Human DNA is made up of 23 pairs of chromosomes: one set from the mother and one set from the father. But sometimes a genetic glitch causes a child to inherit both chromosomes from one parent, an occurrence called uniparental disomy. Angelman and Prader-Willi syndromes are examples of this malfunction on chromosome number 15.

Angelman syndrome occurs when a child’s chromosome 15 pair both come from the father. Named after the British physician Harry Angelman (1915–1996), who first described it in 1965, this genetic disorder causes developmental and intellectual delays. Children with Angelman syndrome often have difficulty walking, speaking, and balancing; they also have happy, excitable personalities prone to outbursts of smiling and laughing. Other symptoms include stiff, jerky movements; seizures; and a small head size. It’s estimated that this fairly rare condition affects 1 in 20,000 births. Children with Angelman syndrome may require antiseizure medication, as well as physical and behavioral therapy.

The sister syndrome to Angelman is Prader-Willi, in which a child inherits both chromosomes 15 from the mother. This condition is named after the Swiss pediatricians Andrea Prader (1919–2001) and Heinrich Willi (1900–1971), who first wrote about it in 1956. Symptoms include poor muscle tone (infants often feel like loose rag dolls when they’re held), underdeveloped sex hormones, and delayed motor development. Children with the condition often have behavioral problems; they tend to be stubborn and can be prone to obsessive-compulsive disorders. Experts believe that Prader-Willi syndrome affects about 1 in 12,000 to 15,000 births.

ADDITIONAL FACTS

  1. Uniparental disomy is a completely random event.
  2. Physicians initially diagnose Prader-Willi and Angelman syndromes based on behavior and distinctive facial features. They confirm them by ordering a karyotype, a genetic test that reveals all chromosomal sets.