Introduction
Much of the previous attention with respect to the ESR Locus has focused primarily on reproductive traits due to the role of steroid hormones such as estrogen in reproduction. Rothschild et al. (1994, 1996) and Short et al. (1995, 1997) published reports identifying an additive effect of the ESR B allele on increasing litter size in Meishan and Large White crossbred populations. The presence of this favorable litter size allele is limited primarily to the Taihu breeds and the domestic Large White and Yorkshire breeds. Its origin in the Large White and Yorkshire breeds is likely from the Taihu breeds due to the lack of segregation in other domestic breeds and the historical interbreeding of pigs of the Taihu group with European pigs that eventually gave rise to the Large White breed prior to 1800 (Porter, 1993, cited in Jones, 1998). While the Taihu breeds such as Meishans are reproductively superior to the European breeds, they are notably inferior in regards to production and carcass traits (Haley et al., 1992). Because of these characteristics and the likelihood that Chinese germplasm is the origin of the ESR B allele, selection for this allele in a marker-assisted selection program may bring about detrimental effects on economically important production and carcass traits.