Punnett Squares:
Short hair (S) is dominant to long hair (s) in cats. What would be the
genotype(s) of a cat with long hair?

The genotype of the long hair will be ss.

Explanation:

A dominant trait can be described as a physical trait which masks the effect of the recessive trait. The alleles for a trait which is being expressed are dominant over the alleles of the trait which are being masked.

As the scenario described in the question, a dominant trait short hair is the dominant trait hence the genotype for short hair will be SS or Ss.

Long hair is the recessive trait and the alleles for it will be ss.