MADISON, Wis. — Johnny Walsh, who is nearly blind, stood before an audience at the Comedy Club on State and joked about whether he wants to have children. “I feel like if I had a kid, it would just be one really long game of Marco Polo,” said Walsh, 32, who is legally blind from a condition called Usher syndrome. His brother, Mike Walsh, 37, has the same genetic disorder, marked by progressive loss of peripheral vision after childhood and significant hearing loss at birth. The condition causes …