Complex science surrounding genetics and homosexuality
It is not a new finding that variation in complex traits such as human sexual behaviour is caused jointly by effects of environmental factors and many genetic variants with small effects, distributed across the genome (Editorial, 31 August). This principle has been understood for many such traits for more than 100 years. Work using modern genomic methods in organisms from fruit flies to humans, such as the study referred to in the editorial, strongly confirms it. To say that a trait has a genetic cause does not imply that non-genetic factors are unimportant, nor that it is controlled by just a single gene. There is no simple dichotomy between genetic and environmental control of a trait, and understanding this has effectively ended the long-running “nature-nurture” debate about human intelligence.
The editorial suggests that the advantage to a gene “will shift as a result of the shift in a gene’s frequency”. This concept was introduced by RA Fisher in 1930 (well before game theory was invented), but operates only in very specific situations. Studies of genetic variability suggest that very few of the several million DNA sequence variants present in human populations are maintained by such “balancing” selective processes. It is possible that some of these contribute to variation in this particular human behaviour, but testing this possibility would require major further research. The vast majority of DNA sequence variants are very infrequent (hence the need for the study’s huge size, of almost half a million people), and most either do not affect, or slightly reduce, survival or reproductive success.Brian Charlesworth andDeborah CharlesworthUniversity of Edinburgh
• Your editorial shows why choice of language is so important when reporting on science that may have social and political consequences. The claim that “The latest effort has been to see if there is a genetic cause for homosexuality and the result is clear. There isn’t” implies a bias that no serious geneticist would accept. Indeed, it is unlikely that they would ever use the word “cause” at all. Genes have varying degrees of influence on outcomes and scientists are interested to know how strong those influences are. Most are small and multifactorial, as you say. But your choice of words plays straight into the hands of those who would claim that being LBGTQ+ is a “lifestyle choice” and even that it can be “cured”. To suggest that it is “profoundly affected by outside circumstances” suggests that environmental factors are a bigger “cause” than someone’s inherent nature. When protesters are lining up outside primary schools to decry the “promotion” of LBGTQ+, I would have hoped for a more considered reflection on what the science is really saying.Richard GilyeadSaffron Walden, Essex
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