Interleukin 6 (IL-6), produced by the IL6 gene, is one such genetic variant involved in muscle growth. It's a cytokine, a messenger molecule used by cells to communicate with each other, and is absolutely essential for muscle growth. Muscle growth increases the size of muscles in two ways: it can either increase the size of individual muscle cells (known as sarcoplasmic hypertrophy), or increase the size of muscle fibres (known as microfibrilar hypertrophy). Your choice of exercise dictates the type of muscle growth you experience. Repetitive submaximal exercises used in bodybuilding increases the size of muscle cells via sarcoplasmic hypertrophy, and it also induces IL-6. But, because of genetic variation not everyone has the same amount of IL-6 in his or her blood.
A subset of us carry an altered or mutated version of the IL6 gene (called rs1800795), and make significantly lower levels of this cytokine compared to those with the normal version of the gene. Studies show that this is disadvantageous for activities that require power or extra muscle strength. The high IL-6 producing version is more frequent among elite power athletes like jumpers and throwers. What's more, inheriting two copies (one copy from each parent) of the normal, high cytokine producing version of the gene greatly enhances the odds of becoming a power athlete. How does this relate to bodybuilding? It means that people with the high IL-6 levels will find it easier to build bigger muscles required to achieve the current bodybuilding standards, than someone who inherited the low cytokine-producing version of IL-6.
Full article here: https://www.nutrabolics.com/blogs/news/the-genetics-behind-bodybuilding-remedying-genetic-obstacles-with-supplements