one can be long in the trunk and short
in the legs, or vice versa, and so on through a great number of
possible combinations. Here is a perfectly measurable character (no one has ever claimed that it is a genetic "unit character" in man although
it behaves as such in some plants) as to the complex basis of which all will agree. And it is known, from common observation as well as from pedigree studies, that it is not inherited as a unit: children are never born in two discontinuous classes, "tall" and "short," as they are with colour blindness or normal colour vision, for example. Is it not a fair assumption that the difference between the apparent unit character of feeble-mindedness, and the obvious non-unit character of height, is a matter of difference in the number of factors involved, difference in
the degree to which they hang together in transmission, variation in the factors, and certainly difference in the method of measurement? Add that the line between normal and feeble-minded individuals is wholly arbitrary, and it seems that there is little reason to talk about
feeble-mindedness as a unit character. It may be true that there is some sort of an inhibiting factor inherited as a unit, but it seems more
!likely that feeble-mindedness may be due to numerous different causes; that its presence in one child is due to one factor or group of factors, and in another child to a different one.[50]

It does not fall wholly into the class of blending inheritance, for it does segregate to a considerable extent, yet some of the factors may
show blending. But at present one can say with confidence of this, as of other mental traits, that like tends to produce like; that low grades of
!mentality usually come from an ancestry of low mentality, and that bright children are usually produced in a stock that is marked by intelligence.

!Most mental traits are even more complex in appearance than feeble-mindedness. None has yet been proved to be due to a single germinal difference, and it is possible that none will ever be so demonstrated.

!Intensive genetic research in lower animals and plants has shown that a visible character may be due to

!Independent multiple factors in the germ-plasm.

!Multiple allelomorphs, that is, a series of different grades of a single factor.

One distinct Mendelian factor (or several such factors), with modifying factors which may cause either (a) intensification, (b) inhibition, or (c) dilution.


!Variation of a factor.

!Or several or all of the above explanations may apply to one case.

!Moreover, the characters of which the origin has been most completely worked out are mostly colour characters, whose physiological development seems to be relatively simple. It is probable that the development of a mental character is much more complicated, and therefore there is more likelihood of additional factors being involved.

To say, then, that any mental trait is a unit character, or that it is
!due to a single germinal difference, is to go beyond both the evidence and the probabilities.

From this standpoint, we return to attack the problem of the relation between parent and offspring. We noted that there is no uniform sequence in a single family, and illustrated this by the case of brown eyes. But
!if a thousand parents and their offspring be selected and some trait, such as eye-colour, or stature, or general intelligence, be measured, a uniformity at once appears in the fact of regression. Its discoverer, Sir Francis Galton, gives this account of it:
!
"If the word 'peculiarity' be used to signify the difference between the
amount of any faculty possessed by a man, and the average of that possessed by the population at large, then the law of regression may be described as follows: each peculiarity in a man is shared by his kinsmen, but on the average in a less degree. It is reduced to a
!definite fraction of its amount, quite independently of what its amount might be. The fraction differs in different orders of kinship, becoming smaller as they are more remote. When the kinship is so distant that its effects are not worth taking into account, the peculiarity of the man, however remarkable it may have been, is reduced to zero in his kinsmen. This apparent paradox is fundamentally due to the greater frequency of mediocre deviations than of extreme ones, occurring between limits separated by equal widths."

As to the application of this law, let Galton himself speak: "The Law of Regression tells heavily against the full hereditary transmission of any gift. Only a few out of many children would be likely to differ from mediocrity so widely as their Mid-Parent [i. e., the average of their two parents], allowing for sexual differences, and still fewer would differ as widely as the more exceptional of the two parents. The more bountifully the parent is gifted by nature, the more rare will be his
good fortune if he begets a son who is as richly endowed as himself, and still more so if he has a son who is endowed yet more largely. But the law is evenhanded; it levies an equal succession-tax on the transmission of badness as of goodness. If it discourages the extravagant hopes of a gifted parent that his children on the average will inherit all his
powers, it not less discountenances extravagant fears that they will inherit all his weakness and disease.


