Practice question

Pax6 homozygous null mice known as Small eye mutants display complete absence of eyes, nose and pancreas defects. Genetic lesion eliminates functional paired and homeodomain, preventing expression of downstream lens, retinal and corneal genes. Optic vesicle forms initially but arrests because retinal progenitor specification fails and surface ectoderm remains incompetent. Contact between vesicle and ectoderm cannot initiate crystallin expression. Heterozygotes show microphthalmia with small lens and iris defects, reflecting dosage sensitivity. Thus loss of both alleles results in anophthalmia rather than isolated lens abnormality, demonstrating master control function across whole eye field.