Co-transduction frequency is inversely proportional to
Co-transduction scoring after P1 or P22 generalized transduction measures how often two bacterial markers transfer together in same phage head. Packaging capacity limits fragment size; genes within same head length co-occur frequently. If distance between genes exceeds headful, they cannot fit together, co-transduction drops to zero. Intermediate separations show recombination between them inside recipient reducing co-inheritance. Therefore frequency is inversely proportional to map distance: very close genes show near 100% co-transduction, farther genes show lower percentages, enabling fine-structure mapping and ordering using Wu formula d = [1 - (cofreq)^(1/3)] × length.
Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 6: Cotransduction Frequency and Distance