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.