Adrenarche: Onset of pubic and axillary hair; normally occurs after age 8 years.

Adrenal glands: Glands that sit on top of the kidneys and regulate stress reactions and some mineral balance.

Adrenocorticotropic hormone (ACTH): A hormone secreted by the pituitary that stimulates the adrenal glands; part of the stress response system.

Allele: One of two or more alternative forms of a gene occupying corresponding sites (loci) on partner chromosomes.

Amenorrhea: Lack of menstrual periods.

Amniocentesis: A procedure in which a small amount of the amniotic fluid surrounding the fetus is drawn off and subjected to genetic and biochemical analysis.

Androgen: Any one of several male hormones.

Angelman syndrome: A rare syndrome reported in 1965 by Dr. H. Angelman and associated with a chromosome 15 deletion similar to that seen in PWS.
In AS, however, the deletion is seen on the chromosome contributed by the mother, whereas those with PWS lack certain genes from the father. The clinical problems and appearance of AS and PWS are distinctly different. These two syndromes represent the first examples in humans of genetic imprinting.

Autosome: Any chromosome that is not a sex chromosome; in humans there are 22 pairs of autosomes and one pair of sex chromosomes.


Chromosome: Package of genes that carries genetic information. Most cells have 46 chromosomes, which are found in the nucleus.

Chromosome abnormality: Any variation from the normal chromosomal pattern. The common abnormalities found in Prader-Willi syndrome are:

  • Deletion: a condition in which a piece of genetic material is lost or missing from a chromosome. Typically it is a small piece of the long arm of the 15th chromosome that is missing.
  • Translocation: a transfer of a fragment of one chromosome to another chromosome.

Cocontraction: The simultaneous contraction of all the muscles around a joint to stabilize it.

Cortisol: A hormone secreted by the adrenal glands in response to stress.

Cryptorchidism: Undescended testicles (testicles are not present in the scrotal sac).


DNA Deoxyribonucleic acid: The genetic code or chemical blueprint for the cell.

de novo new:  Used in genetics to describe a structural alteration of the chromosomes that is present in a child but not in either parent.

Deletion: A missing piece of chromosomal material. In PWS, the deletion is in the proximal long arm of chromosome 15.

Dominant: Capable of expression when carried by only one of a pair of partner chromosomes in the cells of the body.

Down syndrome: A syndrome resulting from three (instead of the usual two) copies of chromosome 21.

Dual-photon absorptiometry (DPA): A gamma radiation technique for measuring bone density.


Electrophoresis: In genetics, the technique for separating materials via the movement of charged particles suspended in a gel under the influence of an applied electric field.


Failure to thrive: Poor or inadequate weight gain during infancy.

FISH Fluorescence in situ hybridization: A test using a fluorescence-tagged segment of DNA that combines with the DNA present in the normal PWS critical region and allows determination by its presence or absence as to whether a 15q deletion is present.