Yes, mutations which are expressed always involve a lack of function with the exception of those mutations which are in 'hot spots', which means they will mutate and back mutate (a back and forth thing) reasonably frequently. This gives rise to the variation that bacteria are capable of, for example. But these mutations are A to B to A to B to A to B, etc. C and D never enter the picture.
There may be a genetic predisposition to homosexuality. But there is also a genetic predisposition to anger. That does not give an excuse to either one. We are born with certain genetic traits, such as a talent in music or the kind of mind which seems to grasp logic intuitively or the ability to learn many different languages with seeming ease. We are also born with some negative tendencies, too. One of the primary jobs of parenting is to teach the child to control the negative and develop the positive.
In other words, if you are going to use genetics as an excuse for homosexuality, you had better be ready to use it as an excuse for a host of other negative factors, such as road rage (uncontrolled anger) and possibly even pathological lying. Are you prepared to do that? For if homosexuals are to be given excuse and status, why not the others?
Differences in how people smell and taste may be genetic but they may also be learned. I hate chili peppers and love curries. Go figure. You think it is genetic? I think it is learned. I was exposed to eastern cooking long before I knew what an enchilada was. When I was pregnant, EVERYTHING smelled different. Hormones. Not genetics. I got back to normal after giving birth.
Some things are learned and some we develop along the way. There is a recent study on identical twins which shows that even they develop differently. The fascinating thing, to me, was that these differences were evidently a combination of learned and genetic material, as although their genes were identical, some of the material between the genes in the chromosomes which governed timing of expression and such were not.
How did that happen? I haven't the slightest. I do know, however, that that would not give either twin an excuse for becoming a sociopath!
Biblically there is an indication that homosexuality might be a genetic tendency. Romans 1 says those who refuse to acknowledge and worship God will be GIVEN OVER to unnatural lusts. You cannot be given over to something that is not there in the first place. However the fact that those who do worship God are NOT given over to it indicates that it is not a necessary condition for those who may have any genetic tendency in that area. The Bible states unequivically that physical relations between members of the same sex are unnatural and wrong. "Unnatural" meaning that we were not created to be that way. Also meaning one can be rescued from that condition by God Himself.