mutation in the fibroblast growth factor receptor (fgfr) gene results in achondroplasia (dwarfism), which is a dominant condition caused by a single base substitution (ggg>agg) that changes the amino acid glycine at position 480 to arginine (g480r) in the fgfr protein. normally, fgfr is active when fgf binds to it and inhibits excessive bone growth, but the mutant fgfr is constantly active even when fgf is not bound to it. how can you create a mouse model for achondroplasia?