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Ch. 4 - Gene Interaction
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 4 - Gene InteractionProblem 8c
Chapter 4, Problem 8c

Two genes interact to produce various phenotypic ratios among F₂ progeny of a dihybrid cross. Design a different pathway explaining each of the F₂ ratios below, using hypothetical genes R and T and assuming that the dominant allele at each locus catalyzes a different reaction or performs an action leading to pigment production. The recessive allele at each locus is null (loss-of-function). Begin each pathway with a colorless precursor that produces a white or albino phenotype if it is unmodified. The ratios are for F₂ progeny produced by crossing wild-type F₁ organisms with the genotype RrTt.
9/16 green : 3/16 yellow : 3/16 blue : 1/16 white

Verified step by step guidance
1
Define the genetic basis of the problem: The two genes, R and T, interact to produce different phenotypes. The dominant alleles (R and T) catalyze reactions leading to pigment production, while the recessive alleles (r and t) are null and do not contribute to pigment production. The phenotypic ratio of 9:3:3:1 suggests a dihybrid cross with epistatic interactions between the genes.
Establish the pathway for pigment production: Start with a colorless precursor. Gene R catalyzes the first reaction, converting the precursor into an intermediate pigment (e.g., yellow). Gene T catalyzes the second reaction, converting the intermediate pigment into the final pigment (e.g., green). If either gene is nonfunctional (homozygous recessive), the pathway is disrupted, leading to different phenotypes.
Assign phenotypes to genotypes: (1) R_T_ (both genes functional) results in the green phenotype because both reactions occur. (2) R_tt (functional R but nonfunctional T) results in the yellow phenotype because the first reaction occurs, but the second does not. (3) rrT_ (nonfunctional R but functional T) results in the blue phenotype because the precursor cannot be converted to the intermediate pigment, but T acts on a different substrate. (4) rrtt (both genes nonfunctional) results in the white phenotype because no reactions occur.
Explain the phenotypic ratios: The 9:3:3:1 ratio arises from the independent assortment of the two genes during meiosis. The genotypes R_T_, R_tt, rrT_, and rrtt correspond to the phenotypes green, yellow, blue, and white, respectively. The probabilities of these genotypes are determined by the Punnett square for a dihybrid cross (RrTt × RrTt).
Summarize the genetic interactions: This problem demonstrates a case of complementary gene action, where the interaction between two genes determines the final phenotype. The dominant alleles at each locus are required for specific steps in the pathway, and the absence of either allele disrupts pigment production, leading to distinct phenotypes.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Dihybrid Cross

A dihybrid cross involves two traits, each controlled by different genes, typically represented by two pairs of alleles. In this case, the genes R and T are being studied, where each gene can have a dominant or recessive allele. The phenotypic ratios observed in the offspring (F₂ generation) arise from the independent assortment of these alleles during gamete formation, leading to a variety of combinations in the progeny.
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Punnet Square

Phenotypic Ratios

Phenotypic ratios describe the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In the given scenario, the ratio of 9/16 green, 3/16 yellow, 3/16 blue, and 1/16 white indicates how the interactions between the alleles of genes R and T influence pigment production. Understanding these ratios is crucial for predicting the outcomes of genetic crosses and the underlying genetic mechanisms.
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Mutations and Phenotypes

Gene Interaction

Gene interaction occurs when the effects of one gene are modified by one or more other genes, leading to a combined effect on the phenotype. In this case, the dominant alleles of genes R and T catalyze different reactions that contribute to pigment production from a colorless precursor. The specific interactions between these genes can result in various phenotypes, as seen in the diverse ratios of the F₂ progeny.
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Related Practice
Textbook Question

The wild-type color of horned beetles is black, although other colors are known. A black horned beetle from a pure-breeding strain is crossed to a pure-breeding green female beetle. All of their F₁ progeny are black. These F₁ are allowed to mate at random with one another, and 320 F₂ beetles are produced. The F₂ consists of 179 black, 81 green, and 60 brown. Use these data to explain the genetics of horned beetle color.

621
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Textbook Question

Two genes interact to produce various phenotypic ratios among F₂ progeny of a dihybrid cross. Design a different pathway explaining each of the F₂ ratios below, using hypothetical genes R and T and assuming that the dominant allele at each locus catalyzes a different reaction or performs an action leading to pigment production. The recessive allele at each locus is null (loss-of-function). Begin each pathway with a colorless precursor that produces a white or albino phenotype if it is unmodified. The ratios are for F₂ progeny produced by crossing wild-type F₁ organisms with the genotype RrTt.

9/16 dark blue : 6/16 light blue : 1/16 white

349
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Textbook Question

Two genes interact to produce various phenotypic ratios among F₂ progeny of a dihybrid cross. Design a different pathway explaining each of the F₂ ratios below, using hypothetical genes R and T and assuming that the dominant allele at each locus catalyzes a different reaction or performs an action leading to pigment production. The recessive allele at each locus is null (loss-of-function). Begin each pathway with a colorless precursor that produces a white or albino phenotype if it is unmodified. The ratios are for F₂ progeny produced by crossing wild-type F₁ organisms with the genotype RrTt.

12/16 white : 3/16 green : 1/16 yellow

381
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Textbook Question

Two genes interact to produce various phenotypic ratios among F₂ progeny of a dihybrid cross. Design a different pathway explaining each of the F₂ ratios below, using hypothetical genes R and T and assuming that the dominant allele at each locus catalyzes a different reaction or performs an action leading to pigment production. The recessive allele at each locus is null (loss-of-function). Begin each pathway with a colorless precursor that produces a white or albino phenotype if it is unmodified. The ratios are for F₂ progeny produced by crossing wild-type F₁ organisms with the genotype RrTt.

9/16 red : 7/16 white

432
views
Textbook Question

Two genes interact to produce various phenotypic ratios among F₂ progeny of a dihybrid cross. Design a different pathway explaining each of the F₂ ratios below, using hypothetical genes R and T and assuming that the dominant allele at each locus catalyzes a different reaction or performs an action leading to pigment production. The recessive allele at each locus is null (loss-of-function). Begin each pathway with a colorless precursor that produces a white or albino phenotype if it is unmodified. The ratios are for F₂ progeny produced by crossing wild-type F₁ organisms with the genotype RrTt.

15/16 black : 1/16 white

377
views
Textbook Question

Two genes interact to produce various phenotypic ratios among F₂ progeny of a dihybrid cross. Design a different pathway explaining each of the F₂ ratios below, using hypothetical genes R and T and assuming that the dominant allele at each locus catalyzes a different reaction or performs an action leading to pigment production. The recessive allele at each locus is null (loss-of-function). Begin each pathway with a colorless precursor that produces a white or albino phenotype if it is unmodified. The ratios are for F₂ progeny produced by crossing wild-type F₁ organisms with the genotype RrTt.

9/16 black : 3/16 gray : 4/16 albino

408
views