Paternal non-disjunction in a 46,XY/47,XXY individual with a fragile 17p12 in the mother.

In a family where the mother carried a fragile site at 17p12, RFLP-analysis with the X-specific probe L1.28 showed that the 46,XY/47,XXY mosaicism detected in her Klinefelter son was due to a non-disjunctional event in paternal meiosis I, followed by a secondary loss of an X-chromosome by a mitotic non-disjunction. Thus, an association between the primary meiotic non-disjunction and the presence of the fragile site could be excluded.