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handout 8 (for students)

handout 8 (for students) - Handout#8 Linkage Linkage...

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Handout #8 Linkage Linkage & Recombination between 2 genes: - 2 scenarios : completely linked (rare or none) & partially linked - testcross used to find whether or not genes are linked & partially or completely linked Complete linkage: Independent assortment: Partial linkage: The further apart between 2 genes, the more likely to cross therefore the more recombinant gametes are expected to see When genes are REAL far apart, genes act as if unlinked. You will see 25% (maximum amount) in each class for test cross with a heterozygous parent. Genetic map is based on recombinant frequency Recombinant frequency between gene A & B (R.f.) =(# of recombinant / total # progeny) *100% Units: map units (mu) or centiMorgan (cM) R.f. indicates the strength of linkage. If 2 genes are tightly linked, the r.f. will be low. If 2 genes are loosely linked, the r.f. will be high. For short map distances, recombinant fraction (RF) is small and approximately equal to map distance. RF = distance in centiMorgans (cM). Coupling phase is when dominant alleles are both on one chromosome & recessive alleles together on other chromosomes Repulsion phase is the opposite condition of coupling phase 3-point Testcross : A 3-point (or 3-factor) testcross is the cross between a parent that is heterozygous for 3 genes (i.e.
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