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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 23b
Chapter 4, Problem 23b

Three strains of green-seeded lentil plants appear to have the same phenotype. The strains are designated G₁, G₂, and G₃. Each green-seeded strain is crossed to a pure-breeding yellow-seeded strain designated Y. The F₁ of each cross are yellow; however, self-fertilization of F₁ plants produces F₂ with different proportions of yellow- and green-seeded plants as shown below.
Table showing parental strains of green and yellow lentil plants, F1 and F2 phenotypes with respective proportions.
Using the allele symbols A and a, B and b, and D and d to represent alleles at segregating genes, give the genotypes of parental and F₁ plants in each cross. 

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Step 1: Understand the phenotypes and crosses. The green-seeded strains G₁, G₂, and G₃ are each crossed with a pure-breeding yellow-seeded strain Y. The F₁ generation in all crosses is all yellow, indicating that yellow is dominant over green.
Step 2: Analyze the F₂ phenotypic ratios to infer the number of genes involved. For G₁, the F₂ ratio is 3:1 (yellow:green), which suggests a single gene with simple dominance (A and a). For G₂, the F₂ ratio is 9:7 (yellow:green), which suggests two genes with complementary gene interaction (A and B). For G₃, the F₂ ratio is 27:37 (yellow:green), which suggests three genes interacting (A, B, and D).
Step 3: Assign genotypes to the parental strains. Since Y is pure-breeding yellow, it must be homozygous dominant for all relevant genes (e.g., AA, BB, DD). The green parental strains must be homozygous recessive for the genes involved in green seed color (e.g., aa, bb, dd).
Step 4: Determine the F₁ genotypes. Since all F₁ plants are yellow, they must be heterozygous for the genes involved (e.g., Aa, Bb, Dd), inheriting dominant alleles from the yellow parent and recessive alleles from the green parent.
Step 5: Summarize genotypes for each cross: For G₁, green parent is aa, yellow parent is AA, F₁ is Aa. For G₂, green parent is aabb, yellow parent is AABB, F₁ is AaBb. For G₃, green parent is aabbdd, yellow parent is AABBDD, F₁ is AaBbDd.

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Key Concepts

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

Mendelian Inheritance and Dominance

Mendelian inheritance explains how traits are passed from parents to offspring through dominant and recessive alleles. In this question, yellow seed color is dominant over green, as all F1 offspring are yellow. Understanding dominance helps predict phenotypic ratios in F2 generations.
Recommended video:
Guided course
04:37
Variations on Dominance

Monohybrid and Dihybrid Cross Ratios

Different F2 phenotypic ratios indicate the number of genes involved. A 3:1 ratio suggests a single gene (monohybrid), 9:7 or 7:9 ratios suggest two genes with epistasis or complementary gene action, and more complex ratios like 37:27 imply multiple genes. These ratios help deduce genotypes of parental strains.
Recommended video:
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10:20
Monohybrid Cross

Genotype-Phenotype Relationship and Allele Symbols

Allele symbols (A/a, B/b, D/d) represent different genes controlling seed color. Assigning genotypes to parental and F1 plants based on phenotypic ratios allows understanding of gene interactions and segregation patterns. This is essential for predicting offspring genotypes and phenotypes.
Recommended video:
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Non-Random Mating
Related Practice
Textbook Question

Xeroderma pigmentosum (XP) is an autosomal recessive condition characterized by moderate to severe sensitivity to ultraviolet (UV) light. Patients develop multiple skin lesions on UV-exposed skin, and skin cancers often develop as a result. XP is caused by deficient repair of DNA damage from UV exposure. Many genes are known to be involved in repair of UV-induced DNA damage, and several of these genes are implicated in XP. What genetic phenomenon is illustrated by XP?

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Textbook Question

Xeroderma pigmentosum (XP) is an autosomal recessive condition characterized by moderate to severe sensitivity to ultraviolet (UV) light. Patients develop multiple skin lesions on UV-exposed skin, and skin cancers often develop as a result. XP is caused by deficient repair of DNA damage from UV exposure.


A series of 10 skin-cell lines was grown from different XP patients. Cells from these lines were fused, and the heterokaryons were tested for genetic complementation by assaying their ability to repair DNA damage caused by a moderate amount of UV exposure. In the table below, '+' indicates that the fusion cell line performs normal DNA damage mutation repair, and '−' indicates defective DNA repair. Use this information to determine how many DNA-repair genes are mutated in the 10 cell lines, and identify which cell lines share the same mutated genes. 

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Textbook Question

Three strains of green-seeded lentil plants appear to have the same phenotype. The strains are designated G₁, G₂, and G₃. Each green-seeded strain is crossed to a pure-breeding yellow-seeded strain designated Y. The F₁ of each cross are yellow; however, self-fertilization of F₁ plants produces F₂ with different proportions of yellow- and green-seeded plants as shown below.

For what number of genes are variable alleles segregating in the G₁ x Y cross? The G₂ x Y cross? In the G₃ x Y cross? Explain your rationale for each answer. 

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Textbook Question

Three strains of green-seeded lentil plants appear to have the same phenotype. The strains are designated G₁, G₂, and G₃. Each green-seeded strain is crossed to a pure-breeding yellow-seeded strain designated Y. The F₁ of each cross are yellow; however, self-fertilization of F₁ plants produces F₂ with different proportions of yellow- and green-seeded plants as shown below.

For each set of F₂ progeny, provide a genetic explanation for the yellow : green ratio. What are the genotypes of yellow and green F₂ lentil plants in the G₂ x Y cross? 

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Textbook Question

Three strains of green-seeded lentil plants appear to have the same phenotype. The strains are designated G₁, G₂, and G₃. Each green-seeded strain is crossed to a pure-breeding yellow-seeded strain designated Y. The F₁ of each cross are yellow; however, self-fertilization of F₁ plants produces F₂ with different proportions of yellow- and green-seeded plants as shown below.

If green-seeded strains G₁ and G₃ are crossed, what are the phenotype and the genotype of F₁ progeny? 

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Textbook Question

Three strains of green-seeded lentil plants appear to have the same phenotype. The strains are designated G₁, G₂, and G₃. Each green-seeded strain is crossed to a pure-breeding yellow-seeded strain designated Y. The F₁ of each cross are yellow; however, self-fertilization of F₁ plants produces F₂ with different proportions of yellow- and green-seeded plants as shown below.

What proportion of the F₂ are expected to be green? Show your work. 

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