Linkage and Mapping in Eukaryotes

Chapter 4: Linkage and Mapping in Eukaryotes

Early Experiments in Drosophila Melanogaster

  • Key Organism: Drosophila melanogaster (fruit fly) is frequently used in genetic studies.

  • Mutant Strains Explained:

    • Banded mutant (designated as bn) crossed with detached mutant (designated as det).

    • Resulting F1 generation phenotypes appeared wild-type (indicating heterozygosity, BbDd).

    • In proper Drosophila nomenclature, wild-type and mutants represented as bn+bn det+ det.

  • Testcross:

    • Testcross with the banded-detached mutant (homozygous recessive).

    • Expected phenotypic ratio from this cross is 1:1:1:1.

Linkage

  • Concept: Not all genes assort independently; those located on the same chromosome tend to be inherited together.

  • Purpose of Chapter:

    • Exploration of linkage.

    • Importance of crossing over.

    • Usage of crossover frequencies to create chromosome maps (to determine relative positions of genes).

Gene Arrangement and Linkage

  • Phenotypic Observations:

    • Deviations from expected 1:1:1:1 ratio indicated linkage.

    • Parental types (same as P1 generation) vs recombinant types (combinations of parental phenotypes).

    • Suggests that alleles can remain together during assortment but may only rearrange at low frequency.

  • Illustration of Testcross Results:

    • F1 testcross results show a majority of non-recombinant types (99.5%) compared to recombinant types (0.5%).

Possibilities for Chromosome Behavior During Meiosis

  1. Genes on Different Chromosomes: All genes assort independently.

  2. Genes Very Close Together: These genes are tightly linked with negligible recombination between them.

  3. Genes Far Apart on the Same Chromosome: Recombination may occur between them with varying frequency.

    • If very close, recombination likelihood decreases.

    • If right next to each other, recombination becomes extremely unlikely.

Experimental Evidence of Crossing Over

  • Creighton and McClintock’s Maize Experiment:

    • Utilized mutant phenotypes on Chromosome 9 to show that chromosome material is swapped during meiosis.

    • Key markers:

    • C = colored (dominant)

    • wx = waxy (recessive)

    • Observed phenotypes:

    • Colorless, non-waxy

    • Colored, waxy

    • Colorless and waxy.

Recombination Throughout Chromosomes

  • Recombination is scattered and can occur anywhere along the chromosome, contributing to genetic diversity in offspring.

Genetic Crosses and Dihybrid Cross Examples

  • Flower Color and Shape Example (P = purple; p = red) - Results in a dihybrid cross of phenotype ratio expected to be 9:3:3:1.

  • Observed Phenotypes:

    • Purple, long: 483 | Expected: 391

    • Purple, round: 390 | Expected: 130

    • Red, long: 393 | Expected: 130

    • Red, round: 1338 | Expected: 435

    • Deviations suggest non-Mendelian inheritance.

Testcross and Results of Different Allele Arrangements

  • Distinguishing between Cis and Trans Configurations:

    • Cis-configuration: One chromosome carrying both mutations, homolog with wild-type.

    • Trans-configuration: Mutations are distributed across different homologs.

  • Linkage Implications:

    • Physical proximity on a chromosome increases overlap in inheritance patterns, driving deviation from expected ratios.

Mapping Genes and Recombination Frequencies

  • Map Units (cM) defined as 1% recombination frequency between genes.

  • Recombination Analysis of Banded and Detached Mutants:

    • Distance calculated: 0.5 mu apart based on testcross results.
      – Units of Distance and Implications:

    • 1 natural unit distance not directly related to a fixed number of base pairs.

    • Gene distances can range significantly: 1 mu generally translates to 100,000 to 10,000,000 base pairs in humans, and about 6000 in yeast.

  • Limitations of Recombination Percentages: Range from 0% (completely linked) to 50% (independent assortment).

Practical Applications in Genetic Mapping

  • Two-point testcross: Cross involving two loci to determine distances between gene pairs.

  • Sturtevant's Linkage Mapping:

    • Developed first linkage map in 1913 using two-point testcross data.

    • Determined gene distances on the X chromosome: white/yellow is 1.5 cM, yellow/miniature is 34.2 cM versus 31.2 cM previously collected estimates.

Creation of Multi-gene Linkage Maps via Three-Point Testcross

  • Using Drosophila, testing multiple loci for wing morphology, body color, and eye color can yield insightful linkage data.

  • Recombination Frequencies calculations yield indicative distances on maps.

    • Example results: Distance between pairs using cross data can inform relative gene locations and distances further aiding in mapping efforts.