Overview
This lecture covers key concepts in chromosome mapping, recombination, and gene linkage, focusing on how geneticists determine gene order and distances on chromosomes.
Chromosome Mapping and Linkage
- Genes located close together on a chromosome are called linked and tend to be inherited together.
- Independent assortment occurs when genes are on different chromosomes or far apart on the same chromosome.
- Linkage can be detected through testcrosses showing parental combination frequencies higher than recombinant types.
Recombination and Crossing Over
- Recombination results from crossing over during meiosis, producing new combinations of alleles.
- The frequency of recombination between two genes correlates with their physical distance on a chromosome.
- One map unit (centimorgan, cM) equals a 1% recombination frequency.
Constructing Genetic Maps
- Geneticists use recombination frequencies from testcrosses to assign a linear order to genes on chromosomes.
- Double crossovers can occur and must be accounted for in three-point testcrosses.
- The sum of adjacent recombination frequencies approximates the distance between distant genes, but double crossovers can lead to underestimation.
Interference and Coincidence
- Interference occurs when one crossover event reduces the likelihood of another nearby crossover.
- Coefficient of coincidence measures the expected versus observed double crossover frequencies.
- Interference = 1 - coefficient of coincidence.
Mapping Example and Calculations
- Three-point testcrosses are used for mapping three genes and determining their order.
- Parental, single crossover, and double crossover progeny must be distinguished by phenotype.
- The gene in the middle is identified by comparing double crossover progeny to parental types.
Key Terms & Definitions
- Linkage — genes located close together on the same chromosome, inherited together.
- Recombination — exchange of genetic material between homologous chromosomes during meiosis.
- Map unit (centimorgan, cM) — distance between genes corresponding to a 1% recombination frequency.
- Interference — phenomenon where one crossover reduces the probability of another crossover nearby.
- Coefficient of coincidence — observed double crossovers divided by expected double crossovers.
Action Items / Next Steps
- Complete assigned three-point testcross problems for practice.
- Review the definitions of linkage, recombination, and genetic mapping for the next class.