Would a trait that has only two distinct phenotypes more likely be a single gene trait or a polygenic trait? how do you know?

Single gene trait

Explanation:

Single gene trait because is only controlled by one gene with two alleles and polygenic traits can have many possible phenotypes

A single gene trait is expressed by possible combination of two alleles of a single gene. A polygenic trait is controlled by several genes and hence it can lead to expression of multiple phenotypes.  

For instance height is a single gene trait while eye color is a polygenic trait.

the answer is sex-linked recessive.