The sonic hedgehog is a toolkit gene, which has multiple roles in development. The regulatory mutations of Sonic hedgehog gene is dominant and viable because it acts as a morphogen. It is a morphogen produced by zone of polarizing activity. The gene Shh regulates organogenesis in vertebrates, as it acts as a morphogen.
Depending on the concentration of Shh, it has various effects on the developing cells of embryo. As it functions with a concentration and time dependent mechanism, it helps in the induction of various transcription factors in the ventral progenitor cells. Cell division of adult stem cells is controlled by shh.
The mutations in the protein coding sequence will have impact on the phenotype. The translation occurs in the coding sequence. It starts from the initiation codon or start codon with the help of initiation factors and nearby sequences.
The mutations cause reduction of the expression of proteins in upstream of open reading frames, which in turn results in widespread defects in the organisms.
The short sequence elements in the UTRs of messenger Ribonucleic acid (mRNA) control the regulation of gene expression. In eukaryotes, the ribosomes load on the 5' cap of mRNA. Then they scan for first start codon that is AUG or methionine. Hence, the upstream open reading frame can affect the translation of downstream coding sequence. This is the cause of widespread defects, if mutations occur in coding sequences.