In the F_2 generation, the genotypic and phenotypic ratios are...

In a Mendelian monohybrid cross, the phenotypic ratio in the $F_2$ generation is $3:1$, whereas the genotypic ratio is $1:2:1$.

In the case of co-dominance (and incomplete dominance), the heterozygous condition produces a distinct phenotype where both alleles are expressed equally. When two heterozygotes are crossed, the $F_2$ generation yields a genotypic ratio of $1:2:1$ (homozygous dominant : heterozygous : homozygous recessive) and a phenotypic ratio of $1:2:1$.

Thus, the genotypic and phenotypic ratios are identical in co-dominance.

Answer: Co-dominance