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