A null allele is a mutant copy of gene that completely lacks that gene's normal function. This can be the result of the complete absence of the gene product (protein, RNA) at the molecular level, or the expression of a non-functional gene product. At the Phenotypic level, a null allele is indistinguishable from a deletion of the entire locus.

The Allele That Is Covered By The Dominant Allele Is The Recessive Allele.

Very small.

You mean SQL?

NULL = anything IS NULL

NULL <> anything IS NULL

...

NULL IS NULL = TRUE

NULL IS NOT NULL = FALSE

The answer is allele