Abstract
Using BrdU‐labeling and acridine orange staining, the behavior of X‐chromosome replication was studied in 28 XXX and 19 XXY digynous mouse triploids. In some of these the paternal and maternal X chromosome could by cytologically distinguished. Such embryos were obtained by mating chromosomally normal females with males carrying Cattanach's X chromosome which contains an autosomal insertion that substantially increases the length of this chromosome. In the XXX triploids there were two distinct cell lines, one with two late‐replicating X chromosomes, and the other with only one late‐replicating X. The XXY triploids were also composed of two cell populations, one with a single late‐replicating X and the other with no late replicating X chromosome. Assuming that the late‐replicating X is genetically inactive, in both XXX and XXY triploids, cells from the embryonic region tended to have only one active X chromosome, whereas those from the extra‐embryonic membranes tended to have two active X chromosomes. The single active X chromosome was either paternal or maternal in origin, but two active X chromosomes were overwhelmingly maternal in origin, suggesting paternal X‐inactivation in extra‐embryonic tissues.