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Chapter 6: Problem 62

A pure-breeding strain of squash that produced diskshaped fruits (see the accompanying illustration) was crossed with a pure-breeding strain having long fruits. The \(\mathrm{F}_{1}\) had disk fruits, but the \(\mathrm{F}_{2}\) showed a new phenotype, sphere, and was composed of the following proportions: long 32 sphere 178 disk 270 Propose an explanation for these results, and show the genotypes of the \(\mathrm{P}, \mathrm{F}_{1},\) and \(\mathrm{F}_{2}\) generations.

Short Answer

Expert verified
The P generation genotypes are \(DD\) (disk) and \(LL\) (long). The F1 genotype is \(DL\) (disk). F2 shows a 9:3:4 phenotypic ratio, indicative of interaction between two genes.

Step by step solution

01

Analyze P Generation Genotypes

In the P generation, there are two pure-breeding strains: one with disk-shaped fruits and another with long fruits. Pure-breeding strains are homozygous, meaning their genotypes contain identical alleles. Let's assign \(DD\) to the disk parent and \(LL\) to the long parent.
02

Predict F1 Generation Genotypes

The F1 generation is produced by crossing the P generation plants. Crossing \(DD\) (disk) with \(LL\) (long) results in all offspring having the genotype \(DL\), as these are the only possible alleles they can inherit from their parents. Phenotypically, these all express disk shape, given the dominance of the disk allele.
03

Understand Inheritance Pattern

The presence of a new sphere phenotype in the F2 generation suggests incomplete dominance or interaction between two genes. However, the proportions hint at a simple dominance and segregation pattern typical of two gene interaction.
04

Determine F2 Generation Ratio and Genotypes

The F2 generation is a result of self-crossing F1 individuals \((DL)\). Performing a Punnett Square, the possible combinations are: \(DD\), \(DL\), \(LD\), and \(LL\). With the ratios from F2: 32 long (\(LL\)), 178 sphere (likely \(LD\) or \(DL\) with incomplete dominance), 270 disk (\(DD\)). This pattern suggests that sphere is an intermediate phenotype between disk (dominant) and long (recessive).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Punnett Square
The Punnett Square is a simple diagram used to predict the genetic outcomes of a cross between two organisms. Although it looks like a grid with four boxes, it helps us understand how alleles from each parent come together in their offspring. To create a Punnett Square, you place one parent's alleles across the top and the other parent's alleles down the side. Each box within the grid represents a possible genotype for the offspring. This visualization is essential for mapping out potential inheritance patterns.
  • It provides a visual way to calculate all possible combinations of alleles and their corresponding traits.
  • Using a Punnett Square helps predict the ratio of different genotypes and phenotypes among offspring.
  • It is especially helpful in understanding simple Mendelian genetics, where traits are determined by single genes with two alleles.
In the exercise, the Punnett Square shows us how alleles from pure-breeding disk and long fruit plants combine in the F1 generation and segregate in the F2 generation.
Homozygous
Homozygous refers to an organism that has two identical alleles for a particular gene. In genetics, alleles are variants of the same gene. When a parent is homozygous, it means there are no alternate variants at that specific gene locus, resulting in predictability regarding offspring traits.
  • Homozygous genotypes are often designated by repeated letters, like DD or LL, based on the alleles.
  • These genotypes are common in pure-breeding lines, which consistently produce offspring with the same phenotype.
  • Being homozygous for a dominant allele ensures that dominant trait expression, as seen in the disk parents of our exercise.
In the given exercise, the P generation consisted of two homozygous plants, ensuring the purity and predictability of trait inheritance in initial crosses.
Dominance and Recessiveness
Dominance and recessiveness describe how certain traits are expressed when different alleles are present. A dominant allele overshadows a recessive one, determining the trait observed in an organism. This concept is pivotal in understanding inheritance patterns.
  • A dominant trait will express even if only one of its alleles is present, such as the disk shape in our squash.
  • Recessive traits require two identical alleles to be expressed, like the long fruit shape appearing only when both alleles are present.
  • Sometimes, when neither allele is completely dominant, an intermediate form like the sphere fruit will appear due to incomplete dominance or gene interaction.
Understanding which alleles exhibit dominance or recessiveness provides insight into the potential phenotypes of offspring, as it did for the F2 generation in our exercise.

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Most popular questions from this chapter

You have been given a virgin Drosophila female. You notice that the bristles on her thorax are much shorter than normal. You mate her with a normal male (with long bristles and obtain the following \(\mathrm{F}_{1}\) progeny: \(\frac{1}{3}\) short-bristled females, \(\frac{1}{3}\) long-bristled females, and \(\frac{1}{3}\) long-bristled males. A cross of the \(\mathrm{F}_{1}\) long- bristled females with their brothers gives only long-bristled \(\mathrm{F}_{2}\) A cross of short-bristled females with their brothers gives \(\frac{1}{3}\) short-bristled females, \(\frac{1}{3}\) long-bristled females, and \(\frac{1}{3}\) long-bristled males. Provide a genetic hypothesis to account for all these results, showing genotypes in every cross.

Mice of the genotypes \(A / A ; B / B ; C / C ; D / D ; S / S\) and \(a / a ; b / b ; c / c ; d / d ; s / s\) are crossed. The progeny are intercrossed. What phenotypes will be produced in the \(\mathrm{F}_{2}\) and in what proportions? [The allele symbols stand for the following: \(A=\) agouti, \(a=\) solid (nonagouti); \(B=\) black pigment, \(b=\) brown; \(C=\) pigmented, \(c=\) albino; \(D=\) nondilution, \(d=\) dilution (milky color); \(S=\) unspotted \(s=\) pigmented spots on white background.

An allele \(A\) that is not lethal when homozygous causes rats to have yellow coats. The allele \(R\) of a separate gene that assorts independently produces a black coat. Together, \(A\) and \(R\) produce a grayish coat, whereas \(a\) and \(r\) produce a white coat. A gray male is crossed with a yellow female, and the \(\mathrm{F}_{1}\) is \(\frac{3}{8}\) yellow, \(\frac{3}{8}\) gray, \(\frac{1}{8}\) black, and \(\frac{1}{8}\) white. Determine the genotypes of the parents.

In sweet peas, the synthesis of purple anthocyanin pigment in the petals is controlled by two genes, \(B\) and \(D\) The pathway is a. What color petals would you expect in a purebreeding plant unable to catalyze the first reaction? b. What color petals would you expect in a purebreeding plant unable to catalyze the second reaction? c. If the plants in parts \(a\) and \(b\) are crossed, what color petals will the \(\mathrm{F}_{1}\) plants have? d. What ratio of purple: blue:white plants would you expect in the \(\mathrm{F}_{2}\) ?

Because snapdragons (Antirrhinum) possess the pigment anthocyanin, they have reddish purple petals. Two pure anthocyaninless lines of Antirrhinum were developed, one in California and one in Holland. They looked identical in having no red pigment at all, manifested as white (albino) flowers. However, when petals from the two lines were ground up together in buffer in the same test tube, the solution, which appeared colorless at first, gradually turned red. a. What control experiments should an investigator conduct before proceeding with further analysis? b. What could account for the production of the red color in the test tube? c. According to your explanation for part \(b\), what would be the genotypes of the two lines? d. If the two white lines were crossed, what would you predict the phenotypes of the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) to be?
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