Genetics is the field of scientific research that studies gene activity in plants, animals, and humans. Genes are segments of DNA (deoxyribonucleic acid) found in each living cell; each of these DNA segments codes for a protein, thereby yielding a phenotypic effect. All life on Earth shares the chemical make-up of DNA, even though each species differs in the number and function of genes. Scientists estimate that human DNA contains between thirty-one and thirty-six thousand genes arrayed over two pairs of twenty-three chromosomes. The forty-six human chromosomes are strands of DNA, with each of the twenty-three strand pairs arranged as a double helix. The DNA strands are composed of four base chemicals: adenine (A), guanine (G), cytosine (C), and thymine (T ). These four bases are typically identified by their single letter abbreviations (A,G,C,T ) and constitute an alphabet, so to speak, that carries...