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In the human, mutations ofOTX2(Orthodenticle homeobox 2transcription factor) translate into eye malformations of variable expressivity (even between the two eyes of the same individual) and incomplete penetrance, suggesting the existence of subtle thresholds in OTX2 activity. We have addressed this issue by analyzing retinal structure and function in six mutant mice with graded Otx2 activity:Otx2+/+,Otx2+/AA,Otx2+/GFP,Otx2AA/AA,Otx2AA/GFPandOtx2GFP/GFP. Null mice (Otx2GFP/GFP) fail to develop the head and are embryonic lethal, and compound heterozygousOtx2AA/GFPmice show a truncated head and die at birth. All other genotypes develop until adulthood. We analyzed eye structure and visual physiology in the genotypes that develop until adulthood and report that phenotype severity parallels Otx2 activity.Otx2+/AAare only mildly affected whereasOtx2+/GFPare more affected thanOtx2+/AAbut less thanOtx2AA/AAmice.Otx2AA/AAmice later manifest the most severe defects, with variable expressivity. Electrophysiological and histological analyses of the mouse retina revealed progressive death of bipolar cells and cone photoreceptors that is both Otx2 activity- and age-dependent with the same ranking of phenotypic severity. This study demonstrates the importance of gene dosage in the development of age-dependent pathologies and underscores the fact that small gene dosage differences can cause significant pathological states.