Examine criteria and guidelines received from universities to which you intend to apply. Some colleges give applicants their maximum word count, the freedom to craft a general statement and offer no suggestions for writing it. Others require a candidate to answer one or more specific questions within a set number of words. Be prepared to write both.
Ask yourself what there is about your life, personality and ambitions that makes you a good candidate for a Ph.D. program. List all impressive, unique personal accomplishments and honors you have earned. Add a list of the people who have most strongly influenced you and explain why. As an exercise in focus, write a sentence about yourself that nobody else can write and you're off to a good start.
Draft a second list composed of academic accomplishments and out-of-classroom experiences that lead you to conclude that gene therapy is the field in which you belong. Don't limit accomplishments to scientific disciplines. You may also possess terrific business or managerial skills learned as an undergraduate, so don't leave them out.
Assemble a final list of examples describing personal growth and ethics. Experiencing physical or monetary difficulties that forced you to leave school for a period may not be the stuff of which personal statements are made in your mind, but admissions committees don't agree. Such experiences show your character, resolve, commitment and conviction. Explain how bumps in the road influenced the way you look at others and yourself. Your strengths as a human being will help you stand out.
Don't be tempted to write a "one-size-fits-all" personal statement and then send it to every school to which you are applying. Academicians agree that the time you've saved isn't worth risking being eliminated because the document has "all-purpose" written all over it. Remember that the folks evaluating your documents have probably been doing this for years.
Avoid going back in time to add weight to your profile. Supporting information about high school experiences are seen as filler interjected by candidates with no recent accomplishments. Politics and religion have no place in your personal statement either. As a rule of thumb, use the name of the school to which you are applying to show that you took time to research the facility, faculty, programs and campus and have concluded it's a good fit for you. The more you match your talents and personality to the facility, the better candidate you will become in the eyes of those making the cuts.
Pretend you are composing an e-mail to very intelligent friend. Tell the story of your life, your ambitions, your hopes and your dreams. Be specific and logical. Avoid using clichés and statements that sound arrogant, preachy, silly, all knowing, off-center and self-serving. Spend the most amount of time crafting your opening and closing paragraphs. Stuff these two sections with the most compelling information, the most notable facts and the best testaments to your aspirations.
Ask at least three people to read the draft. Ideally, one reader should be skilled at spelling, punctuation, syntax, word and sentence structure. Ask a writer friend to help you smooth out the rough spots so your story has logical transitions and moves along effortlessly. Pay particularly close attention to the word limit you have been given.