Table of contents

1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

3. Extensions to Mendelian Inheritance

Epistasis and Complementation

Genetics

3. Extensions to Mendelian Inheritance

Epistasis and Complementation

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Problem 2

Textbook Question

Define and distinguish epistasis and pleiotropy.

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Epistasis refers to a genetic interaction where the expression of one gene is influenced or completely masked by another gene. For example, in coat color inheritance in Labrador retrievers, one gene determines pigment color (black or brown), while another gene determines whether pigment is deposited in the fur. If the second gene prevents pigment deposition, the coat appears yellow regardless of the first gene's alleles.

Pleiotropy occurs when a single gene influences multiple, seemingly unrelated phenotypic traits. For instance, the Marfan syndrome gene affects connective tissue, leading to a variety of symptoms such as long limbs, heart defects, and vision problems.

To distinguish between the two, note that epistasis involves interactions between two or more genes, where one gene's expression affects another's. In contrast, pleiotropy involves a single gene affecting multiple traits.

Epistasis is often studied in dihybrid crosses, where the phenotypic ratios deviate from the expected Mendelian ratios due to gene interactions. For example, a 9:3:3:1 ratio might shift to 9:7 or 12:3:1 depending on the type of epistasis.

Pleiotropy is typically identified through the observation of multiple phenotypic effects caused by mutations in a single gene. This is often studied in the context of genetic disorders or developmental biology.

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Key Concepts

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

Epistasis

Epistasis refers to the interaction between genes where the expression of one gene is affected by one or more other genes. This can result in a masking effect, where the phenotypic expression of a gene (the epistatic gene) overrides that of another gene (the hypostatic gene). For example, in coat color in mice, one gene can determine whether pigment is produced, while another gene can influence the type of pigment, demonstrating how gene interactions can shape phenotypes.

Pleiotropy

Pleiotropy occurs when a single gene influences multiple phenotypic traits. This means that a mutation in one gene can lead to various effects on an organism's phenotype, often seen in conditions like Marfan syndrome, where a single genetic defect affects the connective tissue, leading to diverse symptoms such as tall stature, heart problems, and long limbs. Understanding pleiotropy is crucial for grasping how genetic variations can have widespread effects.

Distinction between Epistasis and Pleiotropy

The key distinction between epistasis and pleiotropy lies in their mechanisms of gene interaction. Epistasis involves the interaction between multiple genes affecting a single trait, while pleiotropy involves a single gene affecting multiple traits. Recognizing this difference is essential for understanding genetic pathways and how traits are inherited, as it influences the complexity of genetic interactions and the resulting phenotypic diversity.

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Related Practice

Multiple Choice

Two heterozygous organisms are crossed, and the F₂ phenotypic ratio is 12:3:1. What form of epistasis do these two genes exhibit?

A cross of white plants and red plants was performed. Using the F2 phenotypic ratio data below, determine what form of gene interaction is taking place.

In the rare Bombay phenotype, a mutation in a second gene can control an individual's what?

In the discussion, we focused on extensions and modifications of Mendelian principles and ratios. In the process, we encountered many opportunities to consider how this information was acquired. On the basis of these discussions, what answers would you propose to the following fundamental questions?

How did geneticists determine that inheritance of some phenotypic characteristics involves the interactions of two or more gene pairs? How were they able to determine how many gene pairs were involved?

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Textbook Question

A researcher interested in studying a human gene on chromosome 21 and another gene on the X chromosome uses FISH probes to locate each gene. The chromosome 21 probe produces green fluorescent color, and the X chromosome probe produces red fluorescent color.

If the subject studied is male, how many green and red spots will be detected? Explain your answer.

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Textbook Question

If the subject studied is female, how many green and red spots will be detected? Explain your answer.

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Textbook Question

In what way does position effect variegation (PEV) of Drosophila eye color indicate that chromatin state can affect gene transcription?

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Textbook Question

Strains of petunias come in four pure-breeding colors: white, blue, red, and purple. White petunias are produced when plants synthesize no flower pigment. Blue petunias and red petunias are produced when plants synthesize blue or red pigment only. Purple petunias are produced in plants that synthesize both red and blue pigment (the mixture of red and blue makes purple). Flower-color pigments are synthesized by gene action in two separate pigment-producing biochemical pathways. Pathway I contains gene A that produces an enzyme to catalyze conversion of a colorless pigment designated to blue pigment. In Pathway II, the enzymatic product of gene B converts the colorless pigment designated to red pigment. The two genes assort independently.

What are the possible genotype(s) for true-breeding blue petunias?

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