Describe one possible effect of a gene mutation in a body cell on an organism and one possible effect on the offspring of that organism

A gene mutation may result in an oncogene. So during that cell's replication the gate keeper genes will no longer be able to suppress it. This results in a mutated daughter cell.  If there is a subsequent mutation an a daughter cell, a tumor will be produced. 

Gene mutation can also cause mosaicism and result in different phenotypes