"It must be clearly understood that there is nothing in these statements to invalidate the general doctrine that the children of a gifted pair
!are much more likely to be gifted than the children of a mediocre pair." To this it should be added that progeny of very great ability will arise more frequently in proportion to the quality of their parents.

It must be reiterated that this is a statistical, not a biological, law; and that even Galton probably goes a little too far in applying it to individuals. It will hold good for a whole population, but not
!necessarily for only one family. Further, we can afford to reëmphasise the fact that it in no way prevents the improvement of a race by selection and assortative mating.

Stature is the character which Dr. Galton used to get an exact measurement of the amount of regression. More recent studies have changed the value he found, without invalidating his method. When large numbers are taken it is now abundantly proved that if parents exceed the average stature of their race by a certain amount their offspring will,
!in general, exceed the racial average by only one-half as much as their parents did. This is due, as Galton said, to the "drag" of the  more remote ancestry, which when considered as a whole must represent very nearly mediocrity, statistically speaking.

The general amount of regression in heredity, then, is one-half. If it be expressed as a decimal, .5, the reader will at once note its identity with the coefficient of correlation which we have so often cited in this book as a measure of heredity. In fact, the coefficient of correlation
!is nothing more than a measure of the regression, and it is probably simpler to think of it as correlation than it is to speak of a Law of Regression, as Sir Francis did.

This correlation or regression can, of course, be measured for other ancestors as well as for the immediate parents. From studies of
eye-colour in man and coat-colour in horses, Karl Pearson worked out the necessary correlations, which are usually referred to as the law of Ancestral Inheritance. Dr. Galton had pointed out, years before, that
!the contributions of the several generations of individuals probably formed a geometrical series, and Professor Pearson calculated this series, for the two cases mentioned, as:

!Parents          Grandparents G-Grandparents G-G-Grandparents

!.6244  .1988   .0630   .0202   ... etc.

In other words, the two parents, together, will on the average of a great many cases be found to have contributed a little more than three-fifths of the hereditary peculiarities of any given individual; the four grandparents will be found responsible for a little less than one-fifth, and the eight great-grandparents for about six hundredths,
and so on, the contribution of each generation becoming smaller with ascent, but each one having, in the average of many cases, a certain definite though small influence, until infinity.


!
It can not be too strongly emphasised that this is a statistical law,
not a biological law. It must not be applied to predict the character of the offspring of any one particular mating, for it might be highly misleading. It would be wholly unjustified, for example, to suppose that a certain man got three-tenths of his nature from his father, because the Law of Ancestral Heredity required it: in point of fact, he might
!get one-tenth or nine-tenths, none or all of a given trait. But, when dealing with a large population, the errors on one side balance the errors on the other, and the law is found, in the cases to which it has been applied, to express the facts.[51]

While, therefore, this Galton-Pearson law gives no advice in regard to individual marriages, it is yet of great value to applied eugenics. In the first place, it crystallises the vague realisation that remote ancestry is of much less importance than immediate ancestry, to an
!individual, while showing that every generation has a part in making a man what he is. In the second place, it is found, by mathematical reasoning which need not here be repeated, that the type of a population may be quickly changed by the mating of like with like; and that this newly established type may be maintained when not capable of further progress. Regression is not inevitable, for it may be overcome by selection.

To put the matter in a more concrete form, there is reason to think that if for a few generations superior people would marry only people on the average superior in like degree (superior in ancestry as well as individuality), a point would be reached where all the offspring would tend to be superior, mediocrities of the former type being eliminated; and this superiority could be maintained as long as care was taken to
avoid mating with inferior. In other words, the Galton-Pearson Law gives statistical support for a belief that eugenic marriages will create an improved breed of men. And this, it seems to us, is the most important implication of that law for eugenics, although it is an implication that
!is generally ignored.

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