Dihybrid Test Cross

In the context of principles of inheritance and variation, a “dihybrid test cross” is a genetic cross used to determine the genotype of an individual showing a dominant phenotype for two different traits. This type of cross involves mating the individual of unknown genotype (usually homozygous dominant for both traits) with a recessive homozygous individual for both traits. The offspring’s phenotypes can then reveal the genotype of the individual in question.

1. Parental Genotypes:

The individual with an unknown genotype (usually homozygous dominant for both traits) is referred to as the “test cross individual” or “individual under test.”

A recessive homozygous individual for both traits is used as the other parent. This individual is known as the “tester” or “test cross parent.”

2. Traits of Interest:

In a dihybrid test cross, two different traits are considered simultaneously. These traits are governed by two different genes located on separate chromosomes.

3. Crossing:

The test cross individual (unknown genotype) is crossed with the tester individual (recessive homozygous for both traits).

4. Offspring Phenotypes:

The phenotypes of the offspring resulting from this cross are observed and recorded.

5. Interpretation:

The phenotypic ratios among the offspring can provide information about the genotype of the test cross individual.

If all the offspring display the dominant phenotype for both traits, it indicates that the test cross individual is most likely homozygous dominant (AA BB) for both traits.

If some offspring display the recessive phenotype for one or both traits, it suggests that the test cross individual is heterozygous (Aa Bb) for one or both traits.

If all the offspring display the recessive phenotype for both traits, it indicates that the test cross individual is homozygous recessive (aa bb) for both traits.

6. Phenotypic Ratios:

The specific phenotypic ratios among the offspring provide clues about the genotype of the test cross individual.