Genetic code. Computer artwork of a baby and a circular DNA sequence. The pattern of bands represents the sequence of chemicals called nucleotide bases that form the genetic code for a section of DNA (deoxyribonucleic acid). The four bases are represented by letters: A (adenine), C (cytosine), G (guanine) and T (thymine), seen through the baby. Each person has a unique genetic code. Certain highly-variable sections of DNA provide an individual genetic fingerprint that may be used in a paternity dispute, for example.