F1 plants in pedigree breeding are not selected because they are:
F1 generation derived from a cross of two genetically distinct pure lines carries both parental genomes in heterozygous state, but every F1 individual receives exactly the same set of alleles: one allele from each parent at every locus. There is no segregation because all F1 plants originate from gametes of homozygous parents, so genotype Aa Bb is identical across the population. Consequently phenotypic variation within F1 is purely environmental, not heritable. Selection among identical genotypes yields no genetic advance, as predicted by breeder's equation R = h²S where genetic variance is zero. Additionally, F1 exhibits maximum heterozygosity and often heterosis, masking recessive traits that breeders aim to expose through segregation. Effective selection must wait until meiosis in F1 shuffles alleles, creating genetic variability in F2 through segregation and recombination, where distinct homozygous and heterozygous combinations become selectable and reveal transgressive segregation useful for further improvement and fixation. Breeder's equation shows zero selection response when genetic variance is absent, confirming that attempting selection in uniform F1 wastes resources and delays generation advance without creating new allelic diversity.
Ref: Hartl & Clark Principles of Population Genetics; Acquaah Chapter 4: F1 uniformity and heterozygosity