Effect of QTL genotype on birth weight. Left panel: Birth weight in litters that do not segregate for the QTL, produced by matings between homozygous parents. Right panel: Birth weight in litters that segregate for the QTL, produced by matings between heterozygous parents. Crosses represent individual pups, and circles represent the average litter mass per female. Average masses are not shown for segregating litters because they contain pups of more than one genotype.