Obviously intelligence has a genetic factor, there is no question about that. Why is cat's intellect different from humans? Due to its DNA. But that is not the point, when it comes to humans, there is no significant enough variation that could lead into large scale intellectual differences. Likewise due to the genetic mixing you couldn't label one traditionally understood "race" as one or the other. Furthermore it makes little sense from evolutionary perspective.

The child genius comment is not laughable. A genius is always tied to a particular skill given in a cultural context. For example playing a piano, or juggling. It is possible that a child is born with certain innate physical qualities that help him in whatever he practices. Like being fair, or strong, tall, etc... But to assume that any 5 year old learned to play piano as some kind of a genetic gift is far more ludicrous. It takes hard practice to perfect your body and mind to work in unison for any particular task, approximately the same for everyone bar perhaps the effects of personal motivation and rigor of the exercise.

With the exception of people with disorders that in some way may make them amazingly good at certain tasks, like having eidetic memory.