The chromosome involved in down syndrome (trisomy 21), which is most frequently due to nondisjunction of chromosome 21, resulting in 3, rather than 2, chromosomes (and making the total 47 chromosomes rather than the normal total of 46). The genetic defect in familial alzheimer’s disease is located on chromosome 21, the same chromosome that has an extra copy in down syndrome. This finding supports the idea that at least one form of alzheimer’s disease is inherited and that a similar genetic defect may occur in both down syndrome and familial alzheimer’s disease.