Elucidating the mechanisms of paternal non-disjunction of chromosome 21 in humans, Human Molecular Genetics 7

Paternal non-disjunction of chromosome 21 accounts for 5–10 % of Down syndrome cases, therefore, relative to the maternally derived cases, little is known about paternally derived trisomy 21. We present the first analysis of recombination and non-disjunction for a large paternally derived population of free trisomy 21 conceptuses (n = 67). Unlike maternal cases where the ratio of meiosis I (MI) to meiosis II (MII) errors is 3:1, a near 1:1 ratio exists among paternal cases, with a slight excess of MII errors. We found no paternal age effect for the overall population nor when classifying cases according to stage of non-disjunction error. Among 22 MI cases, only five had an observable re-combinant event. This differs significantly from the 11 expected events (P < 0.02, Fisher’s exact), suggestin