Consider a bacterium having m+, n+ and q+ genes on its chromosome. As soon as the phage DNA enters inside, the bacterial DNA is fragmented. Each fragment has one gene say m+, n+, and q+ respectively. Bacteriophages reproduce normally. However, an error usually happens during the assembly. Defective phages pack the bacterial genes such as m+, n+ and q+ into their heads. When the progeny phages are released, some may have a normal genome while others may have a bacterial gene m+, n+ or q+ respectively. These phages, later on, infect other bacteria. In this case, the bacterial cells act like recipients since the transducing phages carrying donor genes infect them. Consider a transducing phage carrying an m+ gene segment. When this phage infects a recipient bacterium, there can be a genetic exchange of donor m+ gene with the recipient m gene. It occurs by a double cross-over. The transduced bacterium is stable and is known as a transductant. Suppose we try designing an experiment by selecting the markers in the donor and the recipient cells. Such cells exhibit prototrophic transductants. It is possible to transduce two or more genes simultaneously. A process of simultaneous transduction is known as cotransduction. Two or more marker genes can be cotransduced. There are two types of cotransduction. In one type of cotransduction, the genes closer to each other are packed. In another type of cotransduction, the distant genes are packed due to infection of the bacterial cell by two phages. However, this type of cotransduction is extremely rare. Thus, cotransduction occurs mostly in closely linked genes.