_9-
UCRL-3644
3
zai,
and radiation could account for the entirety of mutation changes in humans.
oN
The fallout of radioactive materials through 1956 has increased the
radiation exposure of gonadal tissue by an amount estimated as approximately
0.004 r/yr (see Table V-Dj (largely from ingested cesium-1374 and deposits on
the ground®),
exposure.
This is an increase of approximately 3% over natural radiation
The recommended limits of radiation exposure in man will be affected
by information on the quantitative relationship between ionization and mutation
and the understanding of the natural mutation burden. Should we estimate the
level of radiation likely to double the natural mutation frequency in man as
25 ror3r, we will be at least 2 to 20 times as concerned about the genetic
problems associated with radiation exposure as we are under the current
assumption that the human mutation rate is doubled by 50 r.
Genetic studies of irradiated Japanese have been carried out by the
Atomic Bomb Casualty Commission at Hiroshima and Nagasaki. A 10-year
study has been analyzed by S. V. Neel and W. S. Schull.
The principal
result is that no measurable increase in mutation rate was observed. They
measured biological characteristics that could reflect genetic state and genetic
change, such as stillbirths, male/female birth ratios, and congenital malformations. The results of all observations of this kind can be interpreted
7
either as demonstrating no measurable increase in these events, which are
associated with mutations, or as showing that, had the true congenital malformation rate been doubled, there would be only 90% probability of discovering even this increase. Thus, a small increase in congenital evidence of
genetic change would not have been detected.
~ oy
The results of the study of the Japanese indicate that the human
genetic effect of radiation is acceptably consistent with the range of response
estimated from mammalian genetic experiments; and it establishes with
certainty that there are no catastrophic genetic effects at low to medium range
of radiation exposure in human beings, although catastrophic effects are predicted at high levels of accumulated radiation exposures to whole populations.
Many new mutations were probably produced in the Japanese exposed to the
atomic bombs; but many of these may have been unobserved because of early
lethality, and the rest are overwhelmingly diluted by the vast number of normal
genes. This dilution was expected; and the statistical odds are known to be
very greatly against the appearance of unfavorable and detectable combinations
of mutant genes in any one generation of offspring.
Genetic change is, of course, basic to the concept of the Darwin
principle of evolution.
For this reason, it is possible that some increase in
the mutation rate might be to human advantage in the long run by providing a
greater pool of variance from which selection could take place, to our final
mK
Reported at the First International Congress on Human Genetics, in
Copenhagen in August 1956.