A detailed genetic linkage map of human chromosome 21 will allow the rapid regional assignment of future genes and DNA markers to this autosome. It will also facilitate the precise mapping of the genes responsible for the DS phenotype and the defect causing FAD. This map can also be used to implicate genes involved with specific features of the DS phenotype by permitting the delineation of finite regions of chromosome 21 which are duplicated in patients with partial trisomy 21, and who manifest select symptoms of the DS phenotype. In the case of FAD, most pedigrees are not ideally structured for genetic linkage analysis since affected individuals die relatively soon (6-8 years) after the onset of symptoms. By first establishing the genetic relationships of DNA markers in the vicinity of the FAD defect, multipoint analyses of these markers can then be performed in FAD families. Multipoint analysis carries a greater probability of identifying markers flanking the FAD locus thereby providing landmarks for cloning attempts aimed at isolating and characterizing the FAD gene defect. We have confirmed our initial findings of a dramatically increased rate of recombination at the telomere in both females and males, and significantly higher recombination in females in the pericentromeric region between D21S1/S11 and D21S13/S16. By comparing patterns of recombination in specific regions of chromosome 21 with regard to both parental sex and age, we have identified a statistically significant downward trend in in the frequency of crossovers in the most telomeric portion chromosome 21 with increasing maternal age. A less significant decrease in recombination with increasing maternal age was observed in the sub-centromeric region of the chromosome where recombination in females is overall higher than in males. Future confirmation of these results in combination with investigations aimed at deciphering the role played by recombination in promoting normal segregation during meiosis should help to elucidate the potential relevance of these findings to the etiological basis of nondisjunction.