What are the odds of inheriting no DNA from a great, great, great grandparent?

[Piquerobi]

Interesting question IMO.

This is the answer I found on the net:

What are the odds of inheriting no DNA from a great, great, great grandparent?

This is a great question and probably one that a lot of people would be curious about. The quick answer is that the odds are pretty close to 100% that you have DNA from your great, great, great grandparent.

If you know anything about how our DNA is passed on, this might seem weird at first. DNA is passed down to the next generation in big chunks called chromosomes.

Every generation, each parent passes half their chromosomes to their child. If nothing happened to the chromosomes between generations, then there would be around a 1 in 8 chance that you would get no DNA from a great, great, great grandparent.

What most people forget, though, is that our chromosomes get mixed and matched before they are passed on. It is because of this "recombination" that your great, great, great grandparent's DNA is almost certainly still lurking in yours.

Mixing DNA Keeps You Connected

Most people have 23 pairs of chromosomes for a total of 46. One of each pair comes from mom and the other from dad. This is why we are 50% related to our moms and 50% to our dads.

Of course our parents got their chromosomes from their parents. This is why you are about 25% related to each grandparent.

If this is all there were to passing on our DNA, then this is what a typical couple of generations might look like:



At first this picture might seem a bit overwhelming but it isn't really. Each grandparent's chromosomes are a different color.

Mom ended up with half of one parent's DNA and half from the other (half green and half light blue). Same thing with dad except he ended up with half red and half dark blue in our picture.

When mom and dad have you, you get a mix of their chromosomes which are a mix of their parent's chromosomes. So you have green, red, light and dark blue chromosomes.

If this is how things worked, then there could even be a small chance you wouldn't be related to your grandparents. All's that would have to happen is something like this:



As you can see, by chance you happened to not get any light green chromosomes. You are totally unrelated to your maternal grandpa!

The odds of something like this happening would be very small...something like 1 in 8.4 million.* It is about the same odds as flipping a coin 23 times and getting all heads.

In fact, the two problems are similar. For a coin, on each flip you can get a head or a tail. For chromosomes, each generation you can get one from grandma or one from grandpa.

If we were to keep playing this game, what would the odds be that you'd get no DNA from one of your great, great, great grandparents? With a few assumptions, it would be about 1 in 8.*

This is not reality though. What really happens is that DNA is swapped between the two chromosomes in each pair before it is passed down. A single generation actually looks like this:



You didn't get any whole chromosomes from either parent. Each chromosome from mom has some DNA from her mom and some from her dad. Same thing with the chromosomes your dad gave you.

This means you don't end up with a 50% chance for a blue and a 50% chance for a red chromosome from dad. Or a 50% chance for a light blue and a 50% chance for a green chromosome from mom. Instead you get a chromosome that is about half red and half blue from dad and one that is half green and half light blue from mom. As you can see this happens for each of your 23 chromosomes.

This is actually how you are 25% related to your grandparents. Because of recombination, each chromosome you got from your mom is a mix of her parents' chromosomes and each chromosome you got from your dad is a mix of his parents' chromosomes. This means you will almost certainly get DNA from all of your grandparents.

And this holds true for later generations too. Here is how much DNA on average you'll share with each generation:



Because of recombination, you can't easily lose big chunks of distant relative's DNA. Instead it is slowly diluted away.

So on average you'll have about 3% of your great, great, great grandparent's DNA. There is a good chance it is a little more or less than this because the DNA swapping in recombination is pretty random. This means that sometimes you'll get a lot of one of the DNA from one chromosome in a pair and a little from the other.

But the odds of getting no DNA from a distant relative over 23 different chromosomes are very small. So small that you almost certainly have at least a bit of their DNA.

In fact, you have DNA from very distant ancestors. For example, scientists can still see Neanderthal DNA in people living today and Neanderthals died out 30 thousand years ago!

Uniquely You

From all of this you might think that none of your DNA is unique...it all came from your ancestors. This isn't true. You have some of your very own DNA that none of your ancestors had.

Where'd this DNA come from? Not aliens or anything like that. No, they mostly came from mutations.

A mutation is a change in our DNA. It can happen because of environmental reasons like radiation, viruses or chemicals or because of a mistake when our DNA gets copied. Since these mutations change the DNA, it will change what we inherit from our parents. As a result we could have a bit of DNA that is different from our ancestors.

Of course this is in addition to the brand new mix of DNA that recombination gave us. You are unique and so is every one of your chromosomes.

* The way you figure out these odds is you apply this formula:

����� ����� �����(1/2)x

In this formula, x is the number of chromosome pairs that have to go a certain way (or the number of coin flips). Basically you are figuring out the odds that for each pair, a chromosome always comes from one grandparent.

In this case, it has to happen for 23 different chromosomes. When we plug 23 for x, we end up with the odds being 1 in about 8.4 million. And here are the rest of the numbers through the generations:

Chances of no DNA from one grandparent: 1 in 8.4 million
Chances of no DNA from one great grandparent: 1 in 4096 (assumes 12 passed down in last generation)
Chances of no DNA from one great, great grandparent: 1 in 64 (assumes 6 passed down in last generation)
Chances of no DNA from one great, great, great grandparent: 1 in 8 (assumes 3 passed down in the last generation)

http://genetics.thetech.org/ask/ask445

[Sikeliot]

I think I inherited less DNA from one side of my family than another, since I have disproportionately low African compared to my mother, and my great aunt (just one generation up from my mother) looks visibly mixed race. My "real" African is probably much higher than what I actually inherited.

[drobbah]

Awale sent me a video a while back with the exact same info.I think this is why I didn't inherit anything from my Yemeni GG grandