law in genetic theory by using twin studies, which allow comparison between monozygotic twins (genes 100% the same) and dizygotic twins (50% the same), with the specification that these twins were brought up together to cancel any environmental discrepancies. Using this method Herman (1959) and Jorm (1983) found that a 100% concordance for monozygotic twins and a 33% concordance for dizygotic twins, suggesting a role for genetic factors. Cardon, Smith, Fulker, Kimberling, Pennington and De Fries (1994), and Grigorenko (1997) go even further in their research to suggest that dyslexia is controlled by chromosome six. However this does not explain how new instances of dyslexia occur in families without and past history or why not everyone in a family with dyslexic history has this condition. The only reliable conclusion that can be drawn, is that genetics appear to play a part in the causation of dyslexia, but they cannot fully explain it. A combination of factors, need to exist for dyslexia to manifest itself.
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