Gene s controls the sharpness of spines in a type of cactus. cactuses with the dominant allele, s, have sharp spines, whereas homozygous recessive ss cactuses have dull spines. at the same time, a second gene, n, determines whether cactuses have spines. homozygous recessive nn cactuses have no spines at all.
a cross between a true-breeding sharp-spined cactus and a spineless cactus (ssnn) would produce what types of offspring?

All the offspring will have sharp-spined phenotype.

Explanation:

By crossing a true-breeding sharp-spined cactus with a spineless cactus, we're crossing a NNSS x nnss. So to determine the offspring's type, we should consider the gametes that would be produced.

The true-breeding sharp-spined cactus can only produce gametes NS, while the spineless one would produce only ns gametes.

As a result, all the individuals of the first generation after the crossing will have the genotype NnSs.

That means, all the cactus of the offspring will have sharp-spined phenotype.

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.