The Inheritance of a Fluctuating Character

In the fall of 1909 Morgan observed that the trident-pattern on the thorax of flies of some of the wild stocks of Drosophila melanogaster varied both in the intensity of the pigmentation and in the extent to which the pattern was spread over the thorax. The degree of pigmentation and the extent of the pattern seemed to vary together. A scale of grades was devised (Fig. 1) by means of which it was possible to describe rather accurately the different populations dealt with. Thus, Table I and Curve a give the distribution of individuals in "Falmouth" wild stock (October, 1911). Morgan selected throughout the fall and winter of 1909-10 for increased trident-pattern, with no certain progress until January, 1910, when a few flies were found that were considerably darker than any hitherto observed. By breeding from these darker flies a race was quickly obtained which showed no individuals as low in grade as those in Classes I and II and very few in Class III. This stock was called "wi th" and mass selection in it was continued for about a year (thirty generations) with no apparent result further than possibly to decrease the numbers in the lower grades. I t seems clear that the various grades observed in the stock prior to January, 1910 were merely the normal fluctuations of the wild type pattern, and that a definite mutation had then occurred which gave rise to higher grades and a characteristic new distribution. A census taken of this new "wi th" mutant population (October, 1911) revealed the condition shown in Table I, and by Curve b, Fig. 2. The most numerous class in the "wi th" stock was VII, which was not represented at all among the wild flies. 639