Test-Cross
• Test-cross – method used by breeders to
determine the genotype of an animal
– Mate the individual with an unknown genotype
with one that you know for sure (HOMOZYGOUS
RECESSIVE)
– Based on the results, you will know the unknown
animal!
Example
• Dwarfism in dogs is a recessive trait
– NN __________
– Nn __________
– nn __________
Example Problem:
• You mate your normal-sized dog with a dwarf dog.
The litter turns out to be half normal, half dwarf. Was
the genotype of your dog NN or Nn?
_____x_____
_____x_____
NN ________________
Nn ________________
nn ________________
_____x_____
_____x_____
• Used by breeders (dogs, cats, horses, etc) to
determine whether or not their animal is purebred
for a trait
• Definition: cross between an individual with an
unknown genotype with an individual with a known
genotype
• The individual with the known genotype is
HOMOZYGOUS RECESSIVE – if you know something
has a recessive trait, you know its genotype (2 little
letters)
• A breeder performs a testcross to determine
whether a dog is homozygous dominant (NN) or
heterozygous (Nn) for a recessive dwarf allele. Half
the offspring appear dwarf. What is the genotype of
the unknown dog? ________