Genetic Linkage, Recombination, and Mapping Study Notes

Genetic Linkage, Recombination, and Mapping

Overview

  • Chapter Focus: Genetic linkage, recombination, and gene mapping

  • Exam Schedule:

    • Exam #2 on Thursday

    • Extra Credit Exam covering Exams #1 -3 on December 4, during class, replacing lecture and Quiz #7.

I. Gene Linkage

  • Definition: Gene linkage refers to the association between genes located on the same chromosome.

  • Key Concept: Genes that are linked tend to be inherited together and do not assort independently in gamete formation.

    • Illustrative Figure: Figure 5-1a shows the chromosomal basis for Mendel's law of independent assortment, where segregating pairs of unit factors assort independently during gamete formation.

II. Crossing Over

  • Definition: Crossing over is the exchange of parts of chromosomes between non-sister chromatids from homologous chromosomes during meiosis.

  • Outcome: Crossing over can lead to recombinant (non-parental) gametes.

  • Process Visualization:

    • Single Crossovers:

    • Occur randomly along non-sister chromatids; likelihood of crossing over increases with the distance between two genes.

    • Close genes = less chance of crossover; far genes = higher chance of crossover.

    • Recombinant gametes frequency approaches but does not exceed 50%.

III. Testing for Gene Linkage and Crossing Over

  • Standard Cross for Linkage Testing:

    • Dihybrid Test Cross: Cross between a double heterozygous individual (AaBb) and a tester homozygous recessive (aabb).

  • Linkage Scenarios:

    1. Complete Linkage:

    • Condition: No crossing over occurs.

    • Outcome: Produces two allele combinations with equal frequency (50% each).

    1. Incomplete Linkage:

    • Condition: Crossing over occurs but parental gametes are more common than recombinant gametes.

    • Outcome: Non-crossover gametes present at >25%; crossover gametes present at <25%.

IV. Gene Mapping

  • Recombination Frequency (RF):

    • Formula: RF = \frac{number\ of\ recombinant\ progeny}{total\ number\ of\ progeny} \times 100

    • Applications: Mapping the distance between linked genes can predict the percent of recombinant offspring.

    • Example Calculation: If there are 40 recombinant out of 200 total progeny, then RF = \frac{40}{200} \times 100 = 20\% .

    • Map Units: An RF of 20% corresponds to 20 map units.