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paradox

the Prader-Willi and Angelman syndromes

Chromosome 15

DNA

fetus

30,000 genes didn't appear enough to explain human complexity. There had to be something they'd missed. The first hints of what was missing lay in the curious paradox of the Prader-Willi and Angelman syndromes, two quite different diseases caused by exactly the same genetic fault. When Pembrey looked at the inheritance pattern for their conditions he noticed something even stranger.
What really mattered was the origin of the Chromosome 15 that had the deletion. If the deletion was on the Chromosome 15 that the child had inherited from father then you had Prader-Willi syndrome. Whereas, if the, er, deletion, er, was inherited from the mother, er, you had the Angelman syndrome.
It was a complete surprise that the same missing strip of DNA could cause one disease when it came from the mother and a completely different disease when it came from the father. It was as if the genes knew where they came from.
You've got a developing fetus manifesting this condition. How does the Chromosome 15 know where it came from? It, there must have been a tag or an imprint placed on that chromosome during either egg or sperm formation in the previous generation to, to say "Hi, I came from mother, I came from father" and we are functioning differently. So that's the key thing that although the DNA sequence is the same the different sets of genes were being silenced depending on whether it came from the mother or from the father.

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