In a plant, black seed color (BB/Bb) is dominant over white seed color (bb). In order to find out the genotype of the black seed plant, with which of the following genotype will you cross it?

To determine the genotype of an organism exhibiting a dominant phenotype (such as a plant with black seeds, which could have either the homozygous dominant genotype BB or the heterozygous genotype Bb), a genetic cross called a test cross is performed.

In a test cross, the individual with the unknown genotype is crossed with an individual that is homozygous recessive for the trait. In this plant, white seed color is recessive, and its genotype is homozygous recessive, represented as bb.

By crossing the black-seeded plant (B_) with the homozygous recessive white-seeded plant (bb), we can identify the unknown genotype based on the offspring's phenotypes:
1. If the black-seeded plant is homozygous dominant (BB):
All offspring of the cross (BB × bb) will be heterozygous (Bb) and display the dominant black seed color phenotype.
2. If the black-seeded plant is heterozygous (Bb):
The offspring of the cross (Bb × bb) will show a 1:1 phenotypic ratio of black-seeded (Bb) to white-seeded (bb) plants.

Therefore, to find out the genotype of the black seed plant, we must cross it with the homozygous recessive genotype, which is bb.