Question

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?

• A. BB
• B. bb
• C. Bb
• D. BB/Bb

Answer 1

The Correct Option is B

The genotype of a black seed plant can be determined via a test cross. Black seeds can result from a homozygous dominant (BB) or heterozygous (Bb) genotype. To distinguish between these, cross the black seed plant with a homozygous recessive plant (bb), which exhibits white seeds. The rationale is as follows:

• If the black seed plant is homozygous dominant (BB):
  All progeny will possess black seeds. This occurs because each offspring will inherit a dominant allele (B) from the black-seeded parent and a recessive allele (b) from the white-seeded parent, yielding the genotype Bb.
• If the black seed plant is heterozygous (Bb):
  The offspring will exhibit a 1:1 ratio of black to white seeds. This is due to half the offspring being Bb (resulting in black seeds) and the other half being bb (resulting in white seeds), contingent upon allele inheritance from the black-seeded parent.

Consequently, crossing the black seed plant with a homozygous recessive (bb) plant allows for the determination of the black seed plant's genotype based on the progeny's phenotypes. This experimental procedure is termed a test cross. The appropriate cross is therefore with bb.