Gene Linkage and Linkage Maps

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Bevorstehend

You will learn how linked genes are inherited and how to create a linkage map from crossing-over frequencies.

Ziele dieses Moduls
  • Sie können erklären, warum gekoppelte Gene nicht unabhängig voneinander vererbt werden
    Verstehen
  • Sie können aus Rekombinationshäufigkeiten eine genetische Karte (Linkage Map) erstellen
    Anwenden
  • Sie können Vererbungsmuster bei gekoppelten Genen vorhersagen und analysieren
    Analysieren

In this module, you will learn the fundamentals of gene linkage. You will understand why genes located on the same chromosome are not inherited independently and how you can create a genetic map from recombination frequencies.

What is Gene Linkage?

Gene linkage means that genes located on the same chromosome are linked and inherited together. They do not follow Mendel's law of independent assortment.

Key Concepts:

  • Linked Genes: Located on the same chromosome and usually inherited together
  • Crossing-Over: Exchange of DNA segments between homologous chromosomes during meiosis
  • Recombination Frequency: Percentage of offspring with recombinant (new) gene combinations

The closer two genes are located on a chromosome, the lower the probability that crossing-over will occur between them.

Example: Drosophila (Fruit Fly)

Thomas Hunt Morgan conducted groundbreaking experiments with fruit flies and discovered gene linkage.

Classic Example:

Two genes for body color and wing length are located on the same chromosome:

  • Gene A: Gray body (dominant) vs. black body (recessive)
  • Gene B: Normal wings (dominant) vs. vestigial wings (recessive)

In a test cross (dihybrid heterozygote × double recessive), you would expect a 1:1:1:1 ratio with independent assortment.

With linked genes, you instead observe:

  • Many offspring with parental type (e.g., gray/normal and black/vestigial)
  • Few offspring with recombinant type (e.g., gray/vestigial and black/normal)

Calculating Recombination Frequency

The recombination frequency indicates how often crossing-over occurs between two genes.

Formula:

Recombination Frequency (%) = (Number of Recombinants / Total Number of Offspring) × 100

Sample Calculation:

You cross a dihybrid fruit fly (AB/ab) with a double recessive (ab/ab) and obtain:

  • Parental type (AB or ab): 900 offspring
  • Recombinant type (Ab or aB): 100 offspring
  • Total: 1000 offspring

Calculation: (100 / 1000) × 100 = 10%

The recombination frequency is 10%, which means the two genes are located 10 map units (cM) apart.

Creating a Linkage Map

A linkage map (genetic map) shows the relative position of genes on a chromosome. Distances are measured in map units (centiMorgan, cM).

1 map unit (cM) = 1% recombination frequency

Procedure:

  1. Determine the recombination frequency between all gene pairs
  2. The gene pair with the highest recombination frequency is located furthest apart
  3. Arrange the genes according to their relative distances

Example:

You have three genes (A, B, C) with the following recombination frequencies:

  • A-B: 15%
  • B-C: 8%
  • A-C: 23%

Analysis:

  • A and C are furthest apart (23 cM)
  • B is located between A and C
  • Order: A --- 15 cM --- B --- 8 cM --- C

Challenge

Practice Problem

You conduct crossing experiments with four genes (W, X, Y, Z) and obtain the following recombination frequencies:

  • W-X: 12%
  • W-Y: 5%
  • W-Z: 18%
  • X-Y: 7%
  • X-Z: 6%
  • Y-Z: 13%

Tasks:

  1. Which two genes are located furthest apart?
  2. Create a linkage map with all four genes
  3. Which gene is located in the middle?

Checklist

0/5

Note

Solution to Practice Problem

1. Furthest apart: W and Z (18 cM)

2. Linkage Map:

The analysis reveals the following arrangement:

  • W and Y are close together (5 cM)
  • X is located between W/Y and Z
  • Total distance: W-Z = 18 cM

Order: W --- 5 cM --- Y --- 7 cM --- X --- 6 cM --- Z

Verification: W-Y (5) + Y-X (7) + X-Z (6) = 18 cM ✓

3. Gene in the middle: Genes Y and X are located between W and Z, with X being most centrally positioned.

Summary

  • Gene Linkage: Genes on the same chromosome are inherited together
  • Crossing-Over: Enables recombination between linked genes
  • Recombination Frequency: Measure of distance between genes (1% = 1 cM)
  • Linkage Map: Genetic map showing relative gene positions
  • Rule: The closer genes are together, the less frequently recombination occurs

These findings are fundamental for understanding inheritance patterns and have practical applications in plant breeding, medicine, and evolutionary genetics.