He spent the first part of the article talking about handedness in music instead of sports, which I have to say almost put me off from reading further.
Although most trainers know how important handedness is, at present they are rather reluctant to make use of the insights scientist like Matthews can give, which I think is rather short-sighted.
That shift is slightly less if you carry one copy, and if you don't carry the Right Shift mutation at all, language and handedness could go to the right or left, just by chance.