Law of Dominance

The core idea of the Law of Dominance is that when two different versions of a trait come together, one version can hide the other. This explains why a cross between two pure-breeding parents (say, tall and dwarf) produces offspring that all look like only one parent (all tall in the F1 generation), and why the hidden version reappears in the next generation (the F2 generation) in a fixed ratio.

Mendel proposed this law based on three key observations he made from his experiments:

• Characters are controlled by discrete units called factors. These factors are what we now call genes. They are not blended or lost when passed from parent to offspring; they remain as separate, independent units.
• Factors occur in pairs. Every individual carries two copies of each factor — one inherited from each parent. For a given character, an organism may have two identical factors (homozygous) or two different factors (heterozygous).
• In a dissimilar pair of factors, one member of the pair dominates (dominant) the other (recessive). When the two factors are different, only the effect of the dominant factor is visible in the organism. The recessive factor's effect is not expressed, but it is not destroyed — it remains hidden.

The law of dominance is used to explain two key patterns seen in a monohybrid cross:

1. Expression of only one parental character in the F1 generation. In a cross between a tall plant (with two dominant factors) and a dwarf plant (with two recessive factors), all F1 offspring inherit one factor from each parent. Because the tall factor is dominant over the dwarf factor, every F1 plant appears tall. The dwarf character is completely masked.
2. Expression of both parental characters in the F2 generation. When the F1 plants (which carry one dominant and one recessive factor) are self-pollinated, the two factors segregate during gamete formation. The resulting F2 generation shows both tall and dwarf plants. The recessive character, which was hidden in the F1, reappears. …