Question

How do we know that height in humans is a polygenic trait?

Answer 1

We know that height in humans is a polygenic trait because it has a lot of variation. The height gene is controlled by at minimum three genes with six different alleles. If all of these alleles are dominant for the tall gene you will be tall if you are dominant for the short gene you will be short if there is no clear dominance between the two genes you will fall somewhere  in the middle. 

Answer 2

Answer: Because there are more than just two heights.

Explanation: When more than one gene is responsible for a given phenotype, the trait is considered polygenic. Human height is a polygenic trait. Studies have uncovered hundreds of loci that influence height, and there are believed to be many more. Due to the high number of genes involved, as well as environmental and nutritional factors, height varies significantly within a given population