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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 8a
Chapter 4, Problem 8a

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

Verified step by step guidance
1
Step 1: Understand the problem. The phenotypic ratio of 9:6:1 suggests that two genes (R and T) interact in a pathway to produce pigment. The dominant alleles (R and T) catalyze reactions, while the recessive alleles (r and t) are null and do not contribute to pigment production. The goal is to design a pathway explaining how these genes interact to produce the observed phenotypes.
Step 2: Define the pathway. Start with a colorless precursor. Gene R catalyzes the first step in the pathway, converting the precursor into an intermediate product. If R is functional (R_), this reaction occurs. If R is null (rr), the precursor remains unmodified, resulting in a white phenotype.
Step 3: Introduce the second gene. Gene T catalyzes the second step in the pathway. If T is functional (T_), it converts the intermediate product into the final pigment. If T is null (tt), the intermediate product remains unmodified, resulting in a light blue phenotype.
Step 4: Explain the phenotypes. The dark blue phenotype (9/16) occurs when both R and T are functional (R_T_). The light blue phenotype (6/16) occurs when R is functional but T is null (R_tt) or when T is functional but R is null (rrT_). The white phenotype (1/16) occurs when both R and T are null (rrtt).
Step 5: Verify the ratio. The genotypic combinations for F₂ progeny of a dihybrid cross (RrTt x RrTt) follow Mendelian inheritance. Calculate the phenotypic ratio by considering the contributions of each gene to the pathway. Confirm that the 9:6:1 ratio aligns with the described pathway and 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 represented by two alleles, allowing the study of inheritance patterns for two genes simultaneously. In this case, the genes R and T are being analyzed for their interactions in producing different phenotypes. The classic Mendelian ratio for a dihybrid cross of two heterozygous parents (RrTt x RrTt) is 9:3:3:1, but here we are examining a modified ratio due to gene interaction.
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Gene Interaction

Gene interaction occurs when two or more genes influence a single trait, leading to phenotypic ratios that differ from those expected from independent assortment. In this scenario, the dominant alleles of genes R and T catalyze different reactions that contribute to pigment production, resulting in a specific phenotypic ratio of 9:6:1. Understanding how these genes interact is crucial for explaining the observed ratios.
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Phenotypic Ratios

Phenotypic ratios represent the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In this case, the ratios of 9/16 dark blue, 6/16 light blue, and 1/16 white indicate how the interactions between the alleles of genes R and T affect pigment production. Analyzing these ratios helps in constructing pathways that explain the genetic mechanisms behind the observed traits.
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Related Practice
Textbook Question

In a type of parakeet known as a "budgie," feather color is controlled by two genes. A yellow pigment is synthesized under the control of a dominant allele Y. Budgies that are homozygous for the recessive y allele do not synthesize yellow pigment. At an independently assorting gene, the dominant allele B directs synthesis of a blue pigment. Recessive homozygotes with the bb genotype do not produce blue pigment. Budgies that produce both yellow and blue pigments have green feathers; those that produce only yellow pigment or only blue pigment have yellow or blue feathers, respectively; and budgies that produce neither pigment are white (albino).

e. The cross of a green budgie and a yellow budgie produces offspring that are 12 green, 4 blue, 13 yellow, and 3 albino. What are the genotypes of the parents?

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Textbook Question

The ABO and MN blood groups are shown for four sets of parents (1 to 4) and four children (a to d). Recall that the ABO blood group has three alleles: IA, IB and i. The MN blood group has two codominant alleles, M and N. Using your knowledge of these genetic systems, match each child with every set of parents who might have conceived the child, and exclude any parental set that could not have conceived the child.

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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.

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

383
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 red : 7/16 white

432
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