Genetics question

[Dr Jekyll]

How closely do you have to be related to someone in order to have more genetically in common with them than with any random stranger?

I've seen a photo of someone who is almost definitely my third cousin and people tell me there is resemblance. But our common ancestor is my great great grandfather (maybe +- one great on his side), from which I presumably only inherited about 6% of my genes. So I wouldn't expect either of us to look like the ancestor never mind each other.

[Dr Jekyll]

Simpo Two said:

Well if you're choosing strangers at random they might be more closely related than you think! (random being a funny thing)

But generally I'd say anyone in your tree is going to be closer to you than a 'stranger'.

But even strangers are likely to be in my tree somewhere. Isn't almost every one in Europe supposed to be descended from the Emperor Augustus?

Most people can see a resemblance with their parents, IE a closer resemblance than with the typical stranger. Also with grandparents. But how far back can you go and still expect to see a genuine resemblance?

[Dr Jekyll]

I avoided putting this in the 'things you've already wanted to know the answer to' thread because I thought the science forum might produce more sensible responses.

[Dr Jekyll]

Nightmare said:

The various characteristics which make up your face are more likely to pop in your family tree, but not necessarily from those closer to you....but more likely than a random stranger

That makes sense, I've got great uncles that look nothing like me, and distant cousins with a resemblance. But there must become a point where someone in my family tree is so far removed that they are no more likely to look like me than a random stranger would.

Googling this topic I found this article.

http://www.dailymail.co.uk/femail/article-1222921/...

It suggests that princess Beatrice looks like Queen Victoria (well maybe). But also that prince William looks like Edward 1st! If he does I reckon it must be coincidence since Edward 1st lived so long ago at least half the country must be descended from him. And probably half the country in 1300 are Williams ancestors so why should he look like the King in particular.

OK resemblances might persist longer with Royals because of the level of inbreeding, but there must be a limit beyond which any resemblance must be coincidental, that's what I'm trying to pin down.