RESUMEN
Genealogies of 66 child with trisomy 21 [Down Syndrome] as diagnosed by chromosome analysis were constructed. The control group included 198 child [three controls for each patient] with no apparent genetic pathology matched in age and sex with the patients. Genealogies of the controls were also constructed, the frequency of consanguineous marriages and average breeding coefficient in parents and grandparents of patients and controls were determined. Parental consanguinity and inbreeding coefficient showed non significant increase in patients than in control. Significant increase in the frequency of consanguinity and in inbreeding coefficient was found only in maternal grand parents of Down patients. Positive family history of Down syndrome in the relatives of the mother's patient was found. The results may suggest that consanguinity has some association with the occurrence of Down syndrome. Homozygosity of autosomal recessive gene may prevent the loss of the trisomic fetus. In addition, the results strengthen the suggestion that homozygosity for autosomal recessive gene may be a predisposing factor to meiotic nondisjunction in mothers who are the offspring of consanguineous parents