Gene mapping from three point cross
Three point cross involves three gene loci. It is used to determine gene order and distance between three genes. From three point cross different recombinant products are obtained because of single and double crossing over between the genes.
Single crossing over between R and S Single crossing over between S and T Double crossing over R S T R ST R ST r S
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
Parental type B m+ yB m y Double crossover Bm+ y Gene order B y m
Single crossing over between B and y Single crossing over between y and m Parent B y+ m+ B y+m Parent 2 Bt y m B+ y m B y m y+ m B+ y+ m+ B+ y m+
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 B and y 100 195+175+130+120+55 40+30+25 (250) +(55) Recombinant frequency between B and y 100 770 305 10039.61cM 770
Genotype Number observed Wild type(++t) Dusky body(d++) Cut wings(tct) White eyes(+tw) Dusky body, cut wings(d,c+) Dusky body, cut wings, white eyes(d,c,w) 22 18 153 6 76 72 Dusky body, white eyes(dtw) Cut wings, white eyes(+cw) 3 150
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