CIE A Level Biology (9700) exams from 2022

Revision Notes

16.2.4 Predicting Inheritance: Test Crosses

Predicting Inheritance: Test Crosses

  • 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

Test Cross Possibility One Table, downloadable AS & A Level Biology revision notes

Predicted ratio of phenotypes of offspring – 1 long ears

Predicted ratio of genotypes of offspring – 1 Dd

Test cross possibility two table

Test Cross Possibility Two Table, downloadable AS & A Level Biology revision notes

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

Exam Tip

Make sure before you start a test cross you think about the following: how many genes are there, how many alleles of each gene are there, which is the dominant allele, what type of dominance is it and is there linkage or epistasis between genes?

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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