PART III. FAMILY STUDIES IN HEREDITY OF BUILD. (continued 4)

It has been repeatedly pointed out that in many cases where multiple factors are invoked to explain the genetical results, the results can be equally well explained on the theory of blended inheritance. The advantage of the factorial theory is that it brings under one type of heredity even these most difficult cases of apparently blending inheritance. More than that, however, the theory of multiple factors explains a number of phenomena that the theory of blending inheritance does not, such as the greater variability of the F₂ as contrasted with the F₁ generation, and the skewness in distribution of progeny in the back-cross. Thus an F × S mating should produce a “medium” progeny with symmetrical distribution of deviating types on the theory of blending inheritance; actually it produces not only mediums, but an excess of fleshy over slender, which is readily accounted for on the hypothesis that fleshiness is partially dominant over slenderness. In this and other ways, especially in focussing attention on gametes, the doctrine of multiple factors is essential to an adequate interpretation of the results of mating between persons of dissimilar build.

D. SUMMARY OF CONCLUSIONS.

1. Two types of variation in build are to be distinguished: (1) the change in average build that accompanies ontogeny and (2) variations in adult build.

2. The popular idea of build is best expressed as the ratio of transverse chest diameter to stature, or (since the chest diameter is rarely known) chest-girth to stature. When only weight and stature are known, the closest approximation to the chest÷stature ratio is given by the weight÷(stature)² ratio, and this is taken as the standard index of build.

3. The correlation between the standard index of build and the relative chest-girth is, for males, about 0.45.

4. The index of build of adult males is slightly greater than of females, because of the relatively greater chest-girth of males. The average index of build for males is 2.52 (35.8) and for females 2.43 (34.5).

5. There are marked racial differences in build; but they are not so great as the differences in ontogenetic stages.

6. There are geographical differences in build; the heavy build of northern peoples may be due to a physiological reaction or, in part, to a selective survival of the fleshier individuals or strains.

7. The ontogenetic curve of build, expressed by relative chest-girth, is expressed by figure 1 and (for infancy) figure 2. It shows that at birth chest-girth is about two-thirds of stature and diminishes in the male to the age of 12 years; thereafter, on the average, it rises to complete maturity.

8. Build declines temporarily during the first month of life, owing to the physiological difficulties attending adjustment to new conditions. It declines temporarily, again, at about 8 months, probably due to the cutting of the incisor teeth.

9. The heavy build of the infant corresponds to that of the short-legged anthropoids. The long-legged, slender-build stage of the boy of 12 years persists in the Nilotic negroes and many low-grade feeble-minded.

10. In adult life the changes in build vary with families. In those characterized by slender build there is typically little change. In those characterized by fleshy build there is typically progressive increase in weight to 50 years. In some families weight fluctuates greatly at different periods of adult life. In general, though with numerous exceptions, a fleshy adult build is foreshadowed in plump build in childhood.

11. Mass studies on adult build give a polygon of distribution which is skew, the mode being toward the slenderer end of the polygon. There is evidence of more than one mode, and hence that there are two or more types of build. For purposes of description five classes of build are recognized—very slender, slender, medium, fleshy, and very fleshy.

12. The diseases associated with very slender and slender build are: tuberculosis, pneumonia, “nervousness,” melancholia. The diseases associated with very fleshy or fleshy build are: diabetes, nephritis and dropsy, apoplexy, and arterio-sclerosis and paralysis accompanying it; also numerous diseases of the alimentary tract.

13. Fleshy parents have, on the average, in our data, larger families than slender parents.

14. Regression toward mediocrity is less striking in the offspring of slender than of fleshy parents, suggesting that fleshy parents carry not only genes for fleshiness but also for slenderness, while slender parents more rarely carry genes for fleshiness.

15. The offspring of two fleshy parents are twice as variable as those of slender parents.

16. The hypothesis is indicated that genetically build is controlled by multiple factors, with fleshiness tending slightly to dominate over slenderness.

17. There is a marked tendency for persons of similar build (or with potentialities for such) to intermarry. Dissimilar builds are selected against.

18. Two slender parents of slender stock have rarely any progeny whose index of build exceeds 2.2, or just above the upper limit of the slender group. In general, the progeny of slender parents are relatively slightly variable.

19. The slender parents are apparently of two kinds: those carrying only one kind of factor for fleshy build and the other two such gametic factors. The progeny of the former are very slightly variable; those of the latter more variable.

20. By comparing coefficients of variability instead of indices, and by making allowance for the greater range of the fleshy class than the slender class, it still appears that the progeny of fleshy parents are relatively and per unit range more variable than the progeny of slender parents. Absolutely the offspring of the fleshy parents are, as stated, twice as variable as of slender parents.

21. The matings of slender parents and fleshy parents of fleshy stock yield a variable progeny, such as is typically found in the F₁ generation when multiple factors are concerned. The variability is between that of the progeny of slender matings and that of the progeny of fleshy matings.

22. The F₁ generation has not a mode of build that is intermediate between that of the parental stocks, but one that is about the same as of the fleshy matings. This is evidence of partial dominance of fleshiness.

23. The mating of two heterozygous (F₁) parents produces, in general, an exceptionally variable progeny. This is one of the strongest evidences of the presence of genetical factors in build.

24. Many, if not most, parents of medium build belong to biotypes with 1 or 2 independent factors for build. Consequently the progeny of medium × medium matings is often strikingly invariable.

25. The “back-cross” of a heterozygous parent with a slender parent, on the one hand, or with a fleshy parent of fleshy stock, on the other, gives a variable progeny, whose mode is shifted toward fleshiness; but which shows a tendency to fall again into the slender (or fleshy) and medium-fleshy grandparental types.

26. Variations in build are not to be accounted for merely by variations in intake and out-go of calories, but also by the endogenous factors that determine the “economy of nutrition” or the cost in energy of adding an additional kilogram of weight to the body. The factors involved in producing differences in these respects are hereditary factors.

27. The hereditary factors probably work through the intermediacy of special organs that influence metabolism, notably the endocrine glands. The latter thus intermediate between the chromosomal constitution, on the one hand, and control of metabolic processes, on the other.

28. The number of factors involved in very fleshy build is, in some cases at least, three independent ones. But the number is probably not the same in all biotypes. Probably in some there is only one, in others four or more.

29. The factors probably influence the functioning of the thyroid, pituitary, and perhaps other glands.