Angelmann and Prader-Willi syndromes are two distinct genetic conditions with different symptoms, both caused by loss of a part of chromosome 15. Children who inherit one copy of this faulty chromosome develop either Angelmann or Prader-Willi syndrome, despite having a normal copy of the chromosome from their other parent. So how does the same mutation (loss of part of chromosome 15) lead to these two different conditions? The answer lies in the discovery that this particular piece of chromosome 15 contains a number of genes that are imprinted, so only the paternal or maternal copy of these gene are expressed; which of the two syndromes appears depends on whether the deletion was in the maternal or paternally inherited chromosome. When the faulty chromosome is inherited from Dad, there is no functional copy of the imprinted genes that are switched off on the maternal chromosome 15 and the result is Angelmann syndrome and vice versa for Prader-Willi syndrome. This is quite unlike most genetic conditions such as cystic fibrosis, where an effect on development or health is only seen when a mutated gene or genes is inherited from both parents.
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