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Chapter 4: Problem 33

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 \(\mathrm{F}_{1}\) generation were rose color. In the \(\mathrm{F}_{2}\) four new phenotypes appeared along with the \(P_{1}\) and \(F_{1}\) parental colors. The following ratio was obtained: \(1 / 16\) crimson \(4 / 16\) rose \(2 / 16\) orange \(\quad 2 / 16\) pale yellow 1/16 yellow \(\quad 4 / 16\) white \(2 / 16\) magenta Propose an explanation for the inheritance of these flower colors.

Short Answer

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Question: Explain the inheritance of flower colors in four o'clock plants, given the observed phenotypic ratios in the F1 and F2 generations. Short Answer: The inheritance of flower colors in four o'clock plants is determined by two interacting genes. The first gene (A) is responsible for pigment production (crimson vs. white), while the second gene (B) controls the intensity or amount of pigment. The observed phenotypic ratios in the F1 and F2 generations can be explained by the genotypes of these two genes and their interactions, resulting in a variety of flower colors.

Step by step solution

01

Understand the Parental (P1) and F1 Generations

The problem states that there are two true-breeding parental strains, one crimson (P1) and the other white (P1). When these are crossed, all F1 generation plants are rose-colored. This suggests that the genes causing the crimson and white colors are co-dominant and create an intermediate color (rose) when both are present.
02

Examine the F2 Generation Phenotypic Ratios

Looking at the F2 generation phenotypic ratio, we have 1 crimson, 4 rose, 2 orange, 2 pale yellow, 1 yellow, 4 white, and 2 magenta, amounting to 16 plants. This ratio can be simplified as 1:4:2:2:1:4:2, which suggests that there are two interacting genes involved in the color inheritance.
03

Assign Alleles for Two Genes

Let's assign alleles for our proposed two-gene model. For the first gene, let's use A for the crimson allele and a for the white allele. For the second gene, let's use B for the pigmented allele and b for the less pigmented allele.
04

Determine the Genotype of F1 Generation

Based on the F1 generation having rose color, we can determine that their genotype is AaBb. Since both parental strains are true-breeding, the crimson parent's genotype would be AABB, and the white parent's genotype would be aabb.
05

Predict the F2 Generation Genotype and Phenotype Ratios

For F1 generation plants with AaBb genotype, when allowed to self-fertilize, we expect the following F2 genotypes and corresponding phenotypes: 1 AABB (Crimson) 2 AABb (Rose) 1 AAbb (Orange) 4 AaBB (Rose) 2 aaBB (White) 4 AaBb (Rose) 2 aabb (Pale Yellow) 1 AAbb (Yellow) 2 AaBB (White) 1 aaBB (Magenta) The above F2 distribution gives us the phenotypic ratio 1:4:2:2:1:4:2 (Crimson:Rose:Orange:Pale Yellow:Yellow:White:Magenta), which matches the observed phenotypic ratio in the exercise.
06

Propose an Explanation for the Inheritance of Flower Colors

Based on the observed phenotypic ratios, we propose that the inheritance of flower colors in the four o'clock plants is determined by two interacting genes. The first gene (A) is responsible for pigment production, with A contributing to the crimson color and a contributing to the white color. The second gene (B) is responsible for the intensity or amount of pigment, with B generating more pigmented flowers and b generating less pigmented flowers. When these genes interact, they create the variety of observed flower colors in the F1 and F2 generations.

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Most popular questions from this chapter

In this chapter, we focused on many extensions and modifications of Mendelian principles and ratios, In the process, we encountered many opportunities to consider how this information was acquired. Answer the following fundamental questions: (a) How were early geneticists able to ascertain inheritance patterns that did not fit typical Mendelian ratios? (b) How did geneticists determine that inheritance of some phenotypic characteristics involves the interactions of two or more gene pairs? How were they able to determine how many gene pairs were involved? (c) How do we know that specific genes are located on the sexdetermining chromosomes rather than on autosomes? (d) For genes whose expression seems to be tied to the sex of individuals, how do we know whether a gene is X-linked in contrast to exhibiting sex-limited or sex-influenced inheritance? (e) How was extranuclear inheritance discovered?

A husband and wife have normal vision, although both of their fathers are red- green color-blind, inherited as an X-linked recessive condition. What is the probability that their first child will be (a) a normal son, (b) a normal daughter, (c) a color-blind \(\operatorname{son},(d)\) a color-blind daughter?

What genetic criteria distinguish a case of extranuclear inheritance from (a) a case of Mendelian autosomal inheritance; (b) a case of \(\mathrm{X}\) -linked inheritance?

Three gene pairs located on separate autosomes determine flower color and shape as well as plant height. The first pair exhibits incomplete dominance, where color can be red, pink (the heterozygote), or white, The second pair leads to the dominant personate or recessive peloric flower shape, while the third gene pair produces either the dominant tall trait or the recessive dwarf trait. Homozygous plants that are red, personate, and tall are crossed with those that are white, peloric, and dwarf. Determine the \(P_{1}\) genotype(s) and phenotype(s). If the \(F_{1}\) plants are interbred, what proportion of the offspring will exhibit the same phenotype as the \(F_{1}\) plants?

In cattle, coats may be solid white, solid black, or black-andwhite spotted. When true-breeding solid whites are mated with true-breeding solid blacks, the \(\mathrm{F}_{1}\), generation consists of all solid white individuals. After many \(\mathrm{F}_{1} \times \mathrm{F}_{1}\) matings, the following ratio was observed in the \(\mathrm{F}_{2}\) generation: \(12 / 16\) solid white \(3 / 16\) black-and-white spotted \(1 / 16\) solid black Rxplain the mode of inheritance governing coat color by determining how many gene pairs are involved and which genotypes yield which phenotypes. Is it possible to isolate a true-breeding strain of black-and-white spotted cattle? If so, what genotype would they have? If not, explain why not.
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