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Incomplete Dominance:

An example of incomplete dominance is seen in snapdragons. Snapdragons have three phenotypes for flower color. Red flowers are homozygous dominant for the red allele (RR), white flowers are homozygous recessive (rr), and pink flowers are heterozygous (Rr).


ATwo pink flowers are crossed. Complete the Punnett square to predict the outcome of the cross between these two heterozygous parents.



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Verified Answer :

  • Identify parent genotypes as \(Rr\) for both pink snapdragons.
  • Use a Punnett square to cross the gametes \(R\) and \(r\) from both parents, resulting in one \(RR\), two \(Rr\), and one \(rr\) offspring.
  • Identify the corresponding phenotypes: \(RR\) is red, \(Rr\) is pink, and \(rr\) is white.
  • The phenotypic outcome of the cross is a ratio of \(1 \text{ Red} : 2 \text{ Pink} : 1 \text{ White}\).

Explanation

  1. Analysis of the Genetic Cross

In snapdragon flowers, color is inherited through incomplete dominance. This means that neither the red allele (\(R\)) nor the white allele (\(r\)) is fully dominant. Instead, the heterozygous genotype (\(Rr\)) produces a phenotype that is a blend of the two, which is pink. The homozygous genotypes (\(RR\) and \(rr\)) represent the red and white phenotypes, respectively. We are crossing two pink-flowered snapdragons, both of which have the heterozygous genotype (\(Rr\)).

  1. Constructing the Punnett Square

To determine the possible outcomes of the cross between two heterozygous pink snapdragons (\(Rr \times Rr\)), we use a Punnett square. Each parent produces two types of gametes, \(R\) and \(r\). By distributing these gametes across the grid, we can visualize the resulting zygote combinations. The Punnett square is constructed as follows:

\[ \begin{array}{|l|l|l|} \ & R & r \\ R & RR & Rr \\ r & Rr & rr \\ \end{array} \]

  1. Predicting the Genotypic and Phenotypic Outcomes

Based on the Punnett square, the offspring genotypes are distributed as follows:

  • 1 cell contains \(RR\), which corresponds to the red phenotype.
  • 2 cells contain \(Rr\), which corresponds to the pink phenotype.
  • 1 cell contains \(rr\), which corresponds to the white phenotype.

Thus, the genotypic ratio is \(1 RR : 2 Rr : 1 rr\), and the phenotypic ratio is \(1 \text{ Red} : 2 \text{ Pink} : 1 \text{ White}\). This means there is a \(25\%\) chance of producing a red flower, a \(50\%\) chance of producing a pink flower, and a \(25\%\) chance of producing a white flower.

Examples

Incomplete dominance helps breeders understand and predict color variations in plants and animals. For instance, if you want to grow a specific proportion of pink roses in a garden, understanding that crossing two pink-flowered plants is necessary to maintain that specific distribution (rather than just crossing red and white) allows you to plan your garden layout more effectively. This concept is fundamental to genetics and agricultural science for managing traits in various species.

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