Genetic epilepsy and developmental disorders in non-affected parents are sometimes de novo disorders, but are also sometimes caused by mosaicism in a parent, which means that the parent has one kind of DNA in some cells and a different kind in other cells. As a result, the risk factor genes may evade detection by normal DNA tests that only look at one, easy to access site. These variants can make their way a child's DNA in vitro after initial fertilization of the egg giving rise to a zygote.
This is discussed in ASHG 2016 Conference Presentations 295, 296 and 297.
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