Dextral/Chance Theory:Proposed by Chris McManus. He hypothesizes a “dextral” (D) allele, which strongly biases handedness in favour of the right hand and control of speech toward the left cerebral hemisphere, along with an alternative “chance” (C) allele, which is presumed directionally neutral. According to this theory, the DD genotype produces only right-handers, the CC genotype produces a random mixture of 50% right-handers and 50% left-handers, and the heterozygotic genotype DC produces 25% left-handers and 75% right-handers. The incidence of left-handedness within “left-handed families” actually appears never to rise above 50%, which supports the idea that what is being inherited is not left-handedness as such, but rather an absence of handedness, so that handedness is then determined by chance.
