Genome scans have enabled the detection of regions on chromosomes that contain genes that affect economic traits, so-called quantitative trait loci (QTL). An example is the genome scan that was conducted at ISU in an F2 cross between the Berkshire and Yorkshire breeds (Malek et al. 2001a,b). This study identified many QTL related to growth performance and meat quality. But this analysis only considered QTL with a Mendelian mode of expression. This implies that an effect of the Berkshire allele on the trait was assumed to be the same whether it was inherited from the F1 sire or from the F1 dam (see Figure 1). As a result, the two heterozygotes (BY and YB) are assumed to have the same effect (d). There is, however, evidence that the expression of some genes depends on their parental origin. For example, with paternal expression, a Berkshire gene for increased meat quality would only be expressed in the F2 progeny if it was inherited from the sire (Figure 2). In that case, individuals with QTL genotype BB and BY are expected to have the same genetic value (Figure 2), as do individuals with genotypes YB and YY. The inheritance mechanism for maternal expression is illustrated in Figure 3. In this case, genotype BB has the same value as YB, as do BY and YY
