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2.4 Genetics and Morgan's Fruit Fly Experiment

Sep 12, 2024

Lecture Notes: Genetics and Thomas Hunt Morgan Experiment

Key Terminology and Concepts

  • Wild Type Phenotypes

    • Phenotypes commonly found in the wild, e.g., red eyes in fruit flies (Drosophila melanogaster).
    • Notation: Wild type alleles may be denoted with a plus symbol (+).

  • Mutant Alleles

    • Deviations from wild type phenotypes, e.g., purple eyes in flies.
    • Can occur naturally or be induced using mutagens (e.g., X-rays, EMS).

  • Genetic Notation

    • PR+: Wild type allele for red eyes.
    • PR: Mutant allele for purple eyes (recessive).
    • VG+: Wild type allele for normal wings.
    • VG: Mutant allele for vestigial wings (recessive).

Thomas Hunt Morgan’s Experiment

Experiment Setup

  • Objective: Track inheritance of eye color and wing size in fruit flies.

  • Traits:

    • Eye color: Red (wild type) vs. Purple (mutant, recessive).
    • Wing size: Normal (wild type) vs. Vestigial (mutant, recessive).

  • Crosses:

    • Cross between a pure-breeding wild type and a double mutant (homozygous recessive for both traits).
    • Produces dihybrid individuals (heterozygous for both traits).

Test Cross

  • Purpose: Determine genotypes and phenotypic ratios.
  • Dihybrid Gametes:
    • Produces PR+ VG+, PR VG, PR+ VG, PR VG+.
  • Tester Gametes:
    • Only PR VG.

Expected Phenotypic Ratios (Independent Assortment)

  • Progeny Expectations: Equal proportions (25%) for each genotype:
    • Red eyes, normal wings (PR+ VG+)
    • Purple eyes, vestigial wings (PR VG)
    • Red eyes, vestigial wings (PR+ VG)
    • Purple eyes, normal wings (PR VG+)

Observed Results

  • Parental Genotypes:

    • Red eyes, normal wings: 1339 individuals (~45%)
    • Purple eyes, small wings: ~1200 individuals (~45%)

  • Recombinant Genotypes:

    • Red eyes, small wings: 151 individuals (~5%)
    • Purple eyes, normal wings: 154 individuals (~5%)

Conclusion: Gene Linkage

  • Linkage Evidence:

    • Recombinant genotypes are rarer than expected by independent assortment.
    • PR+ tends to be linked with VG+, and PR with VG.

  • Reason for Linkage:

    • Genes are located on the same chromosome.
    • Leads to more frequent inheritance of parental genotypes.

Implications

  • Linkage vs. Independent Assortment:
    • Deviations from expected 1:1:1:1 ratio suggest gene linkage.
    • Understanding chromosomal linkage explains inheritance patterns and deviations.

This lecture highlights foundational genetic concepts, including phenotypic expressions, genetic crosses, and the phenomenon of gene linkage as examined by Thomas Hunt Morgan.

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