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Genetics don't just work with the stereotypical Mendel dominant-recessive gene combination (AA, Aa, aa).
Pure altruism is probably a very complex dominant-recessive combination that is so ingrained in that bird's population that it will not go away even when the altruistic gene is initiated, that individual will not produce offspring.
There are lizards that when the fetus is still growing in the egg, the sex is determined by the temperature of the egg. If the egg temperature is above a certain threshold, it will become female, below, a male. That means that a trait can pop up at any time no matter if that trait will cause the individual to produce children or not.
Down Syndrome is when the 23rd allele (I think that is what it is) is copied twice so they have one extra allele than normal. Most people with Down Syndrome do not procreate and the gene still shows up.
If homosexuality is genetic or influenced by genetics, then it would most likely be under one of those three categories or something that I haven't thought of, not a simple Mendel equation.
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