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#heterozygous mutants

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Eye Lens Induction -l

What happens in Pax6 heterozygous mutants?

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.

Ref: Glaser et al. Science 1990; Gilbert Chapter 7: Pax6 dosage - heterozygous small eye formation.

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Eye Lens Induction -ll

What happens in Pax6 heterozygous mutants?

Pax6 exhibits dosage-sensitive haploinsufficient phenotype. Heterozygous mutants (Pax6 +/-) display small eye phenotype known as Small eye in mouse or Sey, microphthalmia, lens hypoplasia, and corneal opacities due to reduced Pax6 dosage insufficient to fully activate lens and retinal targets. Homozygous loss eliminates eye entirely. No extra lens forms, neural retina specification requires Pax6 but small eye includes both lens and retina hypoplasia rather than loss of neural retina alone, reflecting proportionally decreased activation of Six3, Sox2, and crystallin promoters by half dosage of transcription factor.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Pax6 haploinsufficiency – small eye phenotype in heterozygotes.

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