"About half of all Japanese people for a start.
One I know turns bright red and gets completely hammered on a pint."
From experience I wouldn't put it as high as 50%, but yes some Japanese and Chinese people (actually anyone from a culture which historically purified its drinking water by boiling it with tea leaves, rather than brewing beer with it or adding fermented mashed grapes) react badly to alcohol. The problem is that although they have normal alcohol dehydrogenase, they have a less efficient form of aldehyde dehydrogenase, which handles the second step of metabolising alcohol. The result is a build up of aldehydes in the system, which causes the red flush and an evil hangover in short order.
However, getting drunk on small amounts of alcohol is mainly a cultural effect, just as other cultures get morose, look for a fight, sing or get maudlin (or all of the above). If you're a salaryman you have to treat your boss like god during work, but you then go for a drink after work and say exactly what you think of him because you're "drunk". It lets off steam, gives the bosses valuable feedback and is all done in a social environment where normal rules of politeness and deference are suspended. For a westerner trying to do business in Japan it's often the only time you'll get an honest opinion rather than the usual "Yes means maybe, maybe means no" politeness.