Bare-patches, a new sex-linked gene in the mouse, associated with a high production ofXOfemales: II. Investigations into the nature and mechanism of theXOproduction

SUMMARY The high XO production in Bpa + mouse stocks appears to be due, not to Bpa itself, but to a closely linked factor for high XO production ( Fxo ) which may be a gene or a chromosomal aberration. The change of Bpa Fxo to Bpa non- Fxo occurs with a frequency of about 6%; the reciprocal change from Bpa Fxo to Fxo non- Bpa has not been found. There is some evidence that Fxo might involve a structural alteration such as an inversion or a deletion. The X -chromosome loss is due to non-disjunction which occurs in about one-third of the oocytes, probably mainly at meiosis I. Frequencies of other chromosomal abnormalities found in the stocks are given.