# How closely related will your descendants be to you?

I wonder how similar my DNA will be to that of my great-grand-children. If a new person enters my family tree every new generation, he/she will bring in half of his/her DNA, therefore decreasing the amount of DNA that each new generation shares with me. I assume, then, that after several generations, my descendants are going to be no more closely related (genetically) to me than to random people on the streets?

I also once heard that even though the share of my genes will be descreasing, the mutations unique to me will stay. Is that true, and, if so, how is that possible?

And what about all-males descendants? For example, if I have a son and he has XY, then if there always will be a son down the line then will the Y chromosome stay the same through each generation? More specifically, will it still be the Y chromosome from me?

• This hasn't been researched or even thought about at all before posting. I'm voting to close for hwk reasons. Clue: Children have ~50% of either parent. Keep halving the amount every generation. Feb 20, 2017 at 3:53
• This depends on what you mean by genetic similarity. If you mean the fraction DNA that are still direct copies of your DNA in the n'th generation, then the answer is (1/2)^n. But if you mean sequence similarity, then the answer is quite different, and depends on the structure of genetic variation in the population. If some piece of your DNA gets swapped for another piece of DNA, but this DNA has the exact same sequence, do you consider your genetic material to be lost? See en.wikipedia.org/wiki/Identity_by_descent Feb 22, 2017 at 17:48

For how genetically related to you your my grand-grand-children will be....

Well on the surface the answer is simple. Your grand grand children will 12.5% of your DNA.

Dig a little deeper... the question becomes a bit more difficult to answer.

(1) Humans do cheat a small but significantly amount of time. On average about 0.7-3% of all children are unknowingly raised by men who are not their biological father http://theconversation.com/what-are-the-chances-that-your-dad-isnt-your-father-24802. This is less than the 9-30% which has been reported. The reason appears to be that the data was derived from paternity clinics where there was already suspicion on paternity of the child. Not the best data set to represent the population, even if the data set was large and mostly free to researchers.

So which that factored in... it depend on what gender you are, and how said grand grand child is related to you. Are you a woman? Is that grand grand child, the child of your daughter's daughter. A pure maternal descended would guarantee a 12.5% genetic relatedness.

But if you are a man, and the grand grand child, is the child of your son's, son... on average genetic relatedness falls to 11-12% (on average). But since a person is related to your or not, there is a 3%-10% that your grand grand child is not related to you if that child is your son's son's child.

(2)Rural (old world) populations tend to share more genetic material since they have had less outside influence into the genepool. This is a reflection of history, where mobility was limited and families tend to stay for generations. In these places, a random stranger isn't that random after all. In which case genetic relatedness calculations become more complicated. Terms to look up are founder effect, population structure. Large cities, with massive influx of people moving in and out tend to be more mixed. Also note isolation, can also because by social stratification. The nobility is a great example where two random nobles are not as random as you think.

And I also once heard that even though the share of my genes will be descreasing, the mutations unique to me will stay. Is that so and how is that possible?

Not quite. A mutation unique to you has only 50% chance of passing on to a child. (Or a 50% chance of not passing to a child.) So if you have 2 children, that mutation will have a (0.5*0.5=0.25) 25% chance of failing to pass to the next generation. In fact unless that mutation has some sort of selective advantage (awesome X-men powers), or your family has really big families for several generation or you and your families becomes a founder of a new colony/town (founder effect), your unique mutation will likely fallout after a few generations.

And what about all-males descendants? Like, if I have a son and he has XY, then if there always will be a son down the line then Y chromosome is going to stay the same, being the one from me?

yeah they will inherit your Y chromosome.. which will change very little over the generations. It will accumulate a few mutations but that is over thousands of years.

• I am sorry you find that offensive. Most old villagers and towns are inbred. (Some more so then others.) It is very useful for population genetics especially when paired up with the marriages information from the local parish. Also depending on how you look at a population, everyone is inbreed. Go back 1000 years ago, and all europeans are related to each other. This is why population genetics has been such a minefield to work with. I guess I am also not looking at people... but how they are related to each other over centuries. The effective founder population size.The genetic variability Feb 20, 2017 at 4:04
• I think we are imagining different countries and rural communities. Stay specific when possible. Which rural study are you referring to? Feb 20, 2017 at 4:10
• Thank you for your answer. Yes, I heard about that - how populations in countries that are isolated - like, islands, or just small communities tend to be really related to each other. But why does relatedness for all-male descendants fall to 11-12%? If the same Y chromosomes gets passed on. And it looks like for women the relatedness should be smaller - X chromosome can be piged both from mother and father and therefore already grand-daughter of a woman may not have her X chromosome, only that of her grandfather and her father... Feb 20, 2017 at 21:43
• Since at least 99% of all DNA is identical between all humans, it's a little strange to say that only 12.5% will be shared. Feb 21, 2017 at 23:13
• @swbarnes2 You're mixing up inheritance and shared DNA. So whereas 99.9% of the DNA is identical to every other human, 12.5% of your genome is from each of your great-grandparents. Feb 22, 2017 at 5:42