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It's obvious how a misdisjunction can result in klinefelter's syndrome (XXY) but I don't see how this can result in XYY syndrome. Your parents have a collective total of only one Y chromosome so how can any misdisjunction result in a child with two Y chromosomes?

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up vote 7 down vote accepted

It comes from a Y nondisjunction in the father. Anaphase nondisjunction of the Y yields a YY, which goes with one X from the mother to give XYY.

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en.wikipedia.org/wiki/XYY_syndrome#Cause during sperm generation – shigeta Mar 2 '13 at 22:14

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