Coefficient of coincidence and interference
When during crossing over, the chiasma formation in one region of chromosome prevents or reduces the chances of formation of other chiasma just after the immediate adjacent region to the first crossing over, this is termed as interference This leads to lowering of number of double crossing over products then that of expected number
Coefficient of coincidence is the ratio between the number of observed double crossovers and number of expected double crossovers Coefficient of coincidence is used to determine interference.
If there is complete interference i.e. no double crossover products are observed then coefficient of coincidence will be zero. If there is partial interference then coefficient of coincidence will be between 0 and 1 Ifthere is no interference and expected double crossover products are observed then coefficient of coincidence will be 1.
Number of observed crossing over coefficient of coincidence Number of expected crossing over Interfere nce -coefficient of coincidence
Que- In Drosophila, Bar eye(B) is a dominant mutation while miniature wing(m) and yellow body color (y) are recessive mutations. Heterozygous females for these mutations were crossed to normal eyed miniature winged and yellow body colored males. Based on the results obtained determine the gene order and distance between the genes.
Number Phenotype 40 30 175 130 25 195 120 B+ m+ y Bm+ y B m y B m y
Phenotype Bytm+ Bt y m Bt y m+ B yt m By m Number Crossover 195 175 130 120 Parental type Parental type Single crossover between y and m Single crossover between y and m Single crossover between B and y Single crossover between B and y Double crossover Double crossover 40 30 25 By m By m+
No. of single crossing over No. of double crossing over Recombinant frequency between genes Total number of offsprings (55+40) (25+30) Recombinant frequency between B and y 100 195+175+130+120+55 40+30+25 - (95) +(55) Recombinant frequency between B and y 100 770 150 10019.48CM 770
No. of single crossing over No. of double crossing over Recombinant frequency between genes Total number of offsprings (120+130) + (25+30) Recombinant frequency between y and m 100 195+175+130+120+55 40+30+25 (250) +(55) Recombinant frequency between y and m 100 770 305 10039.61cM 770
Expected double crossover Frequency of single crossover between B and y Frequency of single crossover between y and m Expected double crossover 0.19 0.39 0.074
Observed _25+30 double25+3055 crossover Coefficient of coincidence 57.06 0.96 Interference 1- 0.96.04
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