wieviel gene hat der mensch — what are common causes?

The exact number of human genes is not fixed, as the definition of a 'gene' varies (protein-coding vs. non-coding). According to current estimates from the GENCODE project (version 44), there are about 19,000–20,000 protein-coding genes. Additionally, thousands of non-coding RNA genes, pseudogenes, and regulatory elements exist, bringing the total functional gene count to roughly 25,000–30,000. Common causes of genetic differences between individuals include single nucleotide polymorphisms (SNPs), insertions/deletions (indels), and structural variants. These arise from mutations, recombination, and environmental factors. Most are harmless, but some influence disease risk or traits. The provided contexts illustrate examples like rs713598 (bitter taste) or rs3091244 (CRP), which have small effects. Importantly, a DNA test alone cannot diagnose; many variants have unclear or minor clinical relevance. Evidence for gene count is strong (Human Genome Project), and for variation causes it is well-established, but effect sizes are often small.