AFAIK, different sports tend to favour different physical attributes - body shape, type of muscle fibre, flexibility etc. There might be some genetic influence, but only in a statistical sense - as in some countries might have a relatively large group of people with predominantly long muscle fibre (built for long distance running and endurance events) but they will still have individuals with plenty of short muscle fibre also. The increased statistical chance of finding a champion might explain some of the differences across sports, but I doubt that it's very significant.
More likely, I think, is culture. When a country does well in a sport, more people become interested in that sport and the best talent (and funding) becomes available for it - other sports tend to suffer by comparison. Look at the British performances in rowing this year after Redgrave and Pinsent's efforts - not to mention cycling (which just managed to hang onto the Boardman effect via some skillful planning), and I predict a huge surge in swimming after Adlington's performances in the pool. African countries have a strong tradition in long-distance running, but the Caribbean is currently ridiculously strong in sprinting. Within mixed race cultures, there will still be this role-model effect as these countries are still very divided. Linford Christie inspired many young black sprinters - but will Adlington get black women into the pool?
This thread has degenerated so I probably shouldn't have bothered attempting a serious response. NM.
.
