What about lemons in the family? Not all Bernoulli's were top shelf, not all Bach's were JS or PDQ level.
Not all of Bach, Bernoulli, and Euler's children were lemons.
"One bad apple" disproves both extreme nature and extreme nurture theories. See also 2nd generation business inheritors, and statesmen.
I do like the Noether example of offspring going further than parent.