The possible explanations for the fact that men contribute only about twice as many new point mutations to their offspring as do women are as following:
• The X-chromosome is more than twice larger than that of the Y chromosome. Since the number of mutations depends on the size of the chromosome, the Y chromosomes accumulate less mutations that the X chromosomes
• X chromosomes from a female gamete undergo recombination with its homologue, whereas X chromosome from a male gamete does not. The male gametes do not have a homologue of X chromosome. Thus, there are chances that recombination adds to the number of mutations in one of the X chromosomes
• Since male gametes are produced after a number of cell divisions, there is possibility of back mutations to occur. These back mutations reduce the number of point mutations. For example, during cell division an A-T base pair is mutated to G-C base pair. During subsequent cell divisions, if another point mutation occurs at the same base pair, the sequence is reversed to A-T base pair. Such a mutation is called back mutation. Back mutations restore the original nucleotide sequence. Since female gametes do not undergo extensive replication, the probability of a back mutation is rare