How can someone tell if cousins are related, I know they say they are blood relatives,but how much Dna

Question

do first cousins have in common? And is their a guarentee that they have common genes? If, not would they still be considered blood related?

Answer 1

On average, first cousins share around 1/8 of their genetic material.

While there is no guarantee that first cousins have genes in common, odds are pretty good that they do share some.  Similarly, there is no guarantee that siblings share any genes, but on average, they have 1/2 of their genetic material in common.

If two first cousins, by completely random chance, didn't share any genes at all, I suppose you could argue that they aren't technically blood relatives, but for genealogical purposes, they would still be considered as such.  They may not share any genes, but they are not cousins by marriage, and they do share common grandparents.