Monohybrid Cross

In Snapdragon plants, flower color exhibits incomplete dominance, where neither allele is completely dominant over the other. When a red flowered Snapdragon (homozygous dominant, RR) is crossed with a pink flowered Snapdragon (heterozygous, Rr), the phenotypes of the progeny can be determined using a Punnett square.

The resulting genotypes from this cross are as follows: 50% RR and 50% Rr. In terms of phenotype:

• RR results in red flowers.
• Rr results in pink flowers.

Therefore, the progeny will exhibit two phenotypes: red flowered and pink flowered plants.

To determine the genotype of a black seed plant, a test cross can be conducted. The black seed phenotype can be either homozygous dominant (BB) or heterozygous (Bb). To differentiate between these possibilities, cross the black seed plant with a homozygous recessive plant (bb), which has white seeds. Here’s the reasoning:

• If the black seed plant is homozygous dominant (BB):
  All offspring will have black seeds, since they will all inherit one dominant allele (B) from the black seed parent and one recessive allele (b) from the white seed parent, resulting in genotype Bb.
• If the black seed plant is heterozygous (Bb):
  Offspring will show a 1:1 ratio of black to white seeds, since half of the offspring will be Bb (black seeds) and half will be bb (white seeds), depending on which allele is inherited from the black seed parent.

In conclusion, crossing the black seed plant with a homozygous recessive (bb) plant will elucidate the genotype of the black seed plant based on the phenotypes of the progeny. This approach is known as a test cross. Therefore, the correct cross is with bb.