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 fitness across many loci. Now, Armand Leroi:
We may carry 300 deleterious mutations on average, but there is of course a variance associated with that. Not everybody has 300. Some people have more, some people have fewer. If this is true--and statistically it must be true -- then someone in the world has the fewest mutations of all. Someone in the world is the least mutant human of all. Indeed, we can actually calculate, making some assumptions about the shape of the distribution, how many mutations that person has -- and it turns out to be 191 versus the average of 300.