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"Non-Expressing" vs "Hidden"

"Hidden or Masked Merle" is an otherwise longer expressing allele of Ma+ (≥260 bp*), M or Mh whose pattern is being suppressed by the pheomelanin (red/yellow pigment) of either A^y - Sable or e/e - Recessive Red.

NOTE - the Mh allele is sometimes "strong enough" to slightly express on A^y or e/e

When these alleles are inherited by offspring who are not A^y or e/e, they can express a visible merle pattern.
This resulting Mudi litter demonstrates this inheritance quite well.
Sire - e/e, m/M - recessive red hidden/masked merle
Dam- E/E, m/m
Offsprings' E locus of E/e does not mask a merle pattern as the sire's e/e does.

Very occasionally, an otherwise expressing allele can be influenced by "mosaicism" which can disrupt a visible pattern completely.

In this Catahoula example, Aman's M - 266 bp allele has shortened/mutated to Ma - 250 bp.
He has both m/M cells containing the original alleles inherited from his parents and m/Ma cells containing the shortened de novo allele. The chromatogram result shows a higher percentage of the shortened allele than is typical for mosaicism; approximately the same percentage as the original allele. As a result of mosaicism Aman's merle pattern has been hidden/masked. However any offspring are still able to inherit his M allele and express with a visible pattern.

"Non-expressing" alleles of Mc, Mc+, Ma and Ma+(<260 bp*) will not express a pattern on any coat color pigment.
* 260 bp is the most typical length for the Ma+ allele to start expressing a visible pattern.
* 255 - 259 can cause a faux dilute or brownish shading to the coat