The masking of a Mendelian result by the influence of the environment

Abstract

As reported (Oct., 1911) a mutant of Drosophila appeared with a dominant sex-linked character, viz., abnormal abdomen. Typical Mendelian ratios are found in the F₂ offspring if an abundance of food and of moisture is present. As the culture grows older the flies that emerge later gradually change over to the normal type. As a result the Mendelian ratio completely disappears from the surface phenomena. That Mendelian inheritance has actually occurred, but is temporarily masked, is shown by testing the F₂ flies, when the expected number is found (under wet conditions) to transmit the abnormal abdomen. This is best demonstrated by linking the factor for abnormal abdomen A with another sex-linked factor, such as the b factor in the yellow mutant, or the c factor in the white mutant, or to both together as in the two examples given below.

The tables show that in the expected classes for abnormal abdomen A this character at first appears but later is replaced by the normal character N. When the normal flies (of the last sort) were tested they were found to transmit abnormality. The other classes that are genotypically normal remain so in the next generation. Cultures that had produced only abnormal flies for nine generations under wet conditions were allowed to dry out when all the later-hatched flies became normal. These were then placed under moist conditions, and all of their offspring were as abnormal as their ancestors had been.