sample size, general, genetic association, SW

discussion of/ and practical guide on sample size determination - in general
by Lenth, Effective sample size determination
Lenth is author of Java based software for sample size/ power estimation. it is very intuitive
classical web-based calculators of power and sample size for GENETIC ASSOCIATION studies are:
Purcell´s site:
Genetic power calculator

and Derek Gordon´s calculator, allowing researchers to take into account possible genotypic error and/or misclassification errror:

PAWE - Power for Association With Errors

also see Gordon´s article on factors affecting statistical power in the detection of genetic association which was published as part of the JCI review series on genetics of complex diseases.