The controversial study
Last August (2019), a large study published in one of the world’s largest scientific journals, Science, stating that there is not only one gene associated with homosexual behavior, but a collection of several genes.
The publication of the study produced a diversity of news in newspapers, magazines, blogs, etc. The study, the largest ever, is excellent and the results (also the methods) certainly make place for considerable discussion, however, what hit me most was the fact that, despite being about the same study, the news titles and conclusions surrounded through different opinions: for some, genetics has no influence on homosexual behavior, and for others, it does.
I believe that this so basal divergence of opinions – without entering into any ideological, religious or political question – is a result of the lack of knowledge (or forgetfulness) about basic concepts of classical genetics, especially regarding the different inheritance patterns.
If you didn’t read anything about this study, here’s a summary
Harvard and MIT researchers analyzed the genome of about 500,000 people through the method called genome-wide association study (GWAS) looking for single nucleotide polymorphisms (SNPs). The rationale behind these studies is that if many people with the same common trait also share certain SNPs, it is probable that SNPs are somehow associated with this trait.
The study revealed 5 SNPs associated with same-sex sexual behavior (each SNP present in a different gene). However, these five SNPs collectively explained less than 1% of the variation in sexual behavior.
What suggests: There are various additional SNPs (and hence other genes) influencing sexual behavior, many of which could not yet be found. Consequently, the SNPs identified in the study cannot be used to safely prognosticate or predict sexual preferences in any individual because no single gene has a major effect on sexual behaviors.
Generally speaking: homosexual behavior is influenced by an individual’s genetics along with environmental and cultural factors surrounding it.
This “form of inheritance” is not new. We are surrounded by physical, physiological or even mental characteristics, influenced by the interaction between genetics and the environment.
What are the different ways in which a genetic condition can manifest?
- Complex or multifactorial characteristics
Multifactorial (or complex) inheritance refers to diseases and genetic characteristics that occur and are determined by the effects of multiple genes (polygenic) in combination with lifestyle and environmental factors. In several cases, the specific genes involved in these disorders/traits are unknown or their function is poorly described.
It is believed that multifactorial disorders constitute the majority of common features affecting humans, including height, weight, blood pressure, and intelligence, for example. In addition to a wide range of medical (heart disease and diabetes), congenital (congenital defects, including cardiac malformations, neural tube defects, and cleft lip and/or palate) and neuropsychiatric (Autistic Spectrum Disorder, schizophrenia, bipolar disorder) conditions.
Although complex diseases usually cluster in families (i.e., the risk of recurrence is higher among relatives of an affected individual than in the general population, indicating a genetic contribution), they do not have a well-defined inheritance pattern (such as Mendelian, read below). This makes it difficult to determine the risk of inheriting or transmitting these characteristics. Also, it is difficult to study and treat them because the causal factors of most of these disorders have not yet been identified. Because of this, population studies, such as the ones cited here, including a large number of individuals, are so important.
- Monogenic characteristics
In contrast to multifactorial disorders, monogenic disorders are caused by changes in a single gene.
It generally has three main inheritance patterns, which are termed: autosomal dominant, autosomal recessive, or X-linked.
Humans have two copies of each gene, called alleles, one inherited from the father and one from the mother. Diseases with an autosomal recessive inheritance pattern are monogenic disorders that occur due to changes in both gene alleles (the mutation is called to be homozygous).
Diseases with an autosomal dominant inheritance pattern are monogenic disorders that involve changes in only one copy of the gene (the mutation is heterozygous).
X-linked inheritance pattern diseases are monogenic disorders that are linked to changes in genes on the X chromosome, the sex chromosome.
Therefore, monogenic diseases occur regardless of environmental factors. In the presence of the mutation (homozygous or heterozygous, depending on the disease) the individual will manifest the disease and transfer the mutation to their offspring.
There is no gay gene
Returning to the media confusion due to the “gay gene” studies: Studies in the 1990s hypothesized that sexual orientation would be a trait with a strong genetic component. Plus that, specifically men homosexuality, would be associated with a specific region of the genome, region Xq28, containing one or more genes. Additionally, could be possible to test if someone with this change in their genome would be homosexual.
The current study overturned the idea that there would be only one gay gene. As detailed above, homosexual behavior is a multifactorial trait, which, according to the study, is associated with numerous changes in many genes spread throughout the genome and is influenced by environmental factors.
 Large-scale GWAS reveals insights into the genetic architecture of same-sex sexual behavior. Science 30 Aug 2019: Vol. 365, Issue 6456, eaat7693
 No ‘gay gene’: Massive study homes in on genetic basis of human sexuality. Nature 573, 14-15 (2019)
 Evidence for homosexuality gene. Science 16 Jul 1993: Vol. 261, Issue 5119, pp. 291-292