During replication which involved the duplication of the DNA helix, the replication machinery which includes helicases, topoisomerases, single strand binding proteins, DNA polymerase, DNA primase, ligases etc are involved in the synthesis of the new DNA strands. Each of these enzymes play different role in the process. The DNA polymerase performs the functions of proofreading (checking for the incorporation of the correct nucleotide) and also functions in addition of complementary nucleotides to the new DNA strand.
For the answer to the question above,<span> I think the answer is simply because the X chromosome is obtained from both father and mother. they are not now both mixed and it's almost impossible to study the x chromosome patterns only</span>
