What map unit (Centimorgan) is adopted in the construction of genetic maps?
(1) A unit of distance between two expressed genes representing 10% cross over.
(2) A unit of distance between two expressed genes representing 100% cross over.
(3) A unit of distance between genes on chromosomes, representing 1% cross over.
(4) A unit of distance between genes on chromosomes, representing 50% cross over.