If there weren't alleles for different traits, but rather only one version of a gene for different traits, how would humans be different?

Humans would look exactly alike

Explanation:

An allele which is a variation of a gene which expresses a particular trait(Phenotype).

For example, if humans have a gene for eye color but has different alleles for the color being black or brown for example, their phenotypes are different since the alleles are different.

But, if the alleles for the eye color are all the same, the eye color and every other phenotype would be the same. So, there would be no variations in eye color, height, color of skin etc. So, all humans would thus look alike having one set of genes for the different traits.