Experimental studies, however, show that significant exposures to mutagens are not likely to be detected by increased rates of birth defects. Germ cells are more likely to be killed by DNA damage than to be mutated. This loss of cells will lead to infertility. Mutations in germ cells could introduce new genetic conditions into families, but only those conditions that are dominant and whose phenotypes are viable will be seen. Experimental data indicate that most dominant mutations induced in germ cells at high levels of exposure are lethal.
Conceptions carrying dominant lethal genes are likely to abort early or even fail to implant.
Thus the perceived effect is, again, infertility. Recessive mutations induced by mutagens will not be detected until they become homozygous in future generations and many changes in sequence in non-coding regions of the genome will simply remain silent. High dose exposures to mutagens in humans have occurred as the result of nuclear explosions and catastrophes, and continue to occur through cancer radiation and chemotherapy. As predicted by the data discussed above, follow up has not documented increased rates of birth defects, except when exposures occur during pregnancy and can directly affect embryonic tissues. Microcephaly is the only consistent finding in these cases.