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This is a good question and points to the common misconception that if something has a genetic / biological basis then it means that it can be predicted 100% from genes. I'm sure there are many ways that you and your brother are different - in personality, perhaps in some minor aspects of physical appearance, in health status, etc. All of these are driven by a complex interaction of genetic and environmental influences, including influences that you encountered when before you were even born (in utero). Responding to the one person who questioned whether you can have someone submit their DNA and predict their sexual orientation - that's just not the way genes work. Nor can you look at someone's DNA and determine whether they'll be a graceful dancer or a skilled orator. The doctor's point was that if there were no genetic basis, you would see 5% concordance between identical twins, but in reality you see over 50% concordance, showing that there is a large genetic component.