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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 8g
Chapter 4, Problem 8g

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

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Step 1: Understand the problem. The F₂ phenotypic ratio of 13/16 white : 3/16 green suggests that there is an epistatic interaction between two genes (R and T). Epistasis occurs when one gene's expression masks or modifies the effect of another gene. Here, the dominant alleles of R and T are involved in pigment production, while the recessive alleles are null (loss-of-function).
Step 2: Define the pathway. Start with a colorless precursor. Assume that gene R catalyzes the first step in the pathway, converting the precursor into an intermediate product. If R is null (rr), the precursor remains unmodified, resulting in a white phenotype.
Step 3: Add the second gene's role. Assume that gene T acts on the intermediate product produced by R to convert it into a green pigment. If T is null (tt), the intermediate product cannot be converted into the green pigment, resulting in a white phenotype.
Step 4: Explain the phenotypic ratio. The 13/16 white phenotype can be explained by the following genotypes: (1) rrT_ (where R is null, so the precursor remains unmodified), (2) R_tt (where T is null, so the intermediate product cannot be converted to green), and (3) rrtt (where both genes are null). The 3/16 green phenotype occurs only when both R and T are functional (R_T_).
Step 5: Summarize the genetic interaction. This is an example of dominant epistasis, where the presence of a dominant allele at one locus (R or T) can mask the effect of the other gene. The pathway can be summarized as: Colorless precursor --(R)--> Intermediate product --(T)--> Green pigment. If either R or T is null, the pathway is blocked, resulting in a white phenotype.

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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 scenario, 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 various 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 this case, the ratio of 13/16 white to 3/16 green indicates that the majority of the F₂ progeny exhibit a white phenotype, while a smaller proportion shows green. Understanding these ratios helps in deducing the underlying genetic interactions and the effects of dominant and recessive alleles.
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Gene Interaction and Pathways

Gene interaction refers to the way different genes influence each other's expression and the resulting phenotype. In this context, the dominant alleles R and T catalyze distinct reactions that modify a colorless precursor into pigments. The proposed pathways must account for how these interactions lead to the observed phenotypic ratios, illustrating the complexity of genetic control over traits such as color in organisms.
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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 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.

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

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.

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 man with blood types B, Rh+, and N says he could not be the father of a child with blood types O, Rh−, and MN. The mother of the child has blood types A, Rh+, and MN. Is the man correct? Explain.