The effect of a change in mutation rate on the incidence of dominant and X-linked recessive disorders in man.

In order to assess the impact on man of a sustained change in mutation rate that might be caused by ionizing radiation or a chemical mutagen in the environment, it is important to determine the current incidence of genetic disease, the rate at which deleterious mutations arise and the number of generations that mutations persist before eliminated by selection. From these data it should be possible to estimate both the increase in genetic disease in the first generation following the increase in mutation rate, and the rate at which a new equilibrium between mutation and selection would occur. In this paper the results of a survey to determine birth frequency, mutation rate and reproductive fitness for each of the important dominant and X-linked recessive disorders are described. It is estimated that these disorders affect about 0.6% of live-born individuals, including 0.1% of live-borns who carry a newly-arising mutation. These figures are approx. 50% lower than those used by the various committees that have assessed the genetic risk to man from ionizing radiation. If the mutation rate were to permanently double, the frequency of these disorders would be expected in increase in the first generation by 15%, to 0.7% of live-births. The increase in the first 2 generations would be 24% and a 50% increase would occur by the 9th generation. A calculation of the possible increase in dominant and X-linked recessive disorders due to exposure of a population to ionizing radiation indicates that the estimate made in 1977 by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) may be too high by a factor of 2-6 fold.