What is it called when two genes are in a cell and ones gene’s phenotype is expressed and one is not? Explain and give an example.

Let’s use A and a for our alleles in this case.
A is going to represent our dominant trait
a is going to represent our recessive trait

the allele A being dominant means that no matter what if the dominant allele (A) is present in the genotype the phenotype will always express the dominant (A) trait.

If the dominant trait (A) is present in the genotype whether it be AA or Aa the dominant trait will always code for the dominant phenotype. You will never see the recessive phenotype unless there is no dominant allele present (ex. aa)

For example:
Let’s say
Short hair is dominant (A)
Long hair is recessive (a)

If our organisms genotype is Aa they will express the dominant trait over the recessive one. Because their is a dominant trait (A) present.

If our organism’s genotype is Aa that codes for short hair because the dominant allele (A) is present.