I hate to dampen your run as the "grammar guru" here, Neil, but according to the ol' Merriam-Webster Collegiate dictionary, the word "an" may be properly used in the follwing fashions:
"Before unstressed or weakly stressed syllables with initial h both a and an are used in writing <a historic> <an historic> but in speech an is more frequent whether \h\ is pronounced or not. "
So, your statement that you would "NEVER buy from" someone who advertised "an heterozygous" animal may have been a hastily conceived notion (is that "an hastily" conceived one?). LOL
You just have to love grammar discussions, do ya?