hi,

I would do it this way:

1. let q= 1- Na/N the probability of NOT seeing the trait if you pick one sample

2. the probability of not having a single sample with the trait in a sample of n is q^n.

3. You want tokeep this probability below a limit L = say 5% - which is equivalent to a probability y 95% of having one or more peple carrying the trait in your sample, so you can set L = q^n.

So, you just need to solve the equation which gives you n = log L / log q

regrds