The reduced (48%) incidence of schizophrenia in monozygotic twins when the co-twin develops schizophrenia1 has long remained a puzzle and a challenge. Attempts have been made to explain the phenomenon by the use of the concept of incomplete penetrance and a possible postzygotic, mitotic crossover that could account for the discordance. A study conducted by Polymeropoulos et al2 claims "no genetic marker dis-cordances were identified between the co-twins, still leaving the possibility that a tissue-specific, mitotic cross-over may have occurred in one of the discordant twins."
A more promising explanation is suggested by experimental observations of neuronal dendritic development in the sea slugs Aplesias (similar development also is known to occur in the daphnid and the fruit fly). In identical twins of such slugs, their giant neurons are located initially in spatial configurations identical