Discordance and concordance of dementia of the Alzheimer type (DAT) in monozygotic twins indicate heritable and sporadic forms of Alzheimer's disease.

Monozygotic (MZ) twin pairs concordant for dementia of the Alzheimer type (DAT) or for proven Alzheimer's disease (AD) have a significantly higher frequency of a positive family history (DAT or AD in at least one first-degree relative, p less than 0.002) than do discordant MZ twin pairs, consistent with a lesser predicted distribution of proportions of surviving first-degree relatives without DAT (or AD) (p less than 0.001). The results suggest that concordant MZ twin pairs with DAT (or AD) have a heritable form of disease. AD in discordant twins may result from environmental influences or from a somatic chromosomal change following zygotic division.