Mom usually thinks of tabbies as grey or brown cats with black or darker brown stripes orsplotches. However I am an orange tabby. Mom wasn't sure that I really qualified as "tabby", rather thinking of me as a striped "ginger cat". Out of curiosity, Mom looked up "Tabby Cat" on Wikipedia and learned this interesting fact about tabbies, orange tabbies and why you can never get a solid orange cat:
The tabby patterns are due to 3 distinct gene loci and one modifier:
The agouti gene, A/a, controls whether or not the tabby pattern is expressed. The dominant A reveals the underlying tabby pattern, while the recessive non-agouti or "hypermelanistic" allele, a, prevents it. Solid-color (black or blue) cats have the aa combination, hiding the tabby pattern, although sometimes a suggestion of the underlying pattern can be seen (called "ghost striping"). However, the O gene for orange color suppresses the aa genotype, so there is no such thing as a solid orange cat.
This is from:
There is more there if you are interested in Tabby Genetics!