Dear Ahmet Okumuş Starting from an open-pollinated population, continued selfing rapidly separates the population into several lines. During early generations (S2, S3 and S4), several lethal and sub-vital types appear, and selection for such highly heritable traits are highly effective. From S5 generation onwards, one can practice selection for plant height, length of ear shoot, etc (quantitative traits) as the lines attain considerable homozygosity (> 95%), and the effects of dominance and non-allelic interactions have been dissipated. For further details of employing various approaches of selection, you can go through the related chapters in the following books:
1. Principles of Plant Breeding by Robert Allard,
2. Breeding Field Crops by John Poehlman, and
3. Quantitative Genetics in Maize Breeding by Arnel Halluer and JB Miranda Filho