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)?

Use what you have learned and the clues from above to help you complete theses test cross problems.

  1. 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 (%)

  1. 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 (%)

Answer Key

  1. P
  2. p
  3. PP
  4. pp
  5. Pp
  6. PP or Pp
  7. Only pp
  8. 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 f

Ff / ff
Ff / ff

Ff x ff

f

Genotype ratio: 50%Ff 50% ff

Phenotype ratio: 50% floppy 50% straight

f

L L

Ll / Ll
Ll / Ll

LL x ll

l

Genotype ratio: 100% Ll

Phenotype ratio: 100% long tails

l

Grade PSI Inheritance and Variation of Traits