Practice question

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.