A Genetic Test for Radiation Dose-Response

    X-Ray irradiation of a female Drosophila may induce a recessive lethal mutation on one of the two X chromosomes in reproductive cells in her germ line. When mated to an un-irradiated male, some of her daughters will be heterozygous for the chromosome with the recessive lethal (X'X). When these X'X daughters are back-crossed to un-irradiated hemizygous males (XY), half of the male embryos will inherit the un-mutated maternal X and half will inherit the lethal X'. The latter males die in early development, thus X'X females will have only half the expected proportion of male offspring. The assay for induction of X-linked lethals is the proportion of females with this modified sex ratio.

X' X
X' Y

    The graph shows the fraction of lethal mutations induced by various doses of X-Rays (1000 R = 10 Gy). The dose-response curve of X-ray exposure versus rate of induced mutation is linear, which suggests that there is no lower limit (threshold) of radiation exposure that does not produce some mutagenic effect.

    Chronic low-level exposure to radiation has been implicated in a variety of medical conditions in humans, including cancer. The estimated slope of the dose-response curve for increased risk of death from cancer is
0.004% per millisievert (mSv) (0.4% per rem).

HOMEWORK: What is the dose-response rate from mutation in Drosophila? Draw the human and Drosophila dose-response curves on the same graph; use a semi-log plot. Discuss the implications for radiation-induced cancer and mutation.

All text material ©2024 by Steven M. Carr