2a = 8 kg or a = 4 kg. To determine whether the QTL shows dominance, we first need to compute the midpoint value, which is 12 + (20 - 12)/2 = 16 kg. The M1M2 marker class mean is 18 kg and is 2 kg above the midpoint. The degree of dominance is O/a, which in this case is 2/4 = 0.5. Therefore, in this example the QTL allele that increases body mass is 50% dominant to the allele that decreases body mass.

The difference in body mass between the GM M and Gm m marker class means is 20 kg - 12 kg = 8 kg. The total difference in body weight in these dogs is 30 kg - 9 kg = 21 kg. The QTL linked to this genetic marker therefore accounts for 8 kg of the total 21 kg difference in body weight, or 8 kg/21 kg = 38% of the total body mass difference between large and small dog breeds. The percentage of the total difference in the phenotypic value of the two P1 individuals is called the effect size of the QTL. In this hypothetical example, the QTL would be considered a major gene because its effect size is large. The QTL mapping results also tell us that there is partial dominance for the two QTL alleles that have been examined in this study based on the mean phenotypic value of individuals heterozygous for the marker locus.

It is important to notice that the expressions for a and O are both functions of the recombination rate between the QTL and the marker locus. This is because the perceived true effect of the QTL is confounded with the recombination rate in a single-marker QTL mapping analysis. The true additive effect of a QTL (or a) based on a and the recombination rate can be found by rearranging equation 9.35 to give a = ■

Similarly, the true dominance effect of a QTL (or d) based on O and the recombination rate can be found by rearranging equation 9.39 to give d = ■

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