Having long or short spines is controlled by a single gene for a cactus plant. One cactus "parent" plant is homozygous dominant, and has long spines. We are unsure as to the genotype of the second "parent" plant. If all of the cactus babies had long spines, the second parent cactus would be:

Explanation:

Long/short spines is a trait controlled by a single gene.

A homozygous dominant plant (LL) has long spines, so the dominant allele is L which determines long spines.

The cross between the LL parent and another cactus plant gives 100% cactus with long spines (either LL or Ll).

The genotype of the second parent plant cannot be determined from this experiment. Regardless of the genotype of the second plant (LL, Ll or ll), all cactus babies resulting from this cross will always have at least one dominant L allele from the first parent plant, giving all cactus babies with long spines.

the cell cycle is the final answer! : )