Trisomy 21 usually results from nondisjunction during meiosis I. In order to determine whether nondisjunction results from failure of normal chromosome pairing or premature unpairing, recombination frequencies were estimated between DNA polymorphic markers on the long arm of chromosome 21 in families containing one individual with trisomy 21. The recombination frequencies on chromosomes 21 that had undergone nondisjunction were then compared to those on chromosomes 21 that had disjoined normally. The data indicate that recombination is reduced between DNA markers on nondisjoined chromosomes 21. These results are consistent with the hypothesis that reduced chiasma formation predisposes to nondisjunction, resulting in trisomy 21 in humans.

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The current report of a patient with oligodontia and an inversion on chromosome 2 may aid in the identification of novel genes for oligodontia.

We present molecular data from 90 Greek boys with Duchenne or Becker muscular dystrophy using cDNA analysis or mVltiplex PCR or both. Deletions were detected in 63*3% ofpatients and were mainly clustered in two areas of the gene, one in the 3' and one in the 5' end of the gene (exons 3-19 and 44-53