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This short paper is dated 12 March 1902. A few weeks later (7 April 1902), a slightly different version (with the same title) was presented at a Séance of the French Academy of Sciences, and was published in Comptes rendus hebdomadaires des séances de l'Académie des sciences 134: 779-781.

This is the first publication to report Mendelian inheritance in an animal.

Lucien Cuénot was a French biologist based in the University of Nancy, who made pioneering contributions to our understanding of genetics. For a review of some of his contributions, see Hickman and Cairns (2003).

In this 1902 paper, Cuénot describes how he commenced breeding mice in 1900, presumably soon after Mendel’s paper was rediscovered, with the specific aim of investigating whether Mendelian inheritance occurs in animals:

“So far, research on the applications of Mendel's law has all focused on the plant kingdom, and it is not known whether this mode of heredity is also found in animals. For the past two years, I have been experimenting on a very favorable material, which allows me to answer in the affirmative.”

Cuénot’s chosen species was mice, and his crosses were between common “house” gray mice and albino mice. All F₁ animals were gray. When F₁ animals were mated, the resultant 270 F₂ generation comprised 198 gray and 72 albino (26.7%). Matings of F₂ albinos always produced albino: they were true-breeding. When he mated randomly-chosen pairs of F₂ grays, and symbolising the gray allele as “g” and the albino as “b”, he reported that:

“about half of the couples gave me only little grays (189), which proves that one or both parents had only g gametes; the other half of the couples gave me both grays and whites at each litter (162 gray and 57 albino), which proves that each of the two parents had g and b gametes. This time again, according to the probabilities, the number of grays is triple that of albinos (74 and 26%)”

Given that 1/3 of the F₂ grays are expected to be homozygous (gg) and 2/3 to be heterozygous (gb), the expected proportions of the three types of matings are:

gg x gg: 1/3x1/3=1/9: expect only gray offspring

gg x gb: 2(1/3x2/3)=4/9: expect only gray offspring

gb x gb: 2/3x2/3=4/9: expect 3:1 gray:albino ratio in offspring

The “about half” of the mating pairs that gave only gray offspring correspond to the first two mating types (expected frequency = 5/9), giving 189 gray offspring and no albinos; the “other half” of mating pairs correspond to the third mating type (expected frequency = 4/9), giving the expected 3:1 ratio of gray: albino.

Interestingly, Cuénot did not stop with the above satisfying results. He then describes a series of matings in which he tested the influence of ancestral generations. He mated F₁ grays with albinos. From the resultant offspring, he chose grays and mated them with albinos; and so on for a total of five successive generations. In each generation, the proportion of albino ancestry of the gray mice increased in the sequence 1/2, 3/4, 7/8, 15/16, 31/32. In each case, the mating produced roughly equal numbers of gray and albino mice. Cuénot’s commentary on these results (albeit presented here in an inadequate English translation) are remarkable prescient:

“Now, if there is a disjunction of the characters, we have crossed each time gametes of character b (those of the albino), by gametes b and g (those of the gray); and if the genital gland of the latter contains as many gametes of both types, we must always obtain, at each crossing, as many albinos (b + b), than grey (b+g). The experiments are perfectly consistent, this time again, with the theoretical prediction; for five successive generations, the repeated introduction of white blood, to speak the zootechnical language, in no way diminishes the number of gray mice in the litters.”

Cuénot goes on to say that these results make it possible:

“to predict and understand facts that will seem paradoxical to breeders: an albino mouse, whose ancestors, for a number of generations as great as one wants, were gray, is however an albino of absolutely pure breed who will never present gray atavism [reversion]”.

On the other hand, and just as importantly:

“By crossing two gray mice, each containing (n–1)/n of white [albino] blood, n being as large as one wants, one can obtain grays of absolutely pure breed (g + g) which will never present a return to albinism.”

Cuénot does not mention Galton’s ancestral law (with which Bateson was very familiar; see above commentaries on the year 1900), probably because he was not aware of it. However, these results provide a strong refutation of Galton’s law, and strong validation of Mendelian inheritance in animals.

Cuénot went on to publish many other important genetics results obtained from his mice, some of which are described by Hickman and Cairns (2003).

References

Hickman, M. and Cairns, J. (2003) The centenary of the one-gene one-enzyme hypothesis. Genetics 163: 839-841. View this paper