To solve this problem, we need to understand the concept of map units, also known as centimorgans, used in genetic mapping.$\newline \bullet$A centimorgan (cM) is a unit of measure for genetic linkage. It represents the distance between chromosome positions (loci) for which the expected average number of intervening chromosomal crossovers in a single generation is 0.01.$\newline \bullet$This means that a 1% recombination frequency between two genes corresponds to a distance of 1 centimorgan.$\newline \bullet$Therefore, a map unit or centimorgan is defined as the distance between genes on a chromosome that results in a 1% crossover frequency.$\newline$Now, let's evaluate the options:$\newline \bullet$Option 1: A unit of distance between two expressed genes representing 10% crossover.$\newline \bullet$Option 2: A unit of distance between two expressed genes representing 100% crossover.$\newline \bullet$Option 3: A unit of distance between genes on chromosomes, representing 1% crossover.$\newline \bullet$Option 4: A unit of distance between genes on chromosomes, representing 50% crossover.$\newline$The correct definition of a centimorgan is given in Option 3, which states that it represents a 1% crossover.$\newline$Therefore, the correct option is Option 3.