Download Test Cross Review 7th Grade PSI Science Name Use the

Test Cross Review
7th Grade PSI Science
Name _______________________________________
Use the information below to help you with the questions.
The trait for purple petal flowers is dominant to the trait for white colored flowers. The traits can
be represented by the letters P and p.
1. Which letter represents the purple petal trait?
2. Which letter represents the white petal trait?
3. What would be the genotype for a flower that is homozygous dominant?
4. What would be the genotype for a flower that is homozygous recessive?
5. What would be the genotype for a flower that is heterozygous?
6. What genotype(s) would produce a phenotype of purple petals?
7. What genotype(s) would produce a phenotype of white petals?
8. What type of flower are you going to use to test cross with a purple flower that has an
unknown genotype? Why?
9. If you perform a test cross and the offspring come out to be 100% for the dominant trait
what does this tell you about the genotype of the unknown parent (heterozygous or
homozygous)?
10. If you perform a test cross and 50% of the offspring represent the recessive trait, what
does this tell you about the genotype of the unknown parent (heterozygous or
homozygous)?
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7th Grade PSI
Inheritance and Variation of Traits
Use what you have learned and the clues from above to help you complete theses test cross
problems.
A. In dogs, floppy ears are dominant over straight ears. You are a dog breeder and your friend
wants you to breed them a pure bred dog with floppy ears. In your current litter you have
dogs with straight and floppy ears.
REMEMBER: In order to determine which dog has two dominant genes for floppy ears you
perform a test cross involving a homozygous recessive straight ear dog with the selected
floppy ear dog.
If 4 of the 8 puppies have straight ears what would the genotypes of the dogs in the test cross?
(Use “F” and “f”)
X
Genotype ratio (%)
Phenotype ratio (%)
B. In mice, long tails are dominant over short tails. You own a pet shop and sell mice with short
and long tails. Your friend wants to buy a mouse that is pure bred for long tails.
To determine which long tailed mouse is pure bred (homozygous dominant), you perform a
test cross with that mouse.
If 30 of the 30 offspring mice have long tails what would be the genotypes of the mice used in
the test cross? (Use “L” and “l”)
X
Genotype ratio (%)
Phenotype ratio (%)
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7th Grade PSI
Inheritance and Variation of Traits
Answer Key
1.
2.
3.
4.
5.
6.
7.
8.
P
p
PP
pp
Pp
PP or Pp
Only pp
You would use a white flower because you already will know that its genotype is pp.
The only way that the flower could show this recessive trait is if it has two recessive
alleles
9. This would mean that the parent was homozygous dominant and was only able to pass on
the dominant trait
10. This would mean that the parent was heterozygous. In this situation, the parent has a
50/50 chance of passing on either the dominant or recessive trait
F
A.
Ff x ff
f
Ff
ff
Ff
ff
f
Genotype ratio: 50% Ff 50% ff
Phenotype ratio: 50% floppy 50% straight
f
L
A.
LL x ll
L
Ll
Ll
Ll
Ll
l
Genotype ratio: 100% Ll
Phenotype ratio: 100% long tails
l
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7th Grade PSI
Inheritance and Variation of Traits