"You might say, what is this telling us?
The answer is, we don't always know, but I'll tell you one thing-- four of those genes encode different subunits of exactly the same multi-protein complex that is a certain calcium channel in neurons.
That's not an accident.
That tells you in a way you would have never guessed.
And only genetics can tell you that particular L-type calcium channel-- four subunits there-- carry mutations, all of which are associated." 7.00x Intro to Biology- The Secret of Life
"It is up to about 47 genes that have been associated with hereditary risk of breast cancer. Some of them have weak effects.
They explain, oh, I don't know, about a 1/6 of the inheritance.
There are a bunch of strong mutations-- Mendelian-like forms-- they explain about a 1/6 so far. Together, they might explain about 1/3 so far. It's by no means done. But this is what's going on."
"A polygenic disease is a disease that is associated with mutations in more than one gene. For many polygenic diseases, the mutations associated with the disease do not directly result in a disease phenotype but are, instead, correlated with an increase in the risk of having the disease phenotype. "
Eric Lander PhD, "Intro to Biology, the Secret of Life", MOOC MITx , June 2013.