Part 9 (1/2)
”One may venture to guess,” T. H. Morgan says,[47] ”that some of the specific and varietal differences that are characteristic of wild types and which at the same time appear to have no survival value, are only by-products of factors whose most important effect is on another part of the organism where their influence is of vital importance.”
”I am inclined to think,” Professor Morgan continues, ”that an overstatement to the effect that each factor may affect the entire body, is less likely to do harm than to state that each factor affects only a particular character. The reckless use of the phrase 'unit character'
has done much to mislead the uninitiated as to the effects that a single change in the germ-plasm may produce on the organism. Fortunately the expression 'unit character' is being less used by those students of genetics who are more careful in regard to the implications of their terminology.”
[Ill.u.s.tration: THE EFFECT OF ORTHODACTYLY
FIG. 17.--At the left is a hand with the third, fourth and fifth fingers affected. The middle joints of these fingers are stiff and cannot be bent. At the right the same hand is shown, closed. A normal hand in the middle serves to ill.u.s.trate by contrast the nature of the abnormality, which appears in every generation of several large families. It is also called symphalangism, and is evidently related to the better-known abnormality of brachydactyly. Photograph from Frederick N. Duncan.]
[Ill.u.s.tration: A FAMILY WITH ORTHODACTYLY
FIG. 18.--Squares denote males and circles females, as is usual in the charts compiled by eugenists; black circles or squares denote affected individuals. A1 had all fingers affected in the way shown in Fig. 17; B2 had all but one finger affected; C2 had all but one finger affected; D2 had all fingers affected; D3 has all but forefingers affected. The family here shown is a branch, found by F. N. Duncan, of a very large family first described by Harvey Cus.h.i.+ng, in which this abnormality has run for at least seven generations. It is an excellent example of an inherited defect due to a single Mendelian factor.]
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 ill.u.s.tration 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 recognized 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 color 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.
[Ill.u.s.tration: WHITE BLAZE IN THE HAIR
FIG. 19.--The white lock of hair here shown is hereditary and has been traced back definitely through six generations; family tradition derives it from a son of Harry ”Hot-Spur” Percy, born in 1403, and fallaciously a.s.signs its origin to ”prenatal influence” or ”maternal impression.” This young woman inherited the blaze from her father, who had it from his mother, who had it from her father, who migrated from England to America nearly a century ago. The trait appears to be a simple dominant, following Mendel's Law; that is, when a person with one of these locks who is a child of one normal and one affected parent marries a normal individual, half of the children show the lock and half do not. Photograph from Newton Miller.]
[Ill.u.s.tration: A FAMILY OF SPOTTED NEGROES
FIG. 20.--The piebald factor sometimes shows itself as nothing more than a blaze in the hair (see preceding figure); but it may take a much more extreme form, as ill.u.s.trated by the above photograph from Q.
I. Simpson and W. E. Castle. Mrs. S. A., a spotted mutant, founded a family which now comprises, in several generations, 17 spotted and 16 normal offspring. The white spotting factor behaves as a Mendelian dominant, and the expectation would be equal numbers of normal and affected children. Similar white factors are known in other animals. It is worth noting that all the well attested Mendelian characters in man are abnormalities, no normal character having yet been proved to be inherited in this manner.]
Apart from multiple factors as properly defined (that is, factors which produce the same result, either alone or together), extensive a.n.a.lysis usually reveals that apparently simple characters are in reality complex. The purple aleurone color of maize seeds is attributed by R. A.
Emerson to five distinct factors, while E. Baur found four factors responsible for the red color 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.”
In spite of the fact that there is a real segregation between factors as has been shown, it must not be supposed that factors and their determiners are absolutely invariable. This has been too frequently a.s.sumed without adequate evidence by many geneticists. It is probable that just as the multiplicity and interrelation and minuteness of many factors have been the princ.i.p.al discoveries of genetics in recent years that the next few years will see a great deal of evidence following the important lead of Castle and Jennings, as to variation in factors.
Knowing that all the characters of an individual are due to the interaction of numerous factors, one must be particularly slow in a.s.suming 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 fas.h.i.+on. The psychologist H. H. G.o.ddard, 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.
G.o.ddard'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.