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
Y
X'
X' X
X' Y
X
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.
