In this article we will discuss about the use of transduction of genetic mapping in viruses.
Viruses are the very small particles, hence they are considered unsuitable to study inheritance. Because scorable traits are not available with them. However, some of the bacterial characters have been used for inheritance and recombination studies such as plaque morphology (large or small, fuzzy or sharp), host range and virulence.
Plaques are the clear transparent area produced on opaque lawn of bacteria grown on the surface of solid medium in Petri plates. For the first time Alfred D. Hershey attempted to study the inheritance and recombination in viruses by using a cross (h–r+ x h+r–, where h = host range, r:= rough plaque) in bacteriophage T2. A circular genetic map of phage T2 and T4 is given in Fig. 8.22.
The genetic traits studied in this cross were host range and plaque morphology. The h+ infects strain 1 and h– infects both strains 1 and 2. The r+ lyses slowly and produces small plaques, whereas r– lyses rapidly and produces large plaques. Both the types of phages were used to infect E. coli strain 1 to facilitate mixed infection or double infection.
The lysate was spread over a bacterial lawn having the mixture of strains 1 and 2, and analysed. Four types of plaques were recorded: clear and small plaques (h–r+), cloudy and large plaques (h+r–), cloudy and small plaques (h+r+), and clear and large plaques (h–r–). The former two types of plaques have parental phenotypes, while the last two are the recombinants. Out of the four, the recombination frequency (RF) is calculated as below:
RF = (h+r+) + (h–r–)/Total plaques
Recombination in phage T4 has been studied by S. Benzer who resolved the fine structure of rll locus.