Genma began his training under the cruel and perverted Happosai, Master of the Anything Goes School of Martial Arts. With Soun Tendou, he learned most of what there is to being a martial artist. He also learned to be a complete fawning coward in order to appease the wrath of his master.
Later in life, after dumping one girlfriend, he married the much richer and much more cutter Nadoka. Together, they only had a son, Ranma. Being the practical sort, he made arrangements with Soun Tendou, ,who had three young daughters, that Ranma would marry a Tendou daughter and inherit the Tendou Family Anything Goes Dojo.
Then Genma took his son on the first of several training journeys. Nadoka objected to not being able to have time with her son, but Genma convinced her that having a mother's constant tenderness would make Ranma soft, and unfit to be a martial artist. Nadoka grudgingly agreed, but only when Genma promised upon his life that he would make Ranma a 'man among men'.
On these journeys, it quickly became apparent that Genma was not nearly so honest or noble as he would like to believe. He would promise his son in marriage to anyone, if it meant a free meal. He proved to be a ridiculous cheat and spendthrift. Instead of buying boat or plane tickets to make their way across the sea from Japan to China, Genma insisted that he and Ranma could swim the distance. On the last training journey, all this changed. Ranma knocked Genma into the fabled 'Spring of Drowned Panda' and changed his life for ever. Now Genma spends most of his time as a panda, avoiding his wife, whom he is sure is going to enforce his vow.
Remember that pandas can't talk, so he doesn't have to explain his actions. If he does want to communicate, he usually uses a painted sign board (which he uses a weapon, sometimes), although he does resort to charades on occasion.