Mendel’s Monohybrid Cross: Strongest Evidence Against Blending Theory

Mendel’s experiments overturned the blending theory of inheritance, which predicted traits would permanently mix like paint colors. The strongest evidence came from his pea plant crosses, revealing discrete inheritance instead.

Correct Answer

(C) The recessive phenotype showed up in the F2 progeny

This observation directly refutes blending theory, as the recessive trait vanished in F1 but reappeared unchanged in F2, proving traits remain distinct rather than merging permanently. Blending would eliminate the pure recessive form forever, yet Mendel recovered it in a 1:4 ratio among F2 progeny. This recovery of parental traits intact supports particulate inheritance via discrete factors (genes).

Option Explanations

(A) 3:1 Ratio of Phenotypes in F2 Generation

The 3:1 phenotypic ratio (dominant:recessive) in F2 arises from self-pollinating heterozygous F1 plants (e.g., Tt x Tt yielding TT, Tt, tt). While it shows segregation, blending theory could weakly accommodate ratios via uneven mixing, so it’s not the strongest disproof. This ratio confirms dominance and segregation laws but doesn’t uniquely shatter blending like trait recovery does.

(B) All Progeny of F1 Exhibited Dominant Phenotype

F1 hybrids (e.g., Tt) always show the dominant trait, masking recessive ones completely. Blending theory expected intermediate phenotypes (e.g., medium height), so this challenges it somewhat. However, blending proponents might argue temporary dominance, making F1 less conclusive than F2 recovery of pure recessives.

(C) Recessive Phenotype in F2 Progeny (Correct)

As noted, the unchanged return of recessive traits (e.g., white flowers or green seeds) in F2 is decisive. No blending occurs—recessives persist latently and reemerge pure, proving factors don’t dilute irreversibly. This mirrors Mendel’s violet-white flower cross: F1 all violet, F2 3 violet:1 white.

(D) Observation of Incomplete Dominance

Incomplete dominance produces intermediates (e.g., pink flowers from red-white cross), actually mimicking blending. Mendel didn’t observe this in his monohybrid crosses; his traits showed complete dominance. This option supports rather than refutes blending, as seen later in non-Mendelian examples like snapdragons.

This table highlights why (C) stands out for biology exams on Mendelian genetics.