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Heterozygotes for PAX6 mutations have aniridia homozygous for PAX6 mutations devoid (absence) of eye structures CREB cause Rubenstein-Taybi syndrome, pleiotropic set of abnormalities mutations of PAX3 Type I Waardenburg syndrome: Defects include eye color changes, white forelock and deafness . mutations in MITF gene and PAX3 Type II Waardenburg syndrome: Defects include eye color changes, white forelock and deafness . MITF expression is limited to pigment cells PAX3 is required for the expression of MITF HOX genes a special class of transcription factors that can transform one body part into another. ex. Polydactyly= extra digits mutations of Sonic hedgehog morphogen (SHH) (a paracrine factor) can cause holoprosencephaly: Failure of midface and forebrain to develop Leading to cleft lip and palate and absence of forebrain structures Dysmorphology study of birth defects like Cornelia de Lange syndrome Robin sequence Common pathophysiological pattern of defects with a variety of diff. causes Ex. restriction of mandibular growth Teratology= the study of teratogenes ex. Thalidomide Half of all birth defects are polygenic origin