[thedoc (OP)]

Laura Knight asked the Naked Scientists:
   Please settle an argument. Is my niece/nephew a closer blood relation to me than my male/female cousin? I am a female.
What do you think?

[Space Flow]

This is one tough question.
The answer is that you are a closer blood relative to a niece/nephew than to a first cousin.
Why?
A niece/nephew is the product of a sibling. As such he/she would share the genes of both your parents plus one other (your siblings spouse).
Where a first cousin is the product of the sibling of only one of your parents and one other (the spouse of your one parents sibling).

Clearer if we reverse it and look at it from the niece/nephew's point of view.
You as the auntie are closer genetically than the first cousin you are going to produce for that niece/nephew by diluting the blood line with your unrelated spouse.

[evan_au]

Relatedness is a statistical thing, averaged over many genes, and many potential sets of grandparents, children & grandchildren.

When you look at a particular gene, it is possible that a first cousin may inherit a copy of the same gene as you, while a niece or nephew does not. It's just more likely that a niece or nephew will share a particular gene with you.

Mitochondrial DNA is inherited from the mother, so your mother's sister's child would have greater commonality in mitochondrial DNA than your mother's brother's child (or anyone on your father's side). Similarly, your sister's child would share mitochondrial DNA with you, while your brother's child will not.

Other complex patterns hold for sex chromosomes.

[evan_au]

More information:
Laura Knight asked the Naked Scientists:
   Am I closer by blood to my first cousins (my father and their mother were twins) or to my nieces (my brother is father to two and my sister mother to two - my brother and sister are also twins) and my nephews (my other sister is mother to two boys)? We have family fights/discussions over this! I have no children.

[evan_au]

There is quite a clear diagram here that describes relatedness through your cell nucleus:
http://www.joshuakennon.com/how-much-dna-do-you-share-with-your-blood-relatives/

It states that you share 25% of your DNA with your nieces & nephews, but only 12.5% with your first cousins (as an average across a large population). In individual cases, percentages will vary from these population averages, but you won't know the actual relatedness without a DNA test.

Mitochondrial DNA is inherited from the mother, so you share mitochondrial DNA with your brother & sister, and your father's sister. You also share mitochondrial DNA with your aunt's children, and your sister's children, but not your brother's children.

It's best to enjoy all of your family - it's definitely not worth fighting over!