Final answer:
The characteristic of a mutant allele that produces no functional gene product and is inherited as a recessive trait is 'Recessive'. This type of allele is only expressed phenotypically when present in homozygous form.
Step-by-step explanation:
The characteristic of a mutant allele that produces no functional gene product and is usually inherited as a recessive trait is 3) Recessive.
Recessive alleles express a phenotype that is observable only when two copies of the allele are present, as in homozygous recessive genotypes (aa). In cases where an organism inherits one recessive allele and one dominant allele (Aa), the dominant trait masks the presence of the recessive phenotype. It is important to note that the terms 'dominant' and 'recessive' describe the traits or gene products that alleles encode, not the alleles themselves. Thus, when discussing autosomes, the expressed trait in the phenotype of an individual corresponds to the dominant allele; the recessive trait is only visibly expressed when the organism's genotype is homozygous for the recessive allele. In a dihybrid or a monohybrid cross, like the one Mendel studied, a recessive trait would appear in a 3:1 ratio if both parents are heterozygous carriers.