Practice question

Pax6 exhibits haploinsufficiency. Heterozygous Small eye mice carry one null allele, reducing transcription factor dosage to 50 percent. Eye field still forms but retinal progenitor pool proliferation, lens induction and iris development are attenuated. Resulting eyes are significantly smaller, termed microphthalmia, with cataracts, corneal opacification and underdeveloped retina. Dosage threshold differs among targets; lens placode markers require higher Pax6 levels than optic vesicle evagination. Human PAX6 heterozygotes show aniridia, similar dosage effect. Complete absence requires homozygous null, whereas heterozygous produces small eye phenotype reflecting quantitative gene regulation.