Question about phylogenies and DNA sequencing
Genes reflect a phylogeny of their own. So no, the genes for "shape" (if they existed) would be very different between dolphins and sharks. Every single gene in a dolphin is more similar to its homologous counterpart in a cow than to any gene in a shark. Gene by gene, or with the entire genome, it would not matter. The similarities between sharks and dolphins are due to convergence. Convergence make sense in that there is selective pressure to have streamlines bodies to swim through the water. It is possible for genes to converge as well, but the nucleotides would differ, just as the fins on sharks and dolphins differ.