169 5.4 Bacteriophage genetics To make the cross, E. coli strain 1 is infected with both parental T2 phage genotypes. This kind of infection is called a mixed infection or a double infection (Figure 5-25). After an appropriate incubation period, the phage lysate (the progeny phages) is then analyzed by spread-ing it onto a bacterial lawn composed of a mixture of E. coli strains 1 and 2. Four plaque types are then distin-guishable (Figure 5-26). Large plaques indicate rapid lysis ( r ± ) and small plaques slow lysis ( r ² ). Phage plaques with the allele h ± will infect both hosts, forming a clear plaque, whereas h ² results in a cloudy plaque because one host is not infected. Thus the four geno-types can easily be classiﬁed as parental ( h ± r ² and h ² r ± ) and recombinant ( h ² r ² and h ± r ± ), and a recom-binant frequency can be calculated as follows: If we assume that the recombining phage chromo-somes are linear, then single crossovers produce viable reciprocal products. However, phage crosses are subject
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