A mouse with two fathers (left) and a control mouse (right) of the same age and sexZhikun LiMice whose genetic parents are both male have survived to adulthood for the first time. However, achieving this required extensive genetic modification, among other things, that could never be attempted in people.These arent the first mammals with same-sex parents mice with two mothers were first created in 2004, and while unusually small, they appeared to live even longer than normal. But creating mice with two fathers has proved more difficult.