4.2.2.1. Incomplete Dominance

When experiments on peas were repeated using other traits in other plants, it was found that sometimes the F1 had a phenotype that did not resemble either of the two parents and was in between the two.

Incomplete dominance refers to a situation where the phenotypes of the offspring (the F1 generation) do not resemble either parent completely but are an intermediate blend of both. This contrasts with complete dominance, where one trait fully masks the other. In this case, when looking at traits further than just the pea plants, it shows that sometimes genes can interact in a way that neither allele completely dominates the other.

Imagine mixing red paint and white paint: you may get pink paint as the result. This is similar to how incomplete dominance works, where the traits from both parents blend to create a new appearance that is distinct from both.

The inheritance of flower colour in the dog flower (snapdragon or Antirrhinum sp.) is a good example to understand incomplete dominance. In a cross between true-breeding red-flowered (RR) and true-breeding white-flowered plants (rr), the F1 (Rr) was pink.

In the case of snapdragons, crossing one plant with red flowers (RR) and another with white flowers (rr) produced offspring (F1 generation) with pink flowers (Rr). Here, the red trait (R) is not completely dominant over the white trait (r), resulting in the pink phenotype, which is a mixture of the two parental phenotypes. When these pink flowers are self-pollinated, the resulting F2 offspring exhibit a 1:2:1 genotypic ratio but a 1:2:1 phenotypic ratio as well, showing a distinct segregation of traits.

Think of making a smoothie with strawberries and bananas. If you blend them together, you don't just get a banana smoothie or a strawberry smoothie; you get a strawberry-banana smoothie. Similarly, in this genetic example, the mixed inheritance creates a new 'flavor' — the pink color.

When the F1 was self-pollinated, the F2 resulted in the following ratio 1 (RR) Red: 2 (Rr) Pink: 1 (rr) White. Here the genotype ratios were exactly as we would expect in any mendelian monohybrid cross, but the phenotype ratios had changed from the 3:1 dominant : recessive ratio.

In the F2 generation, when the pink-flowered plants (Rr) are crossed, the expected results reveal a ratio of 1 red flowered (homozygous dominant), 2 pink flowered (heterozygous), and 1 white flowered (homozygous recessive). This shows a clear deviation from the typical 3:1 ratio of traits seen in Mendelian dominance. This change points to the nature of gene interaction in incomplete dominance, where the phenotype is a blend rather than one trait completely hiding another.

Consider mixing different fruits together in a fruit salad. When you mix different fruits, you can clearly see pieces of each type, rather than one flavor overpowering the rest. Just as in a fruit salad, where each fruit contributes to the overall flavor, the incomplete dominance in snapdragons allows for a combination of traits, instead of one trait dominating.

What happened was that R was not completely dominant over r, and this made it possible to distinguish Rr as pink from RR (red) and rr (white).

In incomplete dominance, we see that neither allele is completely dominant over the other. The allele for red flowers (R) is not able to completely mask the effect of the allele for white flowers (r). This interplay results in the pink-flowered plants being recognized as distinct from both red and white plants, showcasing a clear example of how gene expression can lead to various visible outcomes that are neither fully one nor the other.

Imagine if you were cooking a dish and decided to use both spicy and mild peppers. If you used too many spicy peppers, it could cover the flavor of the mild peppers completely, resulting in a very spicy dish. However, if the two flavors blend well, creating a nicely balanced dish, it reflects incomplete dominance — both flavors matter, leading to a dish that isn't just spicy or mild but something in between.

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