Linkage information from transformation
Transformation is achieved through the uptake of naked DNA extracted from one strain of bacteria by another strain of bacteria. While extracting this DNA, some breakage into small pieces does take place. If two genes are closer to each other, they have better chance of being carried on the same piece of DNA, thus causing double transformation. On the other hand genes widely separated, will have a better chance of being carried on separate DNA segments and the frequency of double transformation will be the product of single transformation frequencies. Therefore a departure from product rule
of probability for the observed frequency of double recombinants will prove close linkage. Further high resolution mapping will be done by a method given in a subsequent section.
Recombination after gene transfer
When a chromosome segment is transferred from a donor to a recipient strain (or through transduction or transformation), this transferred segment must be integrated into host genome by an exchange mechanism to produce a stable recombinant. The recipient cell at this stage is called a merozygote (partial diploid) which has the complete genome of F-
, called endogenote
and an incomplete genome called exogenote derived from F+
or Hfr. An even number of crossovers, rather than a single or odd number of crossovers, allows incorporation of a part of the genome from exogenote into the endogenote; one of the two products of exchange (i.e. a linear fragment) is usually lost (Fig. 12.15).