Two Models for Human Genetics: Blood Grouping and Psychiatry in Germany Between the World Wars

The methodology of human genetics in the Germany of the twenties and thirties fell into two distinct stylistic groups. On the one hand, there were the mathematical models of Mendelian inheritance of Wilhelm Weinberg and Felix Bernstein, and on the other, the empirical methods of Ernst Rüdin and his group at the Deutsche Forschungsanstalt für Psychiatrie in Munich. Over the course of the 1920s, empiricism began to replace mathematics as the preferred approach of the human geneticists. After the Nazi accession of 1933, mathematical Mendelism almost disappeared from the German literature.

Eugenics often seems to be scientifically rather naïve, a movement rather than a science, whose methods contain glaring flaws. But in the early part of the century, it was the eugenists who were making the attempt to transfer genetic theory, which had been developed with the help of laboratory exemplars such as peas, maize, and fruit flies, to the human species. At a time when eugenists in the United States and Britain were simply pointing to a shocking pedigree with generation after generation of paupers and degenerates to make their point, the German eugenists had already gone well beyond that kind of thing. Even a sophisticated, highly theoretized science, if it deals with the human species, has political implications; but the movement in Germany was as much political as it was scientific.

The German eugenists mathematized their Mendelism very early on. Their leader in this was Wilhelm Weinberg, the Stuttgart hygienist and statistician who in 1910 founded the Stuttgart branch of the Gesellschaft für Rassenhygiene, the German eugenics society. ¹ Weinberg’s publications on the inheritance of twinning and of susceptibility to tuberculosis dated from 1901, before the founding of the first eugenics groups in Germany, and before the popularization of Mendel’s work. ² His statistical charts demonstrating the inheritance of constitution were shown at the Dresden Hygiene Exhibition of 1911, and at the First International Eugenics Congress in London in 1912. ³ Weinberg was among the first to take up Mendel’s theory and begin to develop it mathematically.

To the original Mendelian equation, Weinberg added symbols for the probable frequency of the genes for each trait, and argued that these proportions would hold for every generation as long as there was random mating for the traits in question—which of course is true in a human population only if the people involved cannot see the traits, and cannot set up a preference for or against them (Fig. 1). This was the first of many problems involved in transferring results from peas to people. ⁴

Figure 1.

MENDEL'S BINOMIAL
Assumes that the alleles are present in equal numbers in the population:
AB × AB = A² + 2AB + B²

WEINBERG'S VERSION OF 1908
Incorporates frequencies of the two alleles:
mA + nB = 100%
m²AA + 2mnAB + n²BB = 100%

The binomial theorem in Mendel’s original version and in Wilhelm Weinberg’s generalized version. Adapted from Wilhelm Weinberg, “Uber Nachweis der Vererbung beim Menschen,” Jahrhefte des Vereines für Vater. Naturkunde in Württemberg, 1908, 64: 368–82.

Weinberg pushed his argument further: he worked out expressions for calculating the expected proportions of A and B phenotypes among parents, children, and siblings of a given proband or index case. If a trait was really inherited, the proportions in parents and children should not be the same as the proportions in the siblings (Fig. 2). ⁵

Figure 2.

WEINBERG'S CALCULATION

If a trait is really inherited, the A:B ratio in the parents and children
should be different from that in the siblings of a proband:

For the family of a proband A, ratio of A:B
in parents and children is (1 + mn)A : n²B
in siblings is [4 (1 + mn) + mn²] A : n²(3 + n) B

If there is no significant difference, Mendelian inheritance must be ruled out.

Wilhelm Weinberg’s test of Mendelian inheritance. Adapted from Wilhelm Weinberg, “Uber Vererbungsgesetz beim Menschen,” Z. f. indukt...