Explain how to use a test cross to identify an unknown genotype

Sourav PanNovember 15, 2024

Answered step-by-step

A test cross is a powerful genetic tool used to determine the genotype of an individual exhibiting a dominant phenotype. This method involves crossing the individual with an organism that is homozygous recessive for the trait in question. Here’s how it works and how to interpret the results.

Steps to Perform a Test Cross

1. Identify the Individual of Unknown Genotype:
   • The organism in question displays a dominant trait but has an unknown genotype (it could be either homozygous dominant, e.g., AA, or heterozygous, e.g., Aa).
2. Select a Homozygous Recessive Individual:
   • Choose an individual that is homozygous recessive for the same trait (e.g., aa). This organism’s genotype is known and will only contribute recessive alleles.
3. Conduct the Cross:
   • Cross the individual with the unknown genotype with the homozygous recessive individual.

Analyzing the Results

After performing the test cross, observe the phenotypes of the offspring:

• All Offspring Display Dominant Trait:
  • If all offspring express the dominant phenotype, it indicates that the unknown parent is likely homozygous dominant (AA).
• Offspring Display Both Dominant and Recessive Traits:
  • If there is a mix of phenotypes, specifically if both dominant and recessive traits are observed (e.g., 1:1 ratio), this suggests that the unknown parent is heterozygous (Aa).

Example

Scenario: Flower Color in Pea Plants

• Dominant Trait: Purple flowers (P)
• Recessive Trait: White flowers (p)

1. Unknown Genotype: A purple flowered plant (unknown if it’s PP or Pp).
2. Tester Plant: A white flowered plant (pp).

Punnett Square Analysis

• If Unknown Genotype is Homozygous Dominant (PP):

• Offspring: All purple flowers (Pp).
• If Unknown Genotype is Heterozygous (Pp):

• Offspring: 2 purple flowers (Pp) and 2 white flowers (pp), resulting in a 1:1 ratio.