wie viel gene hat der mensch — what are common causes?

The question about the number of human genes and common causes (e.g., of genetic variation) is not addressed in the provided context. According to general scientific knowledge, humans have approximately 20,000–25,000 protein-coding genes, plus many non-coding regions. Common causes of genetic differences include point mutations, insertions/deletions, copy number variants, and recombination. However, the context lacks specific data on these topics. The evidence for these statements comes from general genetics, not from the given sources. Therefore, the evidence is rated as 'unclear'. For precise answers on gene count or causes, relevant scientific references would be needed.