The y chromosome is believed to have evolved from the x chromosome by a series of steps, including all of the following except: a. suppression of crossing over, keeping the maleness gene associated with other male trait genes. b. mutations at other genes affecting male traits. c. a mutation on a proto-x chromosome causing maleness d. degeneration of other genes due to lack of crossing over. e. actually, all of these are proposed steps in the evolution of the y chromosome.