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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.



Rr

R		□□

r		□□

Verified Answer :

  • Identify the genotypes of the parents as \(Rr\) for both pink flowers.
  • Populate the Punnett square by crossing the alleles \(R\) and \(r\) from each parent.
  • Observe the resulting genotype frequencies: \(1\) \(RR\), \(2\) \(Rr\), and \(1\) \(rr\).
  • Conclude that the offspring genotype distribution is \(\boxed{1 RR: 2 Rr: 1 rr}\).

Explanation

  1. Analyze the genetic problem and inheritance pattern

In genetics, incomplete dominance occurs when the heterozygote phenotype is intermediate between the two homozygous phenotypes. For snapdragons, red color is represented by the allele \(R\), and white color by the allele \(r\). The homozygous dominant genotype \(RR\) produces red flowers, the homozygous recessive genotype \(rr\) produces white flowers, and the heterozygous genotype \(Rr\) produces pink flowers. We are tasked with crossing two pink flowers, which means both parents have the genotype \(Rr\).

  1. Complete the Punnett square for the cross Rr × Rr

To predict the outcomes of a cross between two heterozygous parents (\(Rr \times Rr\)), we use a Punnett square. We place the alleles from the first parent across the top row and the alleles from the second parent along the left side of the grid. Each cell represents a potential genotype for the offspring resulting from the combination of the corresponding row and column alleles.

\(R\)\(r\) \) \(R\)\(RR\)\(Rr\) \) \(r\)\(Rr\)\(rr\) \)
  1. Determine the genotypic and phenotypic outcomes

By analyzing the completed Punnett square, we can determine the ratio of the offspring's genotypes:

  • \(1/4\) are \(RR\) (homozygous dominant, Red)
  • \(2/4\) or \(1/2\) are \(Rr\) (heterozygous, Pink)
  • \(1/4\) are \(rr\) (homozygous recessive, White)

Thus, the predicted phenotypic ratio for the offspring of two pink snapdragons is \(1\) Red : \(2\) Pink : \(1\) White.

Examples

Punnett squares are essential tools in biology for predicting the genetic outcome of crosses. For example, a plant breeder could use this method to estimate the proportion of different colored flowers in a field of snapdragons. If they start with 100 pink snapdragons and cross them randomly, this calculation predicts that approximately 25 will be red, 50 will be pink, and 25 will be white, helping them manage their garden's diversity.

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