answer:(1) Let’s define what you/he mean by “genetic.” You mention “genes” and “gene expression”—these are two separate things. Genes themselves are the wording, so to speak, of the genetic code. Gene expression, however, involves the RNAs and proteins that are the results of those genetic codes, and can be modified by other genes, or by the environment and experience. (2) Because gene expression is such a complex process, and behavior is so complex, it’s very difficult to get a 1:1 relationship between genes and behavior. So if you are looking for a single gene that, when present, makes someone gay, and when absent, makes someone not gay, you are probably not going to find it. (3) That said, abundant evidence points to the existence of some genetic component to homosexuality: (a) Twin studies: perhaps the most recent evidence is this paper published in 2010, finding that genes account for 34–39% of the variance in sexual orientation in adult twin men, and 18–19% in adult twin women. (b) Genetic linkage studies: several studies have pointed to a gene at locus Xq28 that seems to be related to adult male homosexuality, although there is some dispute. The last study linked, however, suggests a potential epigenetic effect at locus 10q26. (c) Some people also believe that differences in brain structures between gay and straight people are evidence of a genetic basis for homosexuality. The problem with this is that the brain is modified by both genes and experience (although as I pointed out above, so is the rest of gene expression). With that caveat, a famous study from 1991 found significant differences in the size of one nucleus of the hypothalamus when comparing gay and straight men—of particular interest because the hypothalamus is involved in sexual behavior. How’s that?