The three-spined stickleback (gasterosteus aculeatus) is a small fish found in both marine and freshwater environments. Marine stickleback populations consist mainly of individuals with amor-like plates covering most of their body surface (completely plated.) Approximately 10,000 years ago, some marine sticklebacks colonized freshwater environments. After many generations in the freshwater environments, the freshwater stickleback populations lacked the armor plating (low plated) typical of marine stickleback populations. A single gene, ectodysplasin (EDA), is thought to be responsible for the variation in the number of armor plates in sticklebacks.
A completely-plated stickleback from a marine population was mated to a low-plated stickleback from a freshwater population. The resulting F1 hybrids all displayed a completely plated phenotype. When the F1 hybrids were allowed to interbreed, the resulting F2 generation included completely plated offspring and low-plated offspring in an approximate 3:1 ratio. Which of the following conclusions is best supported by the results of the breeding experiments