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

Overview of interacting Genes

Genetics

3. Extensions to Mendelian Inheritance

Overview of interacting Genes

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2:05 minutes

Problem 5

Textbook Question

Describe the difference between continuous phenotypic variation and discontinuous variation. Explain how polygenic inheritance could be the basis of a trait showing continuous phenotypic variation. Explain how polygenic inheritance can be the basis of a threshold trait.

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Continuous phenotypic variation refers to traits that show a range of values, such as height or skin color, where intermediate phenotypes are possible. Discontinuous variation, on the other hand, refers to traits that fall into distinct categories, such as blood type (A, B, AB, O) or pea plant flower color (purple or white).

Polygenic inheritance involves multiple genes contributing to a single trait. Each gene may have multiple alleles, and the combined effect of these alleles produces a wide range of phenotypes. For example, in traits like human height, the additive effects of alleles at several loci result in continuous variation.

In polygenic inheritance, the more genes involved, the greater the number of possible allele combinations, leading to a smoother and more continuous distribution of phenotypes. This is often represented as a bell-shaped curve in a population.

A threshold trait is a special case of polygenic inheritance where the trait is determined by the cumulative effect of multiple genes, but the phenotype is expressed only when a certain threshold level of genetic and environmental factors is reached. For example, diseases like Type 2 diabetes may only manifest when the genetic predisposition and environmental factors (e.g., diet, lifestyle) exceed a critical threshold.

In threshold traits, individuals below the threshold appear unaffected, while those above the threshold express the trait. This creates a discontinuous phenotypic outcome (affected or unaffected) despite the underlying polygenic basis, which involves continuous variation in genetic predisposition.

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Key Concepts

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

Continuous vs. Discontinuous Variation

Continuous variation refers to traits that show a range of phenotypes, such as height or skin color, where individuals can have any value within a spectrum. In contrast, discontinuous variation involves distinct categories or classes, such as blood type or flower color, where traits are clearly defined and do not blend. Understanding these differences is crucial for analyzing how traits are inherited and expressed.

Polygenic Inheritance

Polygenic inheritance occurs when multiple genes contribute to a single trait, resulting in a continuous range of phenotypes. Each gene may have a small additive effect, leading to traits that vary smoothly rather than in discrete categories. This mechanism explains why traits like height or skin color exhibit continuous variation, as the cumulative effects of many genes influence the final phenotype.

Threshold Traits

Threshold traits are characteristics that are influenced by multiple genes and environmental factors, where a certain threshold must be reached for the trait to be expressed. For example, a person may have a genetic predisposition for a condition, but it only manifests if environmental factors exceed a specific level. This concept illustrates how polygenic inheritance can lead to traits that are not continuously variable but instead appear once a threshold is crossed.

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For the traits listed in the previous problem, which do you think are likely to be multifactorial traits, with phenotypes that are influenced by genes and environment? Identify two environmental factors that might play a role in phenotypic variation of the traits you identified.

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Compare and contrast broad sense heritability and narrow sense heritability, giving an example of each measurement and identifying how the measurement is used.

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Textbook Question

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Textbook Question

Provide a definition and an example for each of the following terms:

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