In this paper we have compared the breeding performance of Tdy-negative XY, XXY and XYY females to assess the relative importance of the lack of a second X chromosome compared with the presence of a Y chromosome, in reducing fertility. The XY females were of poor fertility, although five of twelve produced at least one offspring. The XXY females had larger, more frequent litters, and a longer reproductive lifespan, implicating the lack of a second X as the major cause of the poor fertility of XY females. Nevertheless, XYY females appeared to be more seriously affected than the XY females, suggesting that the presence of the Y may be a contributory factor. Pachytene analysis demonstrated that the Y is a very inefficient pairing partner for the X during female meiosis. In XY females only 11% of pachytene cells had the X and Y paired; in XXY females the two X chromosomes paired and the Y was almost always a univalent, while in XYY females the X paired with a Y in only 15% of pachytene cells. The presence of unpaired sex chromosomes has previously been implicated as a cause of oocyte loss during pachytene, and the proportion of cells with unsynapsed sex chromosomes decreased as pachytene proceeded, suggesting that they were progressively eliminated. Significantly, protection against elimination was afforded not only by synapsis between sex chromosomes, but also by self-synapsis if a sex chromosome remained as a univalent. It is concluded that sex chromosome univalence leading to pachytene oocyte failure is responsible for the postnatal oocyte deficiency seen in XY females. A separate study has shown that XXY females have a similar level of oocyte deficiency. It is suggested that the presence of a second X chromosome improves the fertility of XXY females, compared with XY females, by improving oocyte quality and by eliminating the production of lethal XY and OY zygotes. The genotype frequencies for the offspring of XY and XXY females differed from those predicted from the pachytene data. The XY females showed a marked deficiency of XO offspring compared with XXY and XYY aneuploid offspring, whereas the XXY females had fewer than expected XXY and XYY aneuploid offspring.