Determine the proportion of offspring phenotypes that would result when two merle dogs mate, if one dog is true-breeding for the long-coat trait and the other dog is true-breeding for the short-coat trait.

heterozygous with genotype "Ll" and phenotype heterozygotic tall.

Explanation:

Let us assume that the trait for long-coat be dominant over the trait of short trait.

Let the allele for long-coat trait be represented by "L" and the allele for short-coat trait be represented by "l"

The genotype of true breeding long-coat dog will be "LL"

and the genotype of true breeding short-coat dog will be "ll"

When these two species are crossed with each other, following offspring are produced

LL * ll

Ll, Ll, Ll, Ll

this means that all the offspring will be heterozygous with genotype "Ll" and phenotype heterozygotic tall.