Cytogenetic study of the heterochromatic polymorphisms in 100 subjects with Down syndrome and their parents.

We report on a cytogenetic study of 100 subjects with Down syndrome (DS), diagnosed from 1980 to 1988, and their parents. Free trisomy was present in 95% of the patients; 5% had trisomy due to an unbalanced translocation. Approximately 60% of patients were born to mothers younger than age 35 years, the highest number of DS births being in the group of mothers between ages 25 and 29 years. These findings are clearly related to the higher number of pregnancies in this group. However, the prevalence of DS births increased significantly in women older than age 35 years, as expected. A cytogenetic study of heterochromatic polymorphisms showed a significant increase of polymorphisms in DS children and in their parents, mostly in mothers, compared with a control group. Since heterochromatic blocks of acrocentric and nonacrocentric chromosomes are frequently associated during interphase, we discuss a possible correlation between nondisjunction and heterochromatic polymorphism.