Subject: How many people would it take to form a viable colony?

I actually need 3 different answers to this question, based on different 
assumptions, for a possible novel.

If I wanted to send an expidition into deep space to establish a colony, how 
many unrelated, fertile mating pairs (from the same ethnic background) would be 
needed to form the basis of a viable colony, assuming that they would never 
have contact with other humans again?  

If the colonist practiced polygamy, with each person having three spouses, how 
many colonists would be needed then?  The assumption here is that no couple 
would be allowed to mate if they had a common ancester within three generations.

Is it possible to create a viable colony with a lower male:female ratio?  What 
would the minimum ratio be?

Thanks!

Re: How many people would it take to form a viable colony?