The following tomato plant traits were studied in a three-point test cross. Norm
ID: 81367 • Letter: T
Question
The following tomato plant traits were studied in a three-point test cross. Normal (round) fruit or ovate (long) fruit, flat leaves or curled leaves and solid color or mottled coloration. F1 plants (heterozygous for all three genes) have normal fruit, flat leaves and solid coloration. F1 plants are crossed with a strain that has ovate fruit, curled leaves and mottled coloration and the following offspring were obtained:
Normal, curled, mottled 3
Normal, curled, solid 26
Normal, flat solid 304
Normal, flat, mottled 68
Ovate, curled, mottled 310
Ovate, curled, solid 60
Ovate, flat, mottled 24
Ovate, flat, solid 5
a) Determine the map of the three genes (show gene order and distance).
b) Sketch the two chromosomes of the trihybrid used to generate these offspring.
c) Calculate the coefficient of coincidence (C) and interference (I).
Explanation / Answer
Answer:
a). flat------7.25 m.u.--------normal----17 m.u.----solid
ovate------7.25 m.u.--------curled----17 m.u.----mottled
b). Normal, flat, solid / Ovate, curled, mottled
c). C = 0.83
I = 0.17
Explanation:
Usually double cross over progeny is more than any recombinant in linkage progeny. So the parental comobinations are Normal, flat, solid / Ovate, curled, mottled
1. If single cross over (SCO) occurs between normal & flat and Ovate & curled
Normal order= normal---------flat & ovate-----curled
After cross over= normal-----curled & ovate------flat
normal-----curled recombinants are 3+26= 29
ovate------flat recombinants are 5+24 =29
Total recombinants = 58
RF = (58/800.)*100 =7.25%
2. If single cross over (SCO) occurs between flat & solid and curled & mottled
Normal order= falt---------solid & curled----mottled
After cross over= flat-----mottled & curled------solid
flat-----mottled recombinants are 68+24= 92
curled------solid recombinants are 60+26= 86
Total recombinants = 178
RF = (178/800)*100 = 22.25%
3. If single cross over (SCO) occurs between Normal & solid and Ovate & mottled.
Normal order= normal---------solid & ovate------mottled
After cross over= normal-----mottled & ovate------solid
normal-----mottled recombinants are 3+68= 71
ovate------solid recombinants are 5+60=65
Total recombinants =136
RF = (136/800)*100 =17%
% RF = Map unit distance
The order of genes is -----
flat------7.25 m.u.--------normal----17 m.u.----solid
RF between flat & normal = 7.25% = 0.0725
RF between normal & solid = 17% = 0.17
Expected double cross overs = 0.0725 * 0.17 = 0.012
Observed double cross overs = 3 + 5 = 8/800 = 0.01
Observed double cross progeny are more than the expected double cross over progeny.
Coefficent of coincidence (CC) = Obeserved dco / Expected dco
CC = 0.01/0.012 = 0.83
Interference = 1-CC = 1-0.83 = 0.17
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