- Joined
- Dec 20, 2022
I am retarded and forgot about crossover; see comments. Original, unedited post below:
For the sake of simplicity, I'm going to make these two (false) assumptions:
With that out of the way, note that you do share 50% of your chromosomes with your mother and father respectively. The same is true with them and their parents. However, it does not follow that you then share 25% with your grandparents. To show why this is, we'll vastly oversimplify the situation and follow the path of a single chromosomal pair:
In this poorly drawn graphic, A through D are your grandparents. M and F are your mother and father. U is you. Dots are chromosomes in a pair and the lines drawn show who inherited what from whom.
As you can see: this scenario outright removes two of your grandparents (B and D) from your genetic heritage entirely. Those two people would have absolutely fuckall to do with you genetically. You are 50% A and C in exactly the same way you are 50% M and F.
This is an oversimplification, of course. You don't have just one chromosome pair, but 23. The chances of you being completely unrelated to a given grandparent is 1/(2^23) or 1 in ~8.4 million. Since you have 4 grandparents, it's 1 in ~2.1 million to be unrelated to any of them. Not particularly likely, but still better odds than winning the lottery. Furthermore: since 23 is not divisible by 2, it is physically impossible to be equally related to all of your grandparents. It's more like 4 inter-related bell curves with 0 and 50% at the fringes and 25% at the peaks.
Now let's talk about maximums. You have 46 chromosomes in total, you cannot be related to more than 46 people in a generation. And even that's optimistic. If you share two chromosomes with one, you have one less genetic ancestor and so on. 6 generations in the past equates to 64 great-great-great-great-grandparents., at least 18 of which would have no relation to you. Assuming a generation length of 25 years (estimates vary, but 25 is a happy medium and makes the math easier), someone born today would find that more than a quarter of their of their ancestors alive 150 years ago aren't actually related to them. That's more recently than the Civil War. This compounds exponentially. 175 years ago, shortly before the Civil war, barely over one third of their contemporary ancestors at most would have anything to do with them. At 250 years ago, just 3 years before the signing of the Declaration of Independence, this person would have 1024 ancestors and still only be related to 46 of them. To give you an idea, pic related is 46 pixels high and 1024 pixels long:
For the sake of simplicity, I'm going to make these two (false) assumptions:
1. No inbreeding
Every breeding pair has a distinct set of ancestors with no overlap.
2. No mutation
Each chromosome is faithfully replicated and those which are passed down are done so without alteration.
Every breeding pair has a distinct set of ancestors with no overlap.
2. No mutation
Each chromosome is faithfully replicated and those which are passed down are done so without alteration.
With that out of the way, note that you do share 50% of your chromosomes with your mother and father respectively. The same is true with them and their parents. However, it does not follow that you then share 25% with your grandparents. To show why this is, we'll vastly oversimplify the situation and follow the path of a single chromosomal pair:
In this poorly drawn graphic, A through D are your grandparents. M and F are your mother and father. U is you. Dots are chromosomes in a pair and the lines drawn show who inherited what from whom.
As you can see: this scenario outright removes two of your grandparents (B and D) from your genetic heritage entirely. Those two people would have absolutely fuckall to do with you genetically. You are 50% A and C in exactly the same way you are 50% M and F.
This is an oversimplification, of course. You don't have just one chromosome pair, but 23. The chances of you being completely unrelated to a given grandparent is 1/(2^23) or 1 in ~8.4 million. Since you have 4 grandparents, it's 1 in ~2.1 million to be unrelated to any of them. Not particularly likely, but still better odds than winning the lottery. Furthermore: since 23 is not divisible by 2, it is physically impossible to be equally related to all of your grandparents. It's more like 4 inter-related bell curves with 0 and 50% at the fringes and 25% at the peaks.
Now let's talk about maximums. You have 46 chromosomes in total, you cannot be related to more than 46 people in a generation. And even that's optimistic. If you share two chromosomes with one, you have one less genetic ancestor and so on. 6 generations in the past equates to 64 great-great-great-great-grandparents., at least 18 of which would have no relation to you. Assuming a generation length of 25 years (estimates vary, but 25 is a happy medium and makes the math easier), someone born today would find that more than a quarter of their of their ancestors alive 150 years ago aren't actually related to them. That's more recently than the Civil War. This compounds exponentially. 175 years ago, shortly before the Civil war, barely over one third of their contemporary ancestors at most would have anything to do with them. At 250 years ago, just 3 years before the signing of the Declaration of Independence, this person would have 1024 ancestors and still only be related to 46 of them. To give you an idea, pic related is 46 pixels high and 1024 pixels long:
1. If we allow for inbreeding, not much changes. Some people would show up multiple times in your family tree, but given the probabilistic nature of genetics, we can effectively treat multiple instances of the same person as two different people without changing a whole hell of a lot.
2. Mutation means you're even less related to your ancestors than this post implies. This assumption would have a greater impact if I had gone into the absurdity of genetic testing to determine your racial heritage but the post was getting a little long anyway so I decided to just omit that bit.
3. 25 year generation length is just meant to be an average. Your ancestors from 10 generations ago likely lived at entirely different times. Some being much younger and others much older.
2. Mutation means you're even less related to your ancestors than this post implies. This assumption would have a greater impact if I had gone into the absurdity of genetic testing to determine your racial heritage but the post was getting a little long anyway so I decided to just omit that bit.
3. 25 year generation length is just meant to be an average. Your ancestors from 10 generations ago likely lived at entirely different times. Some being much younger and others much older.
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