A test cross is an experimental mating used to determine the genotype of an individual displaying a dominant phenotype, by crossing it with a homozygous recessive individual.
Understanding the Test Cross
The test cross, developed by Gregor Mendel, is a powerful tool in genetics used to decipher whether an organism with a dominant trait is homozygous dominant or heterozygous for that trait. Since a dominant phenotype can arise from either a homozygous dominant (AA) or a heterozygous (Aa) genotype, a test cross is necessary to distinguish between the two.
Key Components
- Individual of Unknown Genotype (but Dominant Phenotype): This is the organism whose genotype (AA or Aa) you want to determine.
- Homozygous Recessive Individual (aa): This organism expresses the recessive phenotype because it has two copies of the recessive allele. This individual must be homozygous recessive because any dominant allele from this parent would mask the possible recessive allele from the tested parent.
How It Works
The test cross involves mating the individual with the unknown genotype to a homozygous recessive individual. The offspring's phenotypes reveal information about the unknown parent's genotype.
Possible Outcomes and Interpretations
The phenotypic ratios of the offspring from the test cross provide the key to unlocking the unknown genotype.
| Unknown Genotype | Cross | Offspring Phenotypes | Phenotypic Ratio | Conclusion |
|---|---|---|---|---|
| Homozygous Dominant (AA) | AA x aa | All Dominant Phenotype | 100% | The individual of unknown genotype is homozygous dominant (AA). |
| Heterozygous (Aa) | Aa x aa | Half Dominant, Half Recessive | 1:1 | The individual of unknown genotype is heterozygous (Aa). |
Example:
Let's say you have a pea plant with purple flowers (the dominant trait). You want to know if it's homozygous dominant (PP) or heterozygous (Pp).
- Cross: You cross the purple-flowered plant with a white-flowered plant (pp – the homozygous recessive).
- Observation:
- Scenario 1: All offspring have purple flowers. This suggests the original purple-flowered plant was homozygous dominant (PP).
- Scenario 2: Approximately half the offspring have purple flowers, and half have white flowers. This suggests the original purple-flowered plant was heterozygous (Pp).
Importance
The test cross played a crucial role in Mendel's experiments and continues to be a valuable technique in genetics for:
- Determining the genotype of an individual with a dominant phenotype.
- Analyzing linkage and mapping genes.
- Plant and animal breeding.
In summary, the test cross helps determine whether an individual expressing a dominant trait carries two copies of the dominant allele or one dominant and one recessive allele by observing the phenotypes of its offspring when crossed with a homozygous recessive individual.
