Sensory organ capable of detecting light. F1 generation The first generation of offspring from a cross between two varieties or individuals. In Mendelâ€(tm)s experiments, all the F1 offspring were heterozygous hybrids with a dominant phenotype. F2 generation The second generation of offspring in a breeding experiment; the offspring from a mating between two F1 hybrids. In Mendelâ€(tm)s monohybrid experiments, the ratio of dominant to recessive phenotypes in the F2 generation was 3:1.