Dr. Ruth Ballard
BIO 184
Page 1
Fall 2006
PRACTICE TRIHBRID TEST CROSS MAPPING PROBLEMS
1.
Three recessive traits in garden pea plants are as follows: yellow pods are recessive
to green pods, bluish-green seedlings are recessive to green seedlings, and creeper
(a plant that cannot stand up) is recessive to normal. A true-breeding normal plant
with green pods and green seedlings was crossed to a creeper with yellow pods and
bluish-green seedlings.
a.
Write out this cross and predict the genotype(s) and phenotype(s) of the
F1s.
b.
Write out the cross you would perform to determine the order and map
distance of the genes, assuming they are linked on the same chromosome.
c.
After performing the cross you identified in part b (above), the results are
as follows:
2059
151
281
15
2041
157
282
11
green pods, green seedlings, normal
green pods, green seedlings, creeper
green pods, bluish-green seedlings, normal
green pods, bluish-green seedlings, creeper
yellow pods, bluish-green seedlings, creeper
yellow pods, bluish-green seedlings, normal
yellow pods, green seedlings, creeper
yellow pods, green seedlings, normal
Construct a genetic map that shows the order of the genes along the chromosome
and the distance between adjacent genes.
Dr. Ruth Ballard
BIO 184
2.
Page 2
Fall 2006
In tomato, three genes are linked on the same chromosome. Tall(T) is
dominant to dwarf(t), fruit skin that is smooth (P) is dominant to skin that is
peachy (p), and fruit with a normal tomato shape (O) is dominant to oblate
shape (o). A plant that is heterozygous for all three traits (trihybrid) is test
crossed to produce the following phenotypic distribution among the
offspring:
33
155
11
29
151
12
T,P,O
T,P,o
TpO
t,p,o
t,p,O
t,P,o
Dr. Ruth Ballard
BIO 184
Page 3
Fall 2006
a.
Draw out the original cross that was made between the two pure-breeding
parents to produce the trihybrid. How do you know?
b.
Why are there only six, rather than eight, phenotypes among the offspring
of the test cross?
c.
Draw a genetic map of the chromosome, showing the order and relative
distances (in cM) between the three genes.