Sure, CV. I have the advantage that my father is a biologist!
Cells are made up of a number of components. All our cells actually contain two types of DNA: in the nucleus (nDNA) and in the mitochondria (mtDNA).
The DNA we typically hear referred to is the nDNA, which is our unqiue genetic fingerprint. This comes from our mother via the ovum (egg) and our father via the spermatozoa (sperm). They each contribute 50% of our nDNA.
Within the egg there is also mtDNA. (While there is also mtDNA in the sperm, this appears to be destroyed upon fertilization. This is an area of debate for scientists.) This means that the mtDNA is passed directly from mother to child, unmodified. mtDNA travels unmodified down the maternal side of any family.
To give an example: a woman has two children - a male and a female. They will both have her mtDNA. If they both have children, the female will pass the mtDNA to her child. However, the male's partner will pass the mtDNA to the child. So the child of the female child will have the exact same mtDNA as the original woman, but the child of the male child will not have any of it.
By contrast, the two children of the original woman will have nDNA that is comprised of 50% hers and 50% their father's. Their children will have 50% theirs and 50% their partner's. So both the male and female grandchildren will be only 25% of the original woman.
So if you have a sample of mtDNA you can connect it directly by following the maternal bloodline both forwards and backwards as far as you like. If you have nDNA you can follow it to parents, grandparents and potentially siblings, but it gets harder the further out you go.
I hope that helps!
(I realise for anyone reading this with a biology background that my language and percentages oversimplify genetics, but they are just being used to provide an overview and comparison.)
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