Fly and moth genetic code. Computer illustration showing the genetic code (genome) of a fly (bottom) and moth (top). Research conducted on the genomes of numerous species demonstrates that all living things on Earth are related. If the genetic similarity between two species is large, their kinship is closest. For example, the DNA (deoxyribonucleic acid) of a moth and a fly differs in only 7 percent of the nitrogenous bases (letters) that form their respective sequences, indicating that both species share a common ancestor. DNA is composed of a sequence of four nitrogenous bases: Cytosine (C), Guanine, Adenine (A) and Thymine (T).