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. Author manuscript; available in PMC: 2013 Sep 1.
Published in final edited form as: Mech Ageing Dev. 2012 Jul 31;133(9-10):581–590. doi: 10.1016/j.mad.2012.07.005

Figure 1. Hypothetical norms of reaction.

Figure 1

mean expression levels of a hypothetical gene for two genotypes G1, G2 measured at different ages (after Bell (1990)). a, no interaction: G2 is more highly expressed at all ages, so genotype responses to age have correlation=+1; trait variance does not change with age. b, variance-type interaction: G2 is always more highly expressed (correlation=+1) but genotype means diverge with age (and consequently variance increases with age). c, correlation-type interaction: responses of G1 and G2 do not have correlation=+1 since ranks of G1, G2 change with age; trait variance does not change monotonically with age.