Abstract
In all studies, the effect on body weight is smaller in heterozygous than in homozygous knockout mice, but the exact degree of dominance is not clear. In heterozygous Mc4r+/− animals, body mass is increased on average by about 7–45% and in homozygous Mc4r−/− by 50–100% compared to WT2–5 with substantial overlap between groups. The mutations might have a sex dependent effect, but the results are contradictory. In one study, the effect in males was only about half of that in females.2 However, two studies did not detect a sex by genotype interaction in this Mc4r−/− strain.4,5 One study in a different knockout line of the same inbred strain found a sex by genotype interaction in the opposite direction, with only a marginal effect in heterozygous females whereas heterozygous males had a body weight intermediate between WT and homozygous knockouts.3

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