17. Fine spines (s), smooth fruit (tu), and uniform fruit color (u) are three re
ID: 196060 • Letter: 1
Question
17. Fine spines (s), smooth fruit (tu), and uniform fruit color (u) are three recessive traits in cucumbers, the genes of which are linked on the same chromosome. A cucumber plant heterozygous for all three traits is used in a testcross, and the following progeny are produced from this testcross SUTu s u Tu S u Tu s u tu S U tu s U tu s U Tu s u tu Total 70 21 4 82 21 13 17 230 a. Determine the order of these genes on the chromosome (3 pts.) b. Calculate the map distances between the genes (10 pts)Explanation / Answer
Answer:
a). s tu u
b). s-------------20.87cM-----------tu-------15.65cM----------u
Explanation:
Answer:
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= 21+13=34
S u progeny = 21+17=38
Total this progeny = 72
The recombination frequency between s&u = (number of recombinants/Total progeny) 100
RF = (72/230)100 = 31.3%
2).
If single crossover occurs between u & Tu..
Normal combination: u Tu / U tu
After crossover: u tu / U Tu
u tu progeny= 4+17=21
U Tu progeny = 2+13=15
Total this progeny = 36
The recombination frequency between u&Tu = (number of recombinants/Total progeny) 100
RF = (36/230)100 = 15.65%
3).
If single crossover occurs between s & Tu..
Normal combination: s Tu / S tu
After crossover: s tu / S Tu
s tu progeny= 4+21=25
S Tu progeny = 2+21=23
Total this progeny = 48
The recombination frequency between s&Tu = (number of recombinants/Total progeny) 100
RF = (48/230)100 = 20.87%
Recombination frequency (%) = Distance between the genes (cM)
s-------------20.87cM-----------tu-------15.65cM----------u
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