The use of DNA probes to establish parental origin in Down syndrome.

Molecular analysis was performed to determine the parental origin of the extra number 21 chromosome in 20 couples following the birth of a child with standard trisomy 21. The parent of origin was successfully identified in 9 of 20 (45%) using five chromosome-21-specific DNA probes and eight restriction endonucleases by restriction fragment length polymorphism and dosage analysis; seven were of maternal and two of paternal origin. Utilizing the observed allele frequencies, the expected frequencies of informative homozygous matings [2(p2q2)] approximate 10% for seven of eight enzyme/probe combinations; the eighth, TaqI/pPW231F (D21S3), is 3%. The observed phenotype frequencies for all enzyme/probe combinations tested conform closely to predictions by the Hardy-Weinberg law. Strong linkage disequilibrium was observed between the DNA markers of EcoRI and TaqI with probe pPW236B; identical results were obtained with G-95 alpha 1-11a. We were able to demonstrate that although these two probes are of different size, and hence not identical, they detect the same TaqI and EcoRI polymorphisms; therefore both should be assigned to a single locus, D21S11.