Normally, you inherit 1 copy of each chromosome pair from your biological mother, and the other copy of the chromosome pair from your biological father. Uniparental disomy refers to the situation in which 2 copies of a chromosome come from the same parent, instead of 1 copy coming from the mother, and 1 copy coming from the father. Angelman syndrome (AS) and Prader-Willi syndrome (PWS) are examples of disorders that can be caused by uniparental disomy.
People with Angelman syndrome (AS) have an unusual facial appearance, short stature, severe intellectual disability with a lack of speech, stiff arm movements, and a spastic, uncoordinated walk. They may have seizures and often have inappropriate outbursts of laughter.
Angelman syndrome can result when a baby inherits both copies of a section of chromosome #15 from the father (rather than 1 from the mother and 1 from the father). AS can also occur even when chromosome #15 is inherited normally—1 chromosome coming from each parent. If that section of the mother's chromosome #15 is deleted, only the father's section will be present, allowing AS symptoms to occur. This deletion of a section of the maternally inherited chromosome is the most common cause of AS.
Prader-Willi syndrome (PWS), on the other hand, can result when a baby inherits both copies of a section of chromosome #15 from the mother. As with Angelman syndrome, PWS can also occur even if chromosome #15 is inherited normally. If that section of the father's chromosome #15 is deleted, only the mother's section will be present, allowing PWS symptoms to occur. This latter development happens in 70% of PWS cases.
Babies born with PWS have poor muscle tone and a weak cry. They initially are slow feeders and appear undernourished. The feeding problems improve after infancy. Typically, between 2 to 4 years of age, the child becomes obsessed with food and is unable to control their appetite. The overeating often results in rapid weight gain, obesity, and type 2 diabetes. People with PWS have short stature, small hands and feet, and intellectual disability.
Consult your healthcare provider or genetic counselor for more information on uniparental disomy.