Final answer:
Excess exposure to ultraviolet light and X-rays increases the frequency of sister chromatid exchanges as a result of DNA damage and the subsequent DNA repair processes.
Step-by-step explanation:
Exposure to excess ultraviolet (UV) light and X-rays affects the formation of sister chromatid exchanges by increasing their frequency. UV light can lead to the formation of thymine dimers, causing DNA damage by bonding adjacent pyrimidine nucleotides. This damage can trigger the DNA repair mechanisms that result in sister chromatid exchanges as the cellular machinery attempts to correct the errors.
Similarly, strong ionizing radiation such as X-rays can cause more severe damage to DNA, including single- and double-stranded breaks in the DNA backbone, as well as modification of DNA bases. This severe damage can also increase sister chromatid exchanges during the process of DNA repair. Therefore, the correct answer to the question is (a) Increases sister chromatid exchanges.