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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 8f
Chapter 4, Problem 8f

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

Verified step by step guidance
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Step 1: Understand the problem. The F₂ phenotypic ratio (9/16 black : 3/16 gray : 4/16 albino) suggests that two genes (R and T) interact in a specific way to produce the observed phenotypes. Each dominant allele (R and T) contributes to a step in a biochemical pathway leading to pigment production, while the recessive alleles (r and t) are loss-of-function mutations.
Step 2: Define the pathway. Start with a colorless precursor. Assume that the dominant allele R catalyzes the first step in the pathway, converting the precursor into an intermediate product. This intermediate product is still colorless. Then, assume that the dominant allele T catalyzes the second step, converting the intermediate into the final black pigment. If either step is disrupted (due to the presence of recessive alleles), the pathway will not proceed to completion, resulting in different phenotypes.
Step 3: Assign phenotypes to genotypes. If both R and T are present (genotypes R-T-, where '-' represents either dominant or recessive alleles), the pathway proceeds fully, producing the black phenotype. If R is present but T is absent (genotypes R-tt), the pathway stops at the intermediate stage, producing the gray phenotype. If R is absent (genotypes rrT- or rrtt), the pathway does not proceed at all, and the organism remains albino.
Step 4: Verify the phenotypic ratio. Perform a dihybrid cross (RrTt x RrTt) and use a Punnett square to determine the genotypic combinations of the F₂ progeny. Count the number of progeny with each phenotype based on the pathway defined in Step 3. The expected phenotypic ratio should match the given ratio of 9/16 black : 3/16 gray : 4/16 albino.
Step 5: Summarize the pathway. The pathway can be summarized as follows: Colorless precursor → (R catalyzes) → Colorless intermediate → (T catalyzes) → Black pigment. The phenotypes depend on the presence or absence of functional R and T alleles, with the given ratio explained by the interaction of these two genes in the pathway.

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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 result from the independent assortment of these alleles during gamete formation, leading to a variety of combinations in the F₂ generation.
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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 ratios of 9/16 black, 3/16 gray, and 4/16 albino indicate how the interactions between the dominant and recessive 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 several 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. The specific interactions between these genes can result in various phenotypes, such as black, gray, or albino, depending on the presence or absence of functional alleles.
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Interacting Genes Overview
Related Practice
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

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

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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.

15/16 black : 1/16 white

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.

13/16 white : 3/16 green

Textbook Question

The ABO blood group assorts independently of the rhesus (Rh) blood group and both assort independently of the MN blood group. Three alleles, IA, IB and i, occur at the ABO locus. Two alleles, R, a dominant allele producing Rh+, and r, a recessive allele for Rh-, are found at the Rh locus, and codominant alleles M and N occur at the MN locus. Each gene is autosomal.

A child with blood types A, Rh−, and M is born to a woman who has blood types O, Rh−, and MN and a man who has blood types A, Rh+, and M. Determine the genotypes of each parent.

Textbook Question

The ABO blood group assorts independently of the rhesus (Rh) blood group and both assort independently of the MN blood group. Three alleles, IA, IB and i, occur at the ABO locus. Two alleles, R, a dominant allele producing Rh+, and r, a recessive allele for Rh-, are found at the Rh locus, and codominant alleles M and N occur at the MN locus. Each gene is autosomal.

What proportion of children born to a man with genotype IAIB Rr MN and a woman who is IAi Rr NN will have blood types B, Rh- , and MN? Show your work.