In Language on Vacation, Dmitri Borgmann states that non-pattern words (words with no letter repeated), such as DERMATOGLYPHICS, are just a special case of the isogram, in which each letter is used an equal number of times -- once, twice, three times, etc. Before looking at words where each letter is used more than once, let us exclude the three classes of words not considered legitimate isograms by Mr. Borgmann.
"Pair and Trio Isograms,"
Word Ways: Vol. 15
, Article 3.
Available at: http://digitalcommons.butler.edu/wordways/vol15/iss3/3