This strain contains an abnormal LT/Sv-derived Y Chromosome comprising a spontaneous rearrangement (Eicher et al. 1991), formally designated Y(IsXPAR;Y)Ei and informally called Y*. Burgoyne et. al, (1998), updated the original description of the cytogenetic changes in this abnormal Y Chromosome, as an end-to-end fusion of two pseudoautosomal regions (PAR). Males with this abnormal Y Chromosome (XY*) are fully fertile and can be used to transmit this chromosomal aberration. Duplication of the PAR in Y* males permits generation of X and Y reciprocal translocation products during meiosis (see figure 1 in Eicher et.al 1991). Sperm produced by XY* males contain either a normal X Chromosome, the intact, complete Y^*, a large marker sex chromosome comprising an X Chromosome and most of the Y (Y^*), or a tiny cytogenetic marker sex chromosome (Y^X). Among offspring are the following genotypes and corresponding phenotypes:

XY^*, which are fully fertile males

XX, normal females

XX^Y*, which are sterile males in which gonad development is normal but spermatogenesis fails resulting in smaller testes after puberty. These males can be classified by manually palpating for testes size after 6 weeks of age. The testes are significantly smaller compared with any normal control male of the same age. Also these mice have a very large marker Chromosome (Fig 1, Eicher et al. 1991) easily seen after simple Giemsa staining (G-banding is not necessary. Alternatively these mice have nearly 2 complete X Chromosomes so these mice can be differentiated from normal XY<*> males by an Xist expression assay (Werler et al. 2011).

XY^*X females, which are fertile but litter size and number of litters produced will be less than expected for C57BL/6. These mice have a tiny cytogenetic marker (Fig 1, Eicher et al. 1991) easily seen after Giemsa staining of mitotic metaphase preparations.