Understanding the Phenotypic Ratio of Incomplete Dominance

In genetics, incomplete dominance is a type of inheritance where one allele is not completely dominant over the other. This results in a third phenotype that is a blend or intermediate between the phenotypes of the two homozygous parents.

Here are the key characteristics of incomplete dominance:

• Neither allele is dominant over the other.
• Heterozygous individuals express a phenotype that is distinct from, and often intermediate to, the phenotypes of the homozygous individuals.
• The genotypic ratio and the phenotypic ratio are the same.

Let's consider a classic example, such as flower color in snapdragons, where the allele for red flowers ($\text{R}$) is incompletely dominant over the allele for white flowers ($\text{r}$).

• Homozygous dominant ($\text{RR}$) plants have red flowers.
• Homozygous recessive ($\text{rr}$) plants have white flowers.
• Heterozygous ($\text{Rr}$) plants have pink flowers (an intermediate phenotype).

If we cross a red-flowered plant ($\text{RR}$) with a white-flowered plant ($\text{rr}$):

Parental (P) Generation: $\text{RR}$ (Red) $\times$ $\text{rr}$ (White)

Gametes: $\text{R}$ and $\text{r}$

F1 Generation: All offspring are $\text{Rr}$ (Pink)

Now, if we self-pollinate the F1 generation plants (cross $\text{Rr}$ with $\text{Rr}$):

F1 Generation: $\text{Rr}$ (Pink) $\times$ $\text{Rr}$ (Pink)

Gametes: $\text{R}$ and $\text{r}$ from both parents

To find the genotypes and phenotypes of the F2 generation, we can use a Punnett square:

From the Punnett square, we can see the following genotypes in the F2 generation:

• 1 $\text{RR}$
• 2 $\text{Rr}$
• 1 $\text{rr}$

So, the genotypic ratio is $\text{RR} : \text{Rr} : \text{rr} = 1 : 2 : 1$.

Now let's look at the phenotypes corresponding to these genotypes:

• $\text{RR}$ genotype produces Red flowers.
• $\text{Rr}$ genotype produces Pink flowers.
• $\text{rr}$ genotype produces White flowers.

Therefore, the phenotypic ratio in the F2 generation is Red : Pink : White, which corresponds directly to the genotypic ratio:

Phenotypic ratio = 1 (Red) : 2 (Pink) : 1 (White)

This 1 ∶ 2 ∶ 1 ratio is characteristic of incomplete dominance when crossing two heterozygous individuals.

Comparing this with the given options, the ratio 1 ∶ 2 ∶ 1 represents the phenotypic ratio of incomplete dominance.