Question

In: Biology

Fine spines (s), smooth fruit (tu), and uniform fruit color (u) are three recessive traits in...

Fine spines (s), smooth fruit (tu), and uniform fruit color (u) are three recessive traits in cucumbers. The genes are located on the same chromosome. A cucumber heterozygous for all three traits is used in a test cross and the following progeny are produced:

S       U        Tu             6

s        u         Tu             100

S        u        Tu              50

s         u        tu               2

S        U        tu              110

s        U        tu               59

s         U       Tu              37

S         u        tu               33

a. Determine the order of genes on the chromosome.

b. Calculate the map distances between the genes.

c. Determine the coefficient of coincidence and the interference among these genes.

Solutions

Expert Solution

a). Orger of genes = s—tu--u

b). Gene map:

s-------------29.47cM-----------tu-------19.65cM----------u

c).

Coefficient of Coincidence (COC) = 0.35

Interference = 1-COC = 0.65

Explanation:

Hint: Always recombinant genotypes are smaller than the non-recombinant genotypes.

Hence, the parental (non-recombinant) genotypes is s u Tu / S U tu

1).

If single crossover occurs between s & u..

Normal combination: s u / S U

After crossover: s U/S u

s U progeny= 59+37=96

S u progeny = 50+33 = 83

Total this progeny = 179

Total progeny = 397

The recombination frequency between s&u = (number of recombinants/Total progeny) 100

RF = (179/397)100 = 45.09%

2).

If single crossover occurs between u & Tu..

Normal combination: u Tu / U tu

After crossover: u tu / U Tu

u tu progeny= 33+2=35

U Tu progeny = 6+37 = 43

Total this progeny = 78

The recombination frequency between u&Tu = (number of recombinants/Total progeny) 100

RF = (78/397)100 = 19.65%

3).

If single crossover occurs between s & Tu..

Normal combination: s Tu / S tu

After crossover: s tu / S Tu

s tu progeny= 2+59 = 61

S Tu progeny = 6+50 = 56

Total this progeny = 117

The recombination frequency between s&Tu = (number of recombinants/Total progeny) 100

RF = (117/397)100 = 29.47%

Orger of genes = s—tu--u

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

s-------------29.47cM-----------tu-------19.65cM----------u

Expected double crossover frequency = (RF between s & tu) * (RF between tu & u)

= 29.47%% * 19.65% = 0.05791

The observed double crossover frequency = 2+6/397 = 0.02015

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

= 0.02015 / 0.05791

= 0.35

Interference = 1-COC

= 1-0.35 = 0.65


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