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Interference
- 1.
- 2. WHAT DOES INTERFERENCEMEAN?? • Interference in genetics linkage means “inhibition of further cross over events by a crossover event in a nearby region of the chromosome” Or • “occurrence of an event in a given interval may reduce or enhance the occurrence of an event in its vicinity.
- 3. in simple terms.. •One crossover which occurs at a spot on chromosome will affect the second crossover occurring in the nearby spot, that is will reduce its chances of crossing over.
- 4. • The diagramshows double strand break of the chromosome followed by pairing of homologous chromosomes and repair. • Chromosomal repair leads to cross over and non cross over pairs of the chromosome. • Cross over recombination is shown by DHJ model (branched nucleic acid structure that contains 4 double stranded arms joined) and • non cross over is shown by SDSA model (repair of double strand break without formation of DHJ thus forming homologous pairs). Interference shown in this diagram using the model of meiotic recombination.
- 5. Coefficient of Coincidenceexpress the extent of interference • Interference = 1- coefficient of coincidence (C) Observed DCO C = ----------------- Expected DCO • The values are inversely related. • A value of 1 means the number of the double crossovers that occurs is what would be predicted on the basis of 2 independent events, and there is no interference. • A value of 0 means that none of the expected crossovers occurred, and interference is total.
- 6. Positive interference • “suppression”of recombination events in the vicinity of the cross over segment. • If fewer DCOs than expected occur, I is a positive number and positive interference has occurred. • In C. elegans, for example, only one crossover event per chromosome is observed, and interference along each chromosome is complete (C = 1.0).
- 7. Negative interference • “enhancement”of recombination events. • If more DCOs than expected occur, I is a negative number and negative interference has occurred. • In this case, occurrence of one cross over event increases the likelihood of a second cross over event occurring in the same vicinity.
- 8. Difference b/w positiveand negative interference Positive interference Negative interference 1. One crossover reduces the chance of another crossover in the adjacent region. 1. One crossover enhances the chance of another crossover in the adjacent region. 2. Mostly in eukaryotes 2. Found in some lower organisms 3. In this case coefficient of coincidence is less than one. 3. In this case coefficient of coincidence is always more than one.
- 9. EXAMPLE : GENOTYPE NO.OF OFFSPRING ch b+ cn 105 ch+ b+ cn+ 750 ch+ b cn 40 ch+ b + cn 4 ch b cn 753 ch b+ cn+ 41 ch+ b cn+ 102 ch b cn+ 5 Ques: Determine : 1) gene order 2) gene distance 3) COC 4) interference
- 10. Genotype No. ofoffspring ch b+ cn 105 SCO ch+ b+ cn+ 750 PARENT TYPE ch+ b cn 40 SCO ch+ b + cn 4 DCO ch b cn 753 PARENT TYPE ch b+ cn+ 41 SCO ch+ b cn+ 102 SCO ch b cn+ 5 DCO
- 11. 1)Gene order By comparingparent type and DCO ch+ b+ ch b ch+ b+ ch b cn+ cn cn cn+ cn and cn+ being uncommon should be in centre in order to get a correct gene order. We get, ch+ cn+ b+ or b+ cn+ ch+
- 12. 2)Gene distance Distance betweench and cn ch cn b _______________ ch+ cn+ b+ ch cn+ b+ = 41 ________________ = 40 ch+ cn b Recombinant frequency = sco+sco’+dco+dco’ X 100 total no. of offsprings = 40+41+5+4 X 100 1800 = 5 mu
- 13. Distance between cnand b ch cn b ch cn b+ = 105 _________ _________ = 102 ch+ cn+ b+ ch+ cn+ b Recombinant frequency = 105+102+5+4 X 100 1800 = 12 mu ch cn b 5 12 = 17 mu
- 14. 3) COC =no. of observed DCO no. of expected DCO Observed DCO = 5+4=9 Expected DCO = SCO= 0.05 SCO’= 0.12 = 0.12 x 0.05 x 1800 = 10.8 COC = 09 10.8 = 0.83 4) Interference = 1-COC = 1-0.83 = 0.17