A high frequency of XO offspring from (XY)-Y-Paf* male mice: evidence thatthe Paf mutation involves an inversion spanning the X PAR boundary

It has previously been reported that 19% of the daughters of males carrying the X-linked mutation patchy fur (Paf) are XO with a maternally derived X chromosome. We now report that hemizygous Paf males that also carry the var iant Y chromosome Y*, show a much increased XO production (similar to 40% o f daughters). We hypothesize that the Paf mutation is associated with an in version spanning the pseudoautosomal region (PAR) boundary, and that this l eads to preferential crossing over between the resulting inverted region of PAR and an equivalent inverted PAR region within the compound Y* PAR. This would lead to the production of dicentric X and acentric Y products and co nsequent sex chromosome loss. This interpretation is supported by analysis of the sex chromosome complements at the second meiotic metaphase, which re vealed a high incidence of dicentrics. Another curious feature of the Paf m utation is that mice that are homozygous Paf have more hair than mice that are hemizygous Paf: This can be explained if the Paf mutation is a hypomorp hic mutation that escapes X inactivation because, unlike the wild type alle le, it is now located within the PAR. Copyright (C) 2001 S. Karger AG, Base l.