A, each chain of gene m has 1 free phosphate groups, so gene m contains 2 free phosphate groups, and the number of hydrogen bonds is 1.5N-n, which is an error;

B. The gene is composed of two deoxynucleotide chains. In the figure, A and b*** are the same as the gene M, so the allele M of the gene M cannot be represented by B, and B is wrong.

C. In the double helix structure of DNA, deoxyribose and phosphoric acid are alternately connected and arranged on the outside to form the basic skeleton, and C is wrong;

D and allele are produced by gene mutation, and gene mutation refers to the addition, absence or replacement of base pairs in a gene, so the number of bases or arrangement order of gene M and its allele M can be different, and D is correct.

Therefore, D. 。