Final answer:
The new allele that no longer codes for a functional protein due to a mutation is referred to as a 'mutant' allele. While such an allele is often recessive, it can also act dominantly in certain scenarios.
Step-by-step explanation:
The new allele created by the mutation that results in a nonfunctional protein is called a mutant allele.
When a preexisting allele mutates and loses the ability to code for a functional protein, the resultant allele may not express the desired trait unless it is paired with another similar mutated allele. In such cases, the allele is often termed recessive. However, in some instances, such a nonfunctional allele can exert a dominant effect.
This occurs when even a single copy of the nonfunctional allele can interfere with the normal functionality of the gene product, leading to a phenotype. When speaking about a nonfunctional allele that needs to be present in two copies to express the phenotype, the term 'recessive' is most accurate. Nevertheless, the specific example provided in the question, where a preexisting allele changes and results in a nonfunctional protein, is best described as 'mutant' to reflect the alteration from the original state irrespective of dominance or recessivity.