Predicting Inheritance: Test Crosses (AQA A Level Biology): Revision Note
Exam code: 7402
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, e.g. hh, rr, bb
The resulting phenotypes of the offspring provide sufficient information to suggest the genotype of the unknown individual
Analysing results
For a monohybrid test cross:
If there is no recessive phenotype in the offspring → The unknown individual is homozygous dominant
If there is at least one recessive phenotype in the offspring → The unknown individual is heterozygous
For a dihybrid test cross:
If there are no recessive phenotypes (for either gene) → The unknown is homozygous dominant for both genes
If there is at least one recessive phenotype for one gene only → The unknown is heterozygous for one gene and homozygous dominant for the other
If there is at least one recessive phenotype for both genes → The unknown is heterozygous for both genes
Worked Example
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
Predicted ratio of phenotypes of offspring – 1 long ears
Predicted ratio of genotypes of offspring – 1 Dd
Test cross possibility option 2 Predicted ratio of phenotypes of offspring – 1 long ears : 1 short ears
Predicted ratio of genotypes of offspring – 1 Dd : 1 dd
Analysis:
The breeder observes that at least one offspring has the short ear phenotype (recessive)
This indicates that Floppy must be heterozygous (Dd):
Floppy contributed the recessive d allele
If Floppy were homozygous dominant (DD):
All offspring would have long ears
No short-eared offspring would be possible
Examiner Tips and Tricks
Before constructing a genetic diagram, consider:
How many genes are involved (monohybrid or dihybrid?)
How many alleles exist for each gene
Which alleles are dominant and which are recessive
What type of dominance is present:
Complete dominance
Codominance
Incomplete dominance
Is there any linkage (autosomal or sex-linked)?
Is there epistasis (interaction between genes affecting the same trait)?
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