4. Fine spines (s), smooth fruit (t), and uniform fruit color (u) are recessive

Question

4. Fine spines (s), smooth fruit (t), and uniform fruit color (u) are recessive traits in cucumbers whose genes are linked on the same chromosome. A cucumber that is heterozygous for all three traits is used in a testcross, and the following progeny are produced:

Progeny Phenotype Number of Progeny

S U T 2

s u T 70

S u T 21

s u t 4

S U t 82

s U t 21

s U T 13

S u t 17

total = 230

a. Determine the order of the genes on the chromosome.

b. Calculate the map distance between the genes.

c. Determine the coefficient of coincidence and interference.

d. List the alleles found on the chromosome of the homozygous parents of the heterozygous plant used in the testcross.

Explanation / Answer

A) Nonrecombinants are s u T and S U t.

Double crossovers are s u t and S U T.

Because T differs between the nonrecombinants and the double crossovers, T is the middle gene.

B) S-T distance: recombinants are S T and s t.

RF = (2 + 4 + 21 + 21)/230 = 48/230 = 20.9% or 20.9 m.u.

U-T distance: recombinants are u t and U T.

RF = (2 + 4 + 13 + 17)/230 = 36/230 = 15.6% or 15.6 m.u.

c) Expected dco frequency = (20.9%)(15.6%)=(.209)(.156) =0.0326=3.26%

Observed dco frequency = 6/230 = 2.6%

C.o.C. = observed dcos/expected dcos = 2.6/3.26 = 0.8

I = 1 – C.o.C. = 0.2

d) In the correct gene order for the heterozygous parent: s T u and S t U

For the testcross parent: s t u and s t u

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