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E.2 GENETIC EFFECTS
E.!
It is known that genetic effects result from alterations within genes, called muta-
tions, or from rearrangements of genes within chromosomes. There is no radiation-dose
threshold for the production of mutations, but repair of damage to genetic material can
occur during exposure at low dose rates. This information is reviewed and discussed at
length in the 1977 UNSCEAR Report.
The conventional approach to this problem has been to estimate a "mutation doubling
dose," i.e., the radiation dose required to double the existing mutation rate. The BEIR
Report concludes that this doubling dose for humans lies in the range of 20 to 200 rem. The
UNSCEAR Report considers additional experimental data and opts for a single value of
100 rem. Given a number for the doubling dose, if one can assume that radiation-induced
mutations have the same effect on health as normally occurring mutations and if one knows
the burden of human ill health attributable to such normally occurring mutations, one can
directly estimate the genetic effect of any given radiation dose. Unfortunately, it is not
clear that radiation-induced mutations are equivalent in effect to normally occurring muta-
tions. Nor is there any confidently accepted quantification of the human ill health attrib-
utable to these normally occurring mutations.
Four kinds of specifically recognized genetically associated diseases are usually
distinguished.
1. Autosomal dominant disorders are those caused by the presence of a single gene.
The most common examples are: chondrodystrophy (abnormal cartilage development),
osteogenesis imperfecta (abnormally brittle bones), neurofibromatosis (disease
characterized by multiple soft tumors), eye anomalies including congenital
cataract, and polydactylism (more than 10 fingers or toes) (Trimble and Doughty
1974). It is generally agreed that these disorders will double in frequency if
the mutation rate is doubled (NAS-NRC 1972 and UNSCEAR 1977). There is some dis-
agreement on their normal frequency of occurrence: the earlier data (Stevenson
1959) employed in the BEIR Report indicate a 1% normal incidence, while a more
recent study of and Trimble and Doughty 1974), indicates an incidence of
something less than 0.1%. These new data have not been fully accepted, however,
and the 1977 UNSCEAR Report continues to employ the 1% normal incidence figures.
2. Multifactorial (irregularly inherited) disorders have a more complex and ill-
defined pattern of inheritance. These diseases include a wide variety of congenital
malformations and constitutional and degenerative diseases. Their normal
incidence in the population was estimated in the BEIR Report to be about 4% (NAS-
NRC 1972); however, the newer data of Doughty and Trimble suggest an incidence as
high as 9-10%'(UNSCEAR 1977). The BEIR Report states that, "The extent to which
the incidence of these diseases depends on mutation is not known" but assumes a
"mutational component" of 5 to 50% (p. 56). The 1977 UNSCEAR Report employs a
single figure of 5% and considers 10% to be an upper limit (p. 429). Newcombe has
argued that "the bulk of the most directly pertinent experimental studies thus