section_path: Biology/Genetic characteristics
section_name: Genetic characteristics
paragraph_text: Genetic characteristics
  
  Typically, the cells of female humans contain two X chromosomes, while the cells of male humans have an X and a Y chromosome. During early fetal development, all embryos have phenotypically female genitalia up until week 6 or 7, when a male embryo's gonads differentiate into testes due to the action of the SRY gene on the Y chromosome. Sex differentiation proceeds in female humans in a way that is independent of gonadal hormones. Because humans inherit mitochondrial DNA only from the mother's ovum, genealogical researchers can trace maternal lineage far back in time.