Question

In four o'clock plants, many flower colors are observed. In a cross involving two true-breeding strains, one crimson and the other white, all of the \(P_{1}\) generation were rose color. In the \(F_{2}\), four new phenotypes appeared along with the \(P_{1}\) and \(F_{1}\) parental colors. The following ratio was obtaincd: \(1 / 16\) erimson \(2 / 16\) orange \(1 / 16\) yellow \(2 / 16\) magenta \(4 / 16\) rose \(2 / 16\) pale yellow \(4 / 16\) white Propose an explanation for the inheritance of these flower colors.

Short answer

Answer: The inheritance pattern responsible for the flower colors in this cross is a dihybrid inheritance pattern with incomplete dominance. Two separate genes with incomplete dominance, one for crimson pigment (C) and one for yellow pigment (Y), control the inheritance of the flower colors.

Step-by-step solution

Understanding the initial cross

The two parental strains with crimson and white flowers were true-breeding, meaning they were homozygous for their respective flower color genes. When crossed, the \(P_{1}\) generation showed an intermediate rose color, suggesting incomplete dominance.

Identifying the pattern in the \(F_{2}\) generation

Examine the ratio of flower colors in the \(F_{2}\) generation: \(1 / 16\) crimson \(2 / 16\) orange \(1 / 16\) yellow \(2 / 16\) magenta \(4 / 16\) rose \(2 / 16\) pale yellow \(4 / 16\) white The pattern follows a \(1:2:1:2:4:2:4\) ratio, which is derived from the product of two \(1:2:1\) ratios, suggesting that two separate gene interactions are responsible for the flower color inheritance.

Assigning alleles for flower colors

Assign two pairs of alleles, \(C\) and \(Y\), which control the crimson and yellow pigments in the flowers, respectively. The homozygous recessive genotype for both pairs of alleles will be white (e.g., \(ccyy\)).

Combining the genotypes with phenotypes

Now, combine the genotypes of alleles \(C\) and \(Y\) with the observed phenotypes: \(1 / 16\) crimson: \(CCYY\) \(2 / 16\) orange: \(CCYy\) \(1 / 16\) yellow: \(CCyy\) \(2 / 16\) magenta: \(CcYY\) \(4 / 16\) rose: \(CcYy\) \(2 / 16\) pale yellow: \(Ccyy\) \(4 / 16\) white: \(ccyy\)

Proposing an explanation for the inheritance of flower colors

The inheritance of flower colors in this case appears to be controlled by two separate genes with incomplete dominance, one responsible for crimson pigment (\(C\)) and the other for yellow pigment (\(Y\)). The \(C\) allele determines the degree of crimson pigmentation, with \(CC\) being fully crimson, \(Cc\) being partially crimson, and \(cc\) having no crimson pigment. The \(Y\) allele determines the degree of yellow pigmentation, with \(YY\) being fully yellow, \(Yy\) being partially yellow, and \(yy\) having no yellow pigment. The ratio of flower colors in the \(F_{2}\) generation can be explained by the interactions between these two genes and their alleles, revealing the \(1:2:1:2:4:2:4\) ratio. This suggests a dihybrid inheritance pattern with incomplete dominance at play.

Key concepts

Incomplete Dominance

Incomplete dominance is a fascinating situation in genetics where the phenotype of the offspring is a mix, or intermediate, between the phenotypes of the parents. In the context of four o'clock plants, this is clearly seen in the rose-colored flowers of the \(P_1\) generation. Unlike complete dominance, where one trait completely masks the other, incomplete dominance allows the traits to blend together.

Imagine the crimson and white parent plants as colors being mixed like paint. Crimson represents one pure color (let's call it "A" for full expression) and white represents another pure color ("a" for absence of expression). When combined, the result is neither fully crimson nor fully white, but instead, a beautiful rose color.

This rose color in the \(F_1\) generation represents incomplete dominance where neither the crimson nor the white completely prevail in the offspring. In the genetic symbols used, if \(C\) represents crimson and \(c\) represents lack of crimson, then the \(Cc\) genotype showcases the intermediate rose color.

Flower Color Genetics

The genetics of flower color can be quite intricate, involving not just one but sometimes multiple genes. In the case of our four o'clock plant scenario, two genes contribute to the variety of observed colors.

These two genes, represented here as \(C\) for crimson pigment and \(Y\) for yellow pigment, combine in different ways to produce the spectrum of flower colors seen in the \(F_2\) generation. Each plant can inherit combinations of these genes that lead to a wide range of colors:

• \(CCYY\): Full pigmentation, resulting in deep crimson.
• \(CCYy\): Slightly less pigmentation, appearing as orange.
• \(CCyy\): Only the yellow pigment is visible, giving a yellow flower.
• \(CcYY\): Partial pigmentation shows as magenta, a mix between crimson and rose.
• \(CcYy\): The intermediate rose color, with both pigments present but diluted.
• \(Ccyy\): Shows as pale yellow, less pigmented compared to "deep" shades.
• \(ccyy\): Lack of pigmentation, resulting in white flowers.

Through these combinations, flower color genetics illustrate how variations in genetic makeup can lead to varied phenotypic outcomes.

Gene Interactions

Gene interactions occur when different genes influence a single trait, producing a variety of phenotypes. This is what occurs with the flower colors in our four o'clock plants example. Here, gene interactions between two separate gene pairs create a complex inheritance pattern.

Essentially, each flower color is the result of multiple genes working together. The two gene pairs \(C\) (crimson) and \(Y\) (yellow) contribute to the observed phenotype based on their allelic combinations.

These interactions produce a dihybrid ratio, a classic signpost of such complex genetic behavior. For instance, the resulting \(1:2:1:2:4:2:4\) ratio in the \(F_2\) generation displays the unique interactions of these genes:

• Both dominant genes (\(CCYY\)) produce the most intense crimson shade.
• Presence of both recessive alleles (\(ccyy\)) results in white flowers.
• Intermediate forms (combinations of \(CcYy\), \(CCYy\)) lead to various intermediate colors like rose and magenta.

In this way, gene interactions illustrate the intricate dance genes perform to craft the visible phenotypes that we encounter in nature.