Hedgehog (Hh), and many other signaling pathways, were originally discovered in Drosophila. Hedgehog was so named because of bristles all over the early embryo, resembling a hedgehog. It works, with other genes, to develop the anterior-posterior axis of the embryo.
The mutant develops bristles all over the embryo instead of just in a stripe toward the anterior. Thus Hh is a segment polarity gene. The hedgehog gene family in vertebrates is involved in development of central and peripheral nervous system, skeleton limbs, lungs, skin and germ cells. Shh (Sonic hedgehog) is responsible for major development of brain, spinal cord, and skeleton and is received at the membrane via the patched receptor (PTCH). In the absence of Shh, PTCH inhibits the activity of the pathway, while binding of Shh allows signal transduction to go forward.
In humans, holoprosencephaly is a developmental disorder involving incomplete cleavage of the forebrain, leading to the abnormalities of brain and facial structures. From the above, which abnormalities of genes must be involved?
A. either Shh or PTCH
B. Shh only
C. some other member of the Hh gene family
D. PTCH only
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