Previously it was commonly thought, and published, that all tabby patterns were controlled by alleles of a single gene.  Current evidence indicates that this is not the case.  There are four common tabby patterns in domestic cats, classic (or blotched), mackerel, spotted, and ticked (or unpatterned agouti).  Each of these patterns can be homozygous and breed true.  Some examples include classic pattern American Shorthairs, some mackerel pattern Maine Coon Cats, spotted Ocicats, and ticked Abysinnians.

On the other hand: Mackeral tabbies bred togethjer may be heterozygous and produce classic tabby as well as mackeral tabbies.  Spotted tabbies may be heterozygous and when bred together may produce spotted, mackerel, and classic tabbies in the same litter.  Similarly heterozygous ticked tabbies may produce multiple tabby patterns in the same litter.  Based on data I have collected
There appears to be 3 separate genes controlling tabby pattern, a base pattern of classic or mackerel, then two separate dominant modifying genes.  One, the spotted gene can brealk the pattern into spots.  Two, the ticked gene may break the pattern down even more, producing the even gradient of ticking of the ticked tabby.  In the case of the ticked tabby, cats heterozygous for the ticked allele  have residual striping on the legs and tail that is substantially reduced or eliminated in the homozygous ticked cat.

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Classic Tabbies: the most recessive of tabby patterns.

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Mackerel Tabbies (dominant to classic)

Note: unbroken vertical stripes

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The Spotting Modification: (Appears to be a dominant modifier of mackerel and classic patterns)

Classic pattern spots

note: spots along spine set in rows parallel to the spine,  circular pattern of spots on sides

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Mackerel pattern spots

note: spots set into vertical rows perpendicular to spine, sometimes joining together to form lines.
This cat was registered and shown succesfully as a Mackeral tabby, but was a genetic spotted.

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The Ticked Tabby Modification: (the most dominant modifier)

Heterozygous ticked
  note: presence of leg and tail barring

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Homozygous ticked

note: lack of leg and tail barring

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