Degrees of Separation

I read once that if you go back 500yrs, assuming 25yr average per generation, there will have been 1 million people involved in your future birth. Assuming that each person has 2 parents, 4 grandparents (I have 6, but thats another story) 8 great grandparents and so on. Of those great grandparents, I only met 3, none of whom are alive at present. I didn't meet any gg grandparents. So closely linked (4 degrees of separation) and yet so far.

I met my nana's youngest surviving brother two days ago for the first time. She was the oldest in a family of 8, he was the second to youngest. I was stunned to see how strongly I could see the family resemblance in him, even in the way he held himself, and the intonations he used when he spoke.

In australia, I met a man who has a common ancestor with me - his gg father is my 4th great grandfather. But he told me after meeting me, that I am just like his oldest grand daughter - in looks, mannerisms and way of talking. He could only assume it was through our common line.

So what are these things called genes? And what determines which variants get put into some people in one generation and not others? I apparently *look like my mother, have my fathers expressions, look just like my sister, look nothing like her at all, am just like my mothers mother, have the "ormsby" look, and look like a distant cousin who i've never met before?

If we're all linked to so many people, ancestors, cousins, descendants, like threads in a tapestry woven together, I suppose it would be possible for one ancestors influence to spread exponentially down the line. But i wonder what determines who gets what part of that influence.

*all subjective comments from other people