7.1.4 Predicting Inheritance: Test Crosses

How a Test Cross Works

1. The organism showing the dominant phenotype (genotype unknown) is crossed with an organism that is homozygous recessive (aa).

• The homozygous recessive organism is used because its genotype is known, and any offspring showing the recessive phenotype can reveal whether the unknown organism is heterozygous.

2. Outcomes:

• If all offspring show the dominant phenotype, the organism is homozygous dominant (AA).
• If the offspring show a 1:1 ratio of dominant to recessive phenotypes, the organism is heterozygous (Aa).

Example

1. Phenotype: A pea plant with purple flowers (dominant trait) is tested to determine if it is homozygous (PP) or heterozygous (Pp).
2. Cross:

• Test with a white-flowered pea plant (recessive, pp).

3. Results:

• All offspring are purple → the tested plant is homozygous (PP).
• Offspring are 50% purple, 50% white → the tested plant is heterozygous (Pp).