Question

The recessive sonic-hedgehog mutation (shh) causes a pheno type called syndactyl in which a human hand only grows 1 finger (called a digit). While a nother recessive mutation in gli3 causes a phenotype called polydactyl in which too many fingers form on a single hand (>5). The recessive mutation ling-ling (ll) in a gene important for cilia also causes polydactyl.

a) You intercross shh/+ x gli3/+ animals; your double mutants shh/shh; gli3/gli3 cause polydactyl. What is the relationship between shh and gli3?

b) Shh, Gli3 and IFT (ll) are all part of a signaling pathway which is a dependent pathway. Can you further describe the relationship between shh and gli3?

c) Can you perform the same experiment to determine the relationship between gli3 and ling-ling? Why or why not

d) What would you conclude if instead of shh;shh, gli3/gli3 double mutants displaying polydactyl the y displayed 5 fingers (normal).

Answer 1

a) Shh and gli3 are both control the normal proportion of cells in dorsal and ventral and generate suitable tissue size during the course of brain development. When crossed, the mutants show polydactyl, as the gli3 gene predominantly controls or regulates the Shh target gene. It can be concluded that the relationship between the two is of complete dominance where gli3 dominates and expressed itself over Shh.

b) IFT is known to negatively regulates the Shh pathway in the cilium at a step downstream of the Shh ligand. The disruption of the IFT is also known to accumulate Gli3 at the ciliary tip.

d) If the mutant displayed 5 fingers instead of expressing polydactyl, then it can be concluded that both the genes were repressed.