XXYY Syndrome

In this variation, someone is born with an extra copy of both an X and Y chromosome, resulting in a 48XXYY chromosome pattern instead of the typical 46XY. People with XXYY chromosomes may often have undescended testes and produce lower-than-typical amounts of testosterone, which can cause them to start puberty later than typical and to develop less pronounced secondary sex characteristics like muscle tone and facial hair. They may also experience breast growth and often have other physical features similar to those developed by people with Klinefelter Syndrome (47XXY).

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