Recombination Frequencies in Meiosis

Introduction to Recombination Frequencies

• Focus on recalculating recombination frequencies based on crossover events.
• Stem cells in ovaries or testes undergo meiosis, starting as diploid cells and forming haploid gametes.
• Diploid cells: Two copies of each gene on homologous chromosomes.
  • Copies of genes are called alleles, which can be identical or different.
• Chromosome replication: Each chromosome consists of two identical sister chromatids.
• Prophase I of Meiosis: Tetrads form, allowing crossing over between non-homologous chromatids.

Example of Chromosome with Linked Genes

• Chromosome with genotype: AaBb (DI hybrid, 2N = 2).
• During Interphase, chromosomes duplicate; each now has two identical chromatids.
• Meiosis I: Separation of homologous pairs into two cells, making them haploid.
• Meiosis II: Separation of sister chromatids.
• No crossing over results in gametes identical to parent chromosomes (parental type):
  • Big A Big B and Little a Little b.
  • No recombinants (e.g., Big A Little b or Little a Big B).

Linked Genes and Test Cross

• Test cross: Cross with a completely recessive individual.
• Linked genes prevent certain combinations, e.g., Big A cannot combine with Little b as they’re on the same chromosome.
• Offspring genotypes mirror parental types; no recombinants.
  • Calculate recombination frequency: Recombinants/Total Offspring = 0/100 = 0%.

Crossing Over with Linked Genes

• 100% Crossover Scenario:
  • Chromatids undergo crossing over, creating recombinant chromatids.
  • Parental: AaBb and aabb.
  • Recombinant: Aabb and aaBb.
• Crossing over leads to equal probabilities for parental and recombinant types.

Gene Linkage and Crossover Probabilities

• More than 50% crossing over makes genes statistically behave as unlinked.
• Further apart loci increase crossover likelihood.
  • Example: Gene A and B are far apart, likely to cross over.
  • Genes C and D are close, less likely to cross over.

Morgan’s Mapping of Gene Locations

• Used recombination frequencies to map fruit fly genome.
• Maps show relative, not exact, gene locations.
  • Example of recombination frequencies determining map units:
    • Short antennae and wing shape: 13% recombination.
    • Short antennae and short legs: 31% recombination.
• Larger distances (>50 map units) imply high crossover events.
• Relative distances calculated using recombination percentages (e.g., 54.5 - 48.5 = 6 map units apart).

Conclusion

• Understanding recombination frequencies helps in mapping genes without physical imaging or decoding.
• Sets foundation for further study on the molecular basis of inheritance.