monomorphic

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Chemistry Having only one form, as one crystal form.
• adjective Biology Having or existing in only one form, as having only one allele of a gene or having males and females that look alike.

from The Century Dictionary.

• In zoology, of one and the same (or essentially similar) type of structure; formed much alike; notably uniform in morphic character: said of a number of animals collectively, or of the zoölogical group which they constitute: as, birds are a highly monomorphic class of animals.
• In entomology, having but one form, structure, or morphological character; identical or invariable in form throughout successive stages of development; monomorphous; homomorphous; ametabolic.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Biol.) Having but a single form; retaining the same form throughout the various stages of development; of the same or of an essentially similar type of structure; -- opposed to dimorphic, trimorphic, and polymorphic.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective having or existing in a single shape or form
• adjective genetics, of a gene invariant across a species
• adjective programming, of a function taking only a single data type