A high frequency of XO offspring from X(Paf)Y* male mice: evidence that the 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 variant Y chromosome Y*, show a much increased XO production ( approximately 40% of daughters). We hypothesize that the Paf mutation is associated with an inversion spanning the pseudoautosomal region (PAR) boundary, and that this leads 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 consequent sex chromosome loss. This interpretation is supported by analysis of the sex chromosome complements at the second meiotic metaphase, which revealed a high incidence of dicentrics. Another curious feature of the Paf mutation 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 hypomorphic mutation that escapes X inactivation because, unlike the wild type allele, it is now located within the PAR.