Is a half-sibling the same as a first cousin, from a genetic standpoint?

Yep, same on my father’s side of the family. A set of siblings married a set of siblings. So everybody descended from those marriages is everybody’s cousin in several ways. Sounds incestuous, but it’s not.

On my mother’s side of the family, her two brothers, identical twins (married to completely unrelated women), and a whole bunch of kids between the two of them. So we’re all (that bunch of kids and me and my siblings) first cousins, but apparently some of them are more related, genetically speaking, than me and my siblings. Funny. Never thought of that before. I wonder if the cousins have? I’ll have to ask next time I see any of them.

“Identical twin fathers with identical twin mothers (it could happen)” — I know of identical twins who married identical twins, one couple didn’t have children.

I’m now imagining some movie scenario where identical twins marry identical twins; one couple steals the child of the other couple; DNA tests cannot prove which is the real family, then it turns out that the men switched places with each other at some point; then it turns out the women also switched places…

…Then it turns out there was a triplet on one side…!!!

Sounds more like a soap opera to me!

Then it turns out that they’re not actually twins, but the same person, who travelled back in time!

This almost explains The Patty Duke Show.

You could make a comedy of Shakesperean proportions over these sorts of confusing errors.

The point to take away here is that the statistical bell curve of possible genetic distribution is incredibly pointy. When we talk about “about 50%” in is HIGHLY unlikely it is very much off that. The sigma is quite small.

Just want to pick this out because I think it best illustrates where you are going wrong. The most extreme and impossible chance is that you and your sister share 0%.

Imagine your parents had completely unique DNA-sequences and just two chromosomes. Your father has A1-A2, B1-B2 and your mother has A3-A4, B3-B4. We are also discounting crossover events, so you could get A1-A3, B1-B3 and your sister could get A2-A4,B2-B4 and you would share nothing. In this very simplified scenario there is a 1/16 chance you and your sister will share nothing, and 1/16 you will share everything, but on average you will share 50%.

In reality there are 23 chromosomes and the chance of no crossovers is miniscule, so the chance of sharing nothing or everything is ridiculously small, but the average is still 50%.

I have two nephews - cousins to each other - who married two sisters, making them both first cousins and brothers-in-law, and their children both first and second cousins.

My wife has a first cousin on her mother’s side who married my wife’s second cousin on her father’s side, so their kids and mine are both second and third cousins. (The couple were completely unrelated to each other prior to marriage.)