How can someone share 50% of their DNA with their parents yet all humans share 99.9%?

Question

I have heard that humans share 99.9% of their DNA with other humans. I have also heard that a child shares 50% of their DNA with their parents. How do I resolve this apparent contradiction? It has been really bothering me.

Answer 1

This contradiction can be resolved with identifying what these phrases are being referencing.

The human genome contains 23 pairs of chromosomes. One half of these pairs comes from one parent, the other half of these pairs comes from the other parent. This is what the statement "50% DNA from parents" is referencing.

The statement "all human share 99.9% of their DNA" refers to the entire genome, or all 23 pairs of chromosomes.