What is epigenetics? Genetics and the Human Genome Read more from Chapter DNA, RNA, Chromosomes, and Genes

Just as the term “epidermis” refers to the layer of the skin above (or beyond) the dermis, the term “epigenetics” describes nongenetic causes of a phenotype. For example, in genetic imprinting, a different expression of an allele is produced depending on the parent who transmits it. In humans, an example comes from two medical problems—both that result from a deletion on chromosome 15: In one called the Prader-Willi syndrome—originating from a deletion on the paternal chromosome 15—affected children have small hands and feet, short stature, mental retardation, and are obese. In the Angelman syndrome—originating from a deletion on the maternal chromosome 15—affected children have a large mouth and tongue, severe mental and motor retardation, and a happy disposition, accompanied by excessive laughter.