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.

The completely plated genome is dominant to the low plated gene. It can also be said that the completely plated phenotype is dominant, while the low-plated phenotype is recessive.

Explanation:

This is evident from the initial crossing of the completely plated stickleback with the low-plated ones. This resulted in a 100% F1 generation of completely plated offspring. We can therefore conclude, that the offspring were all heterozygous dominant. This would clearly explain the 3:1 ratio of completely plated to low-plated F2 generation after interbreeding the F1.