Fine spines (s), smooth fruit (tu), and uniform fruit color (u) are three recessive traits in
cucumbers whose genes are linked on the same chromosome. A cucumber
plant heterozygous for all three traits is used in a testcross and the progeny at the top of the
following page are produced from this testcross.
S
U
Tu
2
s
u
Tu
70
S
u
Tu
21
s
u
tu
4
S
U
tu
82
s
U
tu
21
s
U
Tu
13
S
u
tu
17
total
230
(a) Determine the order of these genes on the chromosome (1 pt).
Nonrecombinants are s u Tu and S U tu.
Double crossovers are s u tu and S U Tu.
Because Tu differs between the nonrecombinants and the double crossovers, Tu is
the middle gene.
(b) Calculate the map distances between the genes (2 pts).
S-Tu distance: recombinants are S Tu and s tu.
RF = (2 + 4 + 21 + 21)/230 = 48/230 = 20.9% or 20.9 m.u.
U-Tu distance: recombinants are u tu and U Tu.
RF = (2 + 4 + 13 + 17)/230 = 36/230 = 15.6% or 15.6 m.u.
(c) Determine the coefficient of coincidence and the interference among these genes (1 pt).
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) List the genes, in the correct order, found on each chromosome in the parents used in
the testcross (1 pt).
In the correct gene order for the heterozygous parent: s Tu u and S tu U
For the testcross parent: s tu u and s tu u