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Test Cross



a test cross of an individual of unknown genotype to an
individual that is fully recessive
the phenotypes of the F1 generation of the test cross
reveals whether the unknown genotype is homozygous or
heterozygous
example:
• you have a white ram (white is dominant “W” and
black is recessive “w”) and want to know if it is
heterozygous or homozygous for breeding purposes.

do a test cross by crossing your unknown ram with one
showing a recessive phenotype.
• it must have a recessive genotype (ww)
w
w
?
w?
w?
?
w?
w?
♂
Ww
Test 1
Test 2
♀
♀
ww
Ww
Ww
ww
ww
♂
WW
ww
Ww
Ww
Ww
Ww
If the ram is Heterozygous it will
produce:
If the ram is Homozygous it will
produce:
Phenotypic ratio: 50% white 50%
black, or 2:2 or 1:1
Phenotypic ratio: 100 % White
Genotypic ratio:
2:2 or 1:1 hetero:homo recessive
Genotypic ratio:
100% heterozygous
Analyze:
Analyze:
Heterozygous Seed shape
crossed with a recessive
seed shape
What are the predicted
phenotypes and genotypes?
Phenotypic Ratio
Test 1
♀
•50% round 50% wrinkled or 2:2
or 1:1, ½, 2/4,
Rr
Genotypic Ratio
♂
rr
Rr
rr
Rr
rr
•50% hetero: 50% homo
recessive, 2:2 or 1:1, hetero 2/4
or ½ homo recessive 2/4 or ½
Example Problem
Offspring Numbers
Phenotype
 A horticulturist has seeds from
a cross but does not know the
round-seed
5472
genotype of the phenotype of
peas
the parents. Use the following
wrinkle1850
information to figure out the
seed peas
parental phenotype and
genotype
Solution
 because there is two different phenotypes one the
parents must not be homozygous dominant
Offspring
Phenotype
Numbers
round-seed
peas
5472
wrinkleseed peas
1850
Solution
 because there are two
different phenotypes one
the parents must not be
homozygous dominant
Solution
 5472/1850 = 2.96
2.96/1 or 2.96 : 1
~3:1
to get a 3:1 ratio both parents
must be heterozygous
Tester
♀
♂
R
R
??
R?
R?
R?
R?
Mendelian Genetics
Proof
♀
♂
Rr
Proof
♀
Rr
RR
Rr
Rr
rr
3:1 Phenotypic ratio
♂
Rr
rr
Rr
Rr
rr
rr
1:1 (50%) Phenotypic ratio