Chapter 5- Linkage_ Recombination_Mapping

Basic Principles of Genetic Transmission

Chapter Overview

• Linkage, Recombination, and Gene Mapping

  • 5.1 Gene Linkage and Recombination

  • 5.2 The Chi-Square Test and Linkage Analysis

  • 5.3 Recombination: Crossing-Over During Meiosis

  • 5.4 Mapping Genes Along Chromosomes

  • 5.5 Tetrad Analysis in Fungi

  • 5.6 Mitotic Recombination and Genetic Mosaics

Genetic Linkage

• Definition: Genetic linkage refers to the tendency of DNA sequences that are close together on a chromosome to be inherited together during meiosis, the phase of sexual reproduction.

• Close Proximity: Genetic markers that are physically near each other are less likely to be separated during chromosomal crossover, thus they are more linked than those that are farther apart.

• Inheritance Probability: The nearer two genes are on a chromosome, the lesser the chance of recombination between them, meaning they are more likely to be inherited together.

Gene Linkage and Recombination

• Linked Genes: Genes that are linked on the same chromosome usually assort together.

• Separation Through Recombination: Linked genes can become separated due to recombination, particularly significant if the two genes are farther apart.

• Key Themes:

  1. The greater the distance between two genes, the higher the probability of recombination.

  2. Recombination data aids in creating maps showing the relative locations of genes on chromosomes.

Principle of Independent Assortment

• Diploid and Haploid Cells: Understanding how traits assort independently in diploid reproductive cells compared to haploid gametes.

• Examples: Illustrations of how traits separate during meiosis, impacting the genetic makeup of gametes.

Thomas Hunt Morgan's Work with Drosophila (1912)

• Drosophila melanogaster: Chosen due to:

  • Short generation time (8-14 days).

  • Ease of cultivation and crossing in the lab.

  • Genetic similarity to human disease traits.

  • Chromosome number: 8 (3 pairs of autosomes, 1 pair of sex chromosomes).

• Mutations Identified: e.g., vestigial wings and body color variants.

Evidence for Linked Genes in Drosophila

• Traits Studied: Body color and wing length, observed as traits on the same chromosome:

  • Body Color: Gray (normal) vs. Black (mutant).

  • Wing Length: Normal vs. Vestigial (short).

• Initial Cross: Morgan crossed a true-breeding wildtype fly with a double mutant to observe inheritance patterns.

• Test cross results: Analyzing phenotypic ratios indicates linkage versus independence.

Recombination Frequencies and Mapping

• Recombination Frequency: Calculated as the number of recombinant offspring divided by the total number of offspring, expressed as a percentage, reflecting the linked nature of genes.

• Mapping Genes:

  • A. H. Sturtevant's contribution: correlation between recombination frequency and physical distance

  • 1% recombination = 1 map unit (centiMorgan).

• Use in Genetic Mapping: Calculating the distances between three genes creating a three-point genetic map, identifying the relative positions on chromosomes.

Properties of Linked vs. Unlinked Genes

• Linked Genes: Show less than 50% recombinants, indicating proximity on the same chromosome.

• Unlinked Genes: Show 50% recombinants, indicating independent assortment or presence on separate chromosomes.

• Practical Applications: Understanding gene linkage is crucial for genetic mapping and understanding inheritance patterns.

Conclusion

• Thomas Hunt Morgan's experiments established critical links between gene proximity, recombination probabilities, and inheritance patterns, laying the groundwork for modern genetics.