Why exactly someone is left- or right-handed remains unclear, but scientists in Britain say they\'ve ruled out genetics as having any significant role. A study of the genome of 4,000 twins failed to find a strong genetic factor in determining handedness, they said. If there was a single major genetic determination of handedness, there should be a detectable shift between left- and right-handed twins in the frequency of variants in that part of the genome, but that wasn\'t the case, the researchers reported in the journal Heredity. \"There should be a detectable shift between right- and left-handed people because modern methods for typing genetic variation cover nearly all of the genome,\" University of Nottingham geneticist John Armour said. \"A survey that compared the whole-genome genotypes for right- and left-handed people should leave such a gene nowhere to hide.\" The findings contradict some previous studies suggesting a link between handedness and a network of genes involved in establishing left-right asymmetry in developing embryos. If there are any genetic factors in handedness at all they must be relatively weak and subtle, the researchers said. \"It is likely that there are many relatively weak genetic factors in handedness, rather than any strong factors, and much bigger studies than our own will be needed to identify such genes unambiguously,\" Armour said. \"As a consequence, even if these genes are identified in the future, it is very unlikely that handedness could be usefully predicted by analysis of human DNA.\" About 10 percent of people in Britain are left-handed, a percentage found in most populations around the world.