Here is a comment from a former genetics student:
"My only complaint is in regards to the portions of the exams that involve definitions and story telling. I should have received a much higher score, had it not been for the definition section in Exam I and the "Describe the experiment" portion of Exam II. If you must include them on the exams, they should not be worth so many points that would drop my score by 10%"
My opinion on this comment is: If you do not know the words of genetics (or any other field you are discussing) maybe you do not know genetics well enough. If you can not describe an experiment, maybe you do not know enough about the experiment.
I believe that there are key words which must be in any definition. The choice of the key words tells a lot about your understanding of the word.
The test of a definition might be: Does this definition impart to the reader a clear and concise meaning of the word.
A reference for writing definitions..... http://owl.english.purdue.edu/Files/107.html