This is an english version of something I wrote in Norwegian blog post about X inheritance

  1. Mother always gives all children an X.
  2. Father always gives Y to sons.
  3. Father always gives X to daughters.
  4. Son can never get X from his father. (2)
  5. Daughter can never get Y. (1 + 2)
  6. Fathers always pass on their mother’s X to daughters. (1 + 3)
  7. X is inherited from any parent. (1 + 3)
  8. X is never inherited via two males in line (2)
  9. X is always inherited through female lines. (1)
  10. Father’s X is always passed down unchanged to daughters. (3+4+6 *)
  11. Father’s Y is always passed down unchanged to sons. (2 *)
  12. Females most often pass their X recombined to children. (1) however, sometimes they don’t recombine their X (*)
  13. X brakes down slowly through generations especially if it passes through male lines. Therefore, X may contain identifiable DNA several generations further back than you are used to. (10)
  14. Grandfather’s X never moves on to grandchildren on the paternal side. (8)
  15. Grandfather’s X is passed down on to grandchildren on the maternal side. (10)
  16. X-DNA can be used to hypothesize if (half-)sisters have the same father or not. They will always have a 100% match on X-DNA if they have the same father. Do not always assume halfsisters have the same father unless you have other evidence (NB! Read 12 again)
  17. Full sisters will always have a 100% match on X. (10)
  18. Brothers with the same mother will normally always have a different X from her. (12)
  19. Brothers with same father share same Y (2 *)

*) Recombination requires two identical chromosomes to take place. Minute changes (mutations) in DNA do occur rarely on single Y and X chromosones.