Question

Sol.(b) [Key Concept: Mutagens — X-rays and Crossing Over] Exposure to X-rays (ionising radiation) enhances the frequency of crossing-over (recombination). X-rays cause DNA double-strand breaks, and when these are repaired by homologous recombination, crossing over occurs. Muller (1927) showed that X-rays dramatically increase mutation rates and also increase crossing-over frequency. This is why X-rays are used to map genes by increasing recombination frequencies.