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#unordered tetrad analysis

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Linkage Mapping and Tetrad analysis

Map distance between two genes in unordered tetrad analysis is calculated using

Map distance in unordered tetrads converts tetrad frequencies to recombination fraction based on proportion recombinant chromatids among all chromatids scored. Tetratype ascus contains half recombinant chromatids, nonparental ditype contains all four recombinant chromatids fully recombinant. Simplest formula RF = (TT/2 + NPD)/total tetrads multiplied by 100 gives centimorgan estimate for moderate distances under 30 cM. More accurate mapping including multiple crossover correction uses (TT +6 NPD)/2×total or (TT/2 +3 NPD)/total because NPD may arise from multiple exchange types. Simpler version sufficient for introductory genetics problems.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 5: Unordered Tetrad Map Distance Formula TT/2 + NPD

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Linkage Mapping and Tetrad analysis

In unordered tetrad analysis, linkage is indicated when

In unordered tetrad collection from yeast or other fungus, linkage detection relies on comparing counts PD and NPD. Since PD requires no crossover and NPD requires double crossover involving all chromatids, linked loci favor PD over NPD markedly because single crossover insufficient to generate NPD. Statistical test evaluates whether PD significantly greater than NPD using chi-square or exact test. If equality holds, genes assort independently irrespective TT frequency. Significant excess PD provides evidence linkage even before map distance calculated. Once linkage confirmed, distance calculated weighting TT and NPD contributions appropriately.

Ref: Klug et al., Concepts of Genetics, 12th ed., Chapter 5: Linkage Indicated When PD Greater Than NPD

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