I thought this was something that was generally well known...apparently it is not.
As I'm sure most of you will know most Appaloosa horses carry Appaloosa characteristics. These are mottled skin, roaning, white scelera and striped hooves.
However, not all appaloosa coloured horses carry this, the one's that don't are known as Non-Characteristics (or NC's).
NC appaloosa horses come in all breeds, purebred Appaloosa's, mixed horses and appaloosa coloured breeds. In fact it is estimated that around 35% of all Appaloosa coloured horses are NC's. It's not rare at all!
In fact...all Appaloosa breed books/spotted horse societies etc. note the need for these characteristics, they are seen as desirable. Presumably because they indicate pure genetics, or were thought to and now it has become standard. Horse's are marked down if they do not show them.
Genetics wise nobody is quite sure why NC's arise. Research has been performed (see Harrison, 2006) but it has been inconclusive. At least what this testing did show was that NC's are not linked to the presence or lack of presence agouti 'A' series genes or extension 'e' series genes.
Interestingly having very pure Appaloosa line's with consistent characteristic showers does not guarantee that the horse will be born with characteristics. In fact 67% of all NC's are purebred Appaloosa's with Foundation stock lines.
It would be really great to see more genetic research done into this, it's a fascinating area. Something obviously blocks the characteristics but the question is what is it! Suggestions on a post card (or the comment box :p).