Since identical twins are not always both gay, that proves that homosexuality cannot be genetic?

Hmm, let us think about that for a minute.

Research last year at Duke University found that identical twins both develop Alzheimer's by their late 70s only about 40 percent of the time. In many cases, one does and one does not.

http://www.livescience.com/3905-identical-twins-identical.html

A number of years ago, he showed that if one identical twin gets Hodgkin’s disease then his co-twin is 14 to 15 times more likely to get Hodgkin’s as well, making it the single most inheritable cancer, far more than breast cancer or colon cancer. Yet only a minority of identical twins who get Hodgkin’s disease both get it.

http://www.talk-about-twins.com/html/six_twin_experts_tell_all--tal.html

So, I assume we should conclude that Alzheimers or Hodgkin’s disease does not have any genetic basis since twins do not always both get it?