Three linked autosomal loci were studied in squash: flowering time (late vs. early), fruit shape (elongate vs. ovate) and stem length (long vs. short). Pure breeding early flowering, elongated squash with long stems were mated to pure breeding late flowering, ovated squash with short stems. The F1 were testcrossed and the testcross progeny are given below.

Flowering Fruit Stem Number
time shape Length
Late Elongate Short 628
Early Elongate Short 72
Early Elongate Long 1481
Late Elongate Long 448
Late Ovate Long 57
Early Ovate Long 585
Early Ovate Short 332
Late Ovate Short 1399

1. How many progeny did you look at for this experiment?

2. Which locus is in the center?

3. What is the value for R1? Use two decimal places in your answer.

4. What is the value for R2? Use two decimal places in your answer.

5. What is the coefficient of coincidence? Use four decimal places in your answer.

1. Number of progeny =  5141

2. stem length is the center locus

3. R1= 28.75 cm

4. R2 = 16.53 cm

5. The coefficient of coincidence 0.5726

Explanation:

If single crossover happens between fruit shape and  flowering time ….

Normal combination: Early, elongate / late, ovate

After crossover: Early, ovate / late, enormalate

Early, ovate progeny = 305 + 673= 978

Late, enormalate progeny = 405 + 655 = 1070

Summing up early and late progeny = 978 + 1070 = 2048

Therefore, Total progeny = 5141

The  frequency of recombination (RF) between fruit shape and  flowering time = (number of recombinants/Total progeny) *100

RF = (2048/5141)*100

    =  0.38*100

    =39.84%

2).

If  between fruit shape & stem length…single crossover occurs

Normal combination: elongate, normal / ovate, short

After crossover: elongate, short / ovate, normal

Number of Enormalate, short progeny= 665+64= 729

Number of Ovate, normal progeny = 673+76 = 749

Summing up these progenies, we have 729 + 749 =  1478

The frequency of  recombination between fruit shape & stem length = (number of recombinants/Total progeny) *100

RF = (1478/5141)*100 = 28.75%

3).

If  between flowering time & stem length….single crossover occurs

Normal combination: Early, normal / late, short

After crossover: Early, short / late, normal

Early, short progeny=64+305=369

Late, normal progeny = 76+405=481

Summing up these progenies, we have 369 + 481 = 850

Recall that total progeny = 5141

The  frequency of recombination between flowering time & stem length = (number of recombinants/Total progeny) 100

RF = (850/5141)*100 = 16.53%

Recombination frequency (%) = Distance between the genes (cm)

Fruit shape28.75cMstem length16.53cMflowering time

The expected double crossover frequency = (RF between stem length fruit shape ) * (RF between flowering time and stem length)

= 28.75% * 16.53 = 0.0475

The observed double crossover frequency = 64+76/5141 = 0.0272

Coefficient of Coincidence (COC) = Observed double crossover frequency / Expected double crossover frequency

= 0.0272 / 0.0475 = 0.5726

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