Allele that is hidden by a dominant allele?

null allele

That's called a recessive allele.

The recessive allele.

An allele is a small piece of DNA responsible for some characteristic that plugs into a gene locus (or position) along a chromosome. Suppose the particular gene locus is responsible for flower colour...

It is called a recessive allele. An example of this is when a blue-eyed gene is masked by a brown-eyed gene.