• A test cross can be used to?deduce the?genotype?of an unknown individual that is expressing a dominant?phenotype
• The individual in question is crossed with an individual that is expressing the?recessive phenotype
• This is because an individual with a recessive phenotype has a known genotype
• The resulting phenotypes of the offspring provides sufficient information to suggest the genotype of the unknown individual

Results

• For a monohybrid test cross:
  • If?no?offspring exhibit the recessive phenotype then the unknown genotype is?homozygous dominant
  • If?at least one?of the offspring exhibit the recessive phenotype then the unknown genotype is?heterozygous
• For a dihybrid test cross:
  • If no offspring exhibit the recessive phenotype for either gene then the unknown genotype is?homozygous dominant?for both genes
  • If at least one of the offspring exhibit the recessive phenotype for one gene but not the other, then the unknown genotype is?heterozygous for one gene and homozygous dominant for the other
  • If at least one of the offspring exhibit the recessive phenotype for both genes then the unknown genotype is?heterozygous for both?genes

Worked example: Test crosses

• Rabbits have a single gene for ear length that has two alleles:
  • D, a dominant allele that produces long ears
  • d, a recessive allele that produces shorter ears
• A breeder has a rabbit called Floppy that has long ears and they want to know the genotype of the rabbit
  • There are two possibilities:?DD?or?Dd
• The breeder crosses the long-eared rabbit with a short-eared rabbit
  • A rabbit displaying the recessive short ear phenotype has to have the genotype?dd

Test cross possibility one table

Predicted ratio of phenotypes of offspring – 1 long ears

Predicted ratio of genotypes of offspring – 1 Dd

Test cross possibility two table

Predicted ratio of phenotypes of offspring – 1 long ears : 1 short ears

Predicted ratio of genotypes of offspring – 1 Dd : 1 dd

• The breeder identifies the different phenotypes present in the offspring
• There is at least one offspring with the short ear phenotype
• This tells the breeder that their rabbit Floppy has the genotype?Dd
• If Floppy was genotype?DD?none of the offspring would have short ears