Lecture 18: 3 Point Gene Mapping

Recombination Overview

• Meiosis involves homologous pairs of chromosomes lining up during prophase I and II.

• Crossover events occur between non-sister chromatids, leading to genetic variation.

  • Example: Crossover occurs between yellow and blue chromosomes, resulting in recombinant DNA.

Crossover Events

• Single Crossover: Involves swapping a segment between two non-sister chromatids.

  • Leads to new combinations of alleles in gametes.

  • Still produces parental gametes alongside recombinant ones.

• Double Crossover: When two crossover events happen between the same two genes (a and b).

  • If crossovers are far apart, the outcome remains unaffected concerning a and b.

  • If two crossovers occur on either side of a and b, there's still no change to the a and b parental gametes.

Mechanisms of Crossover

• The location of crossover events is crucial for determining the outcome.

• Single Crossovers vs. Double Crossovers: A double crossover can revert some parts back to parent configurations, leading to potential recombination of alleles, but it may not change the outcome for certain genes.

Crossover Frequencies

• The likelihood of crossover events can be calculated based on the frequency of specific offspring types.

  • Example: Calculating crossover frequencies for fruit fly traits using previous data (e.g., 9% frequency between b and cinnabar).

Crossover Calculations

• Independent assortment occurs when genes are located far apart on the chromosome and recombination frequency approaches 50%.

• The distance between genes can be assessed in centimorgans (cM), reflecting linkage.

Three-Point Crosses

• An approach that allows mapping of three genes simultaneously to determine their order on a chromosome.

  • The parental chromosome configuration is established using pure breeding individuals.

  • Expectation is that most offspring will appear like either parental type, with fewer showing recombinant types.

Observed vs. Expected Crossovers

• Interference: The effect one crossover event may have on the occurrence of another.

  • Calculate expected double crossovers using the probabilities derived from the distances between genes.

  • Compare observed double crossovers to expected to determine if there's positive or negative interference.

Key Concepts in Calculating Recombinant Frequencies

• Take both single and double crossover events into account by adding crossover frequencies and matching with total offspring.

• Each distance calculated between genes contributes to establishing genetic maps and understanding gene linkage.

• The final step often involves verifying the order of genes based on changes observed in parental and double crossover offspring.