Why siblings can be so different, according to science

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Across
  1. 3. (adj) a synonym for opposite
  2. 6. (v) to punish in an attempt to correct behaviour
  3. 8. (adj) a synonym for oldest
  4. 10. (n) room or distance
  5. 12. (adj) a synonym for satisfying
  6. 13. (v) to receive from parents at birth or death
Down
  1. 1. (adj) a little bad, nothing serious
  2. 2. (adj) someone who is a serious student and studies a lot
  3. 4. (adj( a synonym for difficult
  4. 5. (adj) referring to the mother's side of the family
  5. 7. (adj( exact with no difference
  6. 9. (v) to wonder or be amazed
  7. 11. (noun) a molecule that instructs genes how to develop