OCD: Deficiencies, not genes
Special Note: If you're inspired by this article to try natural therapies for any mental diagnosis you may have, please read this article first: http://www.drugawareness.org/Archives/Miscellaneous/MRAfter.html

This, and a few other articles on drugawareness.org, cover the withdrawal problems of psychiatric drugs. Be sure to consult a health care professional before attempting to switch to natural methods, and note the recommendation for a GRADUAL withdrawal.

OCD (Obsessive-Compulsive Disorder) is a mental disorder with a similar chemistry to schizophrenia. Both diseases involve seratonin deficiency (or a perceived deficiency) in the brain. However, symptoms of the two diseases are somewhat different, as are their reputed causes and remedies.

Schizophrenia, based on an excellent 1952 scientific study by Abram Hoffer, PhD, MD, is primarily a Vitamin B-3 deficiency, and is controlled well with B-3 supplementation. Other researchers like Donald Rudin, MD, have shown that flaxseed oil supplements also help schizophrenia in most cases, which may simply be due to the fact that Omega-3 fatty acids are so important for brain health in general. In his Rare Earths book (pp. 90, 165), Wallach treats schizophrenia with the same method for bipolar, and mentions both Vitamin B-3 and Essential Fatty Acids (including Omega-3's), among several other vitamins, minerals, and other factors like food allergies and hyper/hypoglycemia. (He doesn't cover OCD.)

With OCD, I haven't been able to find a study on causation which I'd consider to be definitive so far, but a number of fragmented studies agree that a deficiency is involved. This may be a more complex deficiency, as a number of things are usually mentioned, namely calcium, magnesium, zinc, Vitamin C, Vitamin D, Vitamin B-12, Folic Acid, Inositol, and other B vitamins. Specifically, a "cerebral calcium deficiency" is sometimes cited. I'll try to make sense of this.

First of all, some articles disagree on whether it's a calcium deficiency or calcium excess. Here's the issue probably involved: in order to ABSORB calcium, it's important to take anywhere from half as much magnesium as calcium, all the way up to just as much magnesium as calcium. In other words, calcium and magnesium should be taken in a 2:1 to 1:1 ratio. A lack of magnesium with adequate or excess calcium could be what is meant by "excess" calcium -- calcium that can't be utilized because magnesium is absent, thereby creating the same result as a calcium deficiency. A lack of magnesium would obviously contribute to a calcium deficiency, as magnesium is needed to properly absorb calcium. Both minerals work together, and an imbalance usually results in a calcium deficiency.

For people who treat their OCD successfully with calcium and/or magnesium, Vitamin C and Vitamin D are almost always mentioned as part of their supplementation package. (Note that Vitamin D is another factor in calcium absorption.) Sometimes B Vitamins like B12 and Folic Acid are also mentioned, as is supplementation with flaxseed oil (Omega-3 fatty acids).

A number of alternative sites on OCD tout various formulas for natural treatment of the disease, but almost all mention deficiencies of some kind, for example:
http://www.alternativementalhealth.com/articles/OCD.htm

Strep Throat is also mentioned as a causal factor for OCD in some childhood cases of the disease. However the physiology behind that observation/hypothesis is debated.

In essence, many people treat their OCD with simple mineral and vitamin supplementation, and claim their symptoms haven't returned since they started the supplementation. Does that sound like a genetic disorder to you?

Environment, not genes
The bottom line is, most diseases are caused by environmental factors (diet, lifestyle, deficiencies, pollutants, microorganisms). Heart attacks run in a family? Well, who taught the kids what to eat or how to cook it? Parents give their children basic training in life, and included in that package are all of their lifestyle habits. Children are exposed to most of the same pollutants as their parents during their childhood, and also as adults if they live near the same location or continue to use the same products. parasites and microorganisms can be passed from generation to generation, and from spouse to spouse. Scientists can cry for research grants, claiming they've found the Breast Cancer gene, but does that gene produce the cancer? Of course not. It's just a ploy for more money. So be skeptical of the onslaught of genetic claims these days, very skeptical. It's just a fad, after all.