lecture recording on 12 March 2025 at 12.12.40 PM

Mapping Genes in Chromosomes

• Understanding the distances between genes on chromosomes is crucial for genetic mapping.

• Old school methods rely on analyzing parental and recombinant genotypes to estimate the relative distances between genes.

• Each chromosome in the body cells contains two dominant alleles (e.g., A allele) rather than one dominant and one recessive (e.g., G allele).

• This dominant-recessive structure is constant across body cells for traits being studied.

Methodology of Genetic Mapping

• Genetic mapping is an indirect measurement method that uses recombination frequencies to estimate gene distances.

• The concept of map units is employed, where one map unit (or centiMorgan, cM) corresponds to a 1% recombination frequency between genes on the chromosome.

Calculating Genetic Distances

• To calculate distances, divide the number of recombinant offspring by the total number of offspring and multiply by 100 to express the result in map units.

• Example Calculation: 0.05 (recombinant frequency) divided by 2839 (total offspring) equals 0.74, and multiplying by 100 gives the distance in map units.

Example Problem in Genetic Mapping

• A problem presents a cross between two different fly phenotypes:

  • Parental Types: Brown body with normal wings crossed with black body with short wings.

  • This example works effectively when the wild type alleles are linked.

Phenotype and Genotype Analysis

• Key phenotype counts from the offspring included:

  • Brown body normal wings: 842

  • Brown body short wings: 818

• Determining parental sequences involves identifying which offspring genotypes correspond to the original parent phenotypes.

Parental and Recombinant Statistics

• Parental Sequence: Brown body normal wings, black body short wings.

• Recombinant Sequence: Brown body short wings, black body normal wings.

• The identification of linked alleles helps illustrate how genes can swap between parent chromosomes during recombination.

Important Considerations

• The mapping method requires a double heterozygous individual crossed with a double homozygous recessive individual for accuracy.

• A successful mapping requires the dominant alleles to originate from the same parent.

• Phenotype counts not only provide insights into the genotype but also help track the inheritance patterns of traits.

Conclusion

• Genetic mapping is a fundamental technique used to determine the distances between genes on chromosomes.

• Using parental and recombinant genotype ratios can yield mapping unit distances essential for understanding genetic relationships.