A Genetic Test
X-Ray irradiation of a female
Drosophila may induce an 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 the un-irradiated hemizygous males
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 X-linked lethals is the proportion of females with this
modified sex ratio.
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, including
cancer. The estimated slope of the dose-response curve for
increased risk of death from cancer is 0.004%
per mSv (0.04% 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.