Final answer:
Gene conversion can be caused by both DNA mismatch repair and DNA gap repair, which are mechanisms involved in correcting errors during DNA replication or repairing DNA damage.
Step-by-step explanation:
The mechanisms that can cause gene conversion include DNA mismatch repair and DNA gap repair. DNA mismatch repair is the correction of errors that skipped the proofreading step during DNA replication. These errors usually involve base substitutions. The mismatch repair machinery recognizes the mismatch and repairs it, which can lead to gene conversion if it involves heteroduplex DNA, with one strand containing one allele and the complementary strand containing a different allele. DNA gap repair also can lead to gene conversion during the repair of DNA gaps that can occur during replication or recombination processes involving heteroduplex DNA. Lastly, the resolution of a Holliday junction, a four-stranded DNA structure that forms during homologous recombination, can also result in gene conversion if sister chromatids exchange genetic information. However, the resolution of Holliday junctions is not part of the options provided. Therefore the correct answer is D, Both a and b can result in gene conversion.