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Overview of Recombination and Linkage Analysis

Apr 22, 2025

Recombination and Linkage Analysis Overview

Introduction

  • Recombination and linkage analysis are essential in genetics.
    • Recombination: Exchange of genetic material between homologous chromosomes during meiosis, increasing genetic diversity.
    • Linkage Analysis: Maps gene locations on chromosomes based on recombination frequency.

Learning Aims and Outcomes

  • Appreciate recombination and its molecular mechanisms.
  • Understand linkage analysis in gene mapping.
  • Explain linkage analysis details, including:
    • Recombination frequencies.
    • Parental and recombinant classes.
    • Consequences of multiple crossovers.
  • Understand mapping locations of linked genes based on recombination frequencies.

The First Linkage Map: Alfred Henry Sturtevant

  • Sturtevant's proposition:
    • Genes are fixed linearly along chromosomes.
    • Used Morgan's fruit flies for linkage mapping.
    • Map based on observed recombination frequencies.
  • Linkage map shows relative gene positions:
    • Distant genes have higher recombination likelihood.

Linkage Symbolism and Terminology

  • Linkage Symbolism: Notation for gene arrangement on chromosomes.
    • Alleles on different chromosomes separated by semicolon.
    • Alleles on the same homolog are in "coupling" or "cis conformation".
    • Alleles on alternate homologs are "uncoupled" or in "trans conformation".

Importance of Recombination

  • Essential for genetic diversity and separating mutations.
  • Without it, chromosome content would be fixed.
  • Occurs during exchange of DNA fragments between chromatids.

Two-Point Test Cross

  • Used to determine linkage between two genes.
  • Involves heterozygous and homozygous recessive individuals.
  • Genotypes and recombinants:
    • AB/ab (Wild type): 10%
    • Ab/aB (Wild type): 10%
    • AB/AB (Wild type): 0%
    • ab/ab (Mutant): 0%

Three-Point Test Cross

  • Determines linkage between three genes.
  • Involves heterozygous and homozygous recessive individuals.
  • Genotypes and recombinants:
    • ABC/abc (Wild type): 5%
    • AbC/aBc (Wild type): 5%
    • ABc/abC (Wild type): 5%
    • abc/abc (Mutant): 0%

Map Distances

  • Refers to gene distance on a chromosome, measured in centimorgans (cM).
  • Determined by crossover frequency.
  • Example Map Distances:
    • A to B: 10 cM
    • B to C: 5 cM
    • A to C: 15 cM

Application of Genetic Linkage Analysis

  • Gene Mapping: Locating genes on chromosomes.
  • Trait Mapping: Identifying genes for specific traits.
  • Disease Mapping: Identifying genes causing diseases.
  • Forensic Analysis: Using DNA evidence in crime-solving.