usually a sample size calculation is the same for agriculture studing, try to use the neyman formula: n= (N(sxt)²) / ((N-1)d² + (sxt²) where, n: sample size; N: population size; s:standard deviation, d: acceptebal erreur (5%); t: confidence limit (1.96)
you can perform phenotypic and phenotypic correlation along with path coefficient analysis to identify what traits has important role in the expression of genes in the genome.