A test cross is carried out to

The correct option is B

determine the genotype of a plant at F₂

The correct option is B.

Explanation of the correct option:

1. It is difficult to know the genotypic composition of a dominant phenotype by simply looking at it.
2. A test cross is an experiment that is used to find out the genotype of a dominant plant of the F₂ generation.
3. It is performed by crossing the dominant offspring with the homozygous recessive parent.
4. A tall plant (unknown genotype) is crossed with a dwarf plant (known genotype). If both dominant and recessive offspring are produced in an equal ratio, then the tall plant is heterozygous. However, if all the offspring are tall, then the tall plant is homozygous.

Explanation of the incorrect option:

Option A:

1. Recombination frequency is calculated from the genetic cross data.
2. A linkage map can then be made to predict whether two genes that determine particular traits are linked.

Option C:

Interbreeding or crossbreeding involves crossing two species or varieties to generate a hybrid that exhibits all the desirable characteristics.

Option D:

Allele frequency is used to assess the relative frequency of a specific allele at a certain locus in a population.

Final answer: A test cross is carried out to determine the genotype of a plant at F₂.