Lecture 5: Genetic interactions and epistasis A. Epistasis ... · 1 Lecture 5: Genetic interactions...

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1 Lecture 5: Genetic interactions and epistasis A. Epistasis in a biochemical pathway B. Epistasis in a regulatory pathway C. Additive interactions D. Synergistic interactions E. Suppressions

Transcript of Lecture 5: Genetic interactions and epistasis A. Epistasis ... · 1 Lecture 5: Genetic interactions...

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Lecture 5: Genetic interactions and epistasis

A. Epistasis in a biochemical pathwayB. Epistasis in a regulatory pathwayC. Additive interactionsD. Synergistic interactionsE. Suppressions

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C2

red

CHI

F3H

A1 A2 BZ1

BZ2

GLUCOSIDE

ANTHOCYANINS

Mt1, Mt2

DIHYDROFLAVONOLFLAVAN-3,4-DIOL

FLAVANONECHALCONE

Peonidin-3-(p-coumaroyl)-rutinoside-5-gluciside

epistasis analyses (genetic interactions among different mutations)

A. Flavonoid biosynthetic pathway in maize

bronze

2

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WT: RedMutations in c2, a1, a2: ColorlessMutations in bz1, bz2: bronze

Double mutants

C2/a1: colourless-but uninformativebz1/a1: colorless-a1 comes before bz1bz2/a1: colorless-a1 comes before bz2

For biosynthetic pathways, the phenotype of the earlier gene in the pathway shows in the double mutant.

ie. the earlier-step mutant is epistatic to the late-step mutant

Determine relationship between a1 and c2 by feeding experiment:add flavanone (naringenin): c2+naringenin = red

a1+naringenin = colorless

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Fig. 7.20

Biochemical Pathways

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Fig. 7.20

See p214-215 ofthe textbook

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B. Regulatory pathways

Signal A B C D gene expression

Positive action-stimulate next step. Null mutation makes insensitive to signal

Negative action-represses next step. Null mutation makes the gene turned on at all time (constitutively)

d-: gene expression constitutively oneven in the absence of signal

b-: gene expression never turned on even in the presence of the signal

b-d- = d- : constitutively on

For regulatory pathways, the phenotype of the later-acting genes shows in the double mutant. ie. the later-acting mutant is epistatic to the earlier-acting mutant 5

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etr1

wtethylene air ethyleneair

ctr1wt ein2

Ethylene CTR1 (Kinase) EIN2triple response

ctr ein2 :?

For regulatory pathways, the phenotype of the later-acting genes shows in the double mutant. ie. the later-acting mutant is epistatic to the earlier-acting mutant

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C. Additive pathways

Double mutants of dissimilar phenotypes produce a combination of both phenotypes

Indicate that the two mutations are in genes acting in separate pathways

ap2-2 (flower abnormal) X gl (no trichome)

ap2-2 gl double mutantabnormal flower and no trichome

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ap2-2

gl1

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D. Synergistic interactions (enhancement)

Two genes may act at the same step of a pathwayOr in parallel or (redundant) pathways

E. Suppression Intrgenic suppressorsExtragenic suppressors

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ap1-1

ap1-1 cal-1 ap1-1 cal-110

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E. Suppression

Intrgenic suppressors

Extragenic suppressors

Allele-specific suppression