From The Genetics of Human Populations:
Considering only recessive genes for which the hetrozygotes are at present distinguishable [present is 1971]...only about 30 percent of the human population would be eligible for reproduction. These genes are, however, only a small minority of the recessive deleterious genes in existence...The probability that an individual would be free of such deleterious recessives would be e^-4 or only 1/55. It might be even much less than this figure, which is computed on the assumption that all recessive deleterious genes have full penetrance, as the expected number of deleterious genes per zygote would be higher if they had incomplete penetrance.
Two points a) we are close (in the next 5-10 years) to full genome sequencing for individuals being feasible. b) most "masking" of a deleterious recessive is nearly complete, but there is usually some difference. The details vary, but consider the cummulative effects of .98 ...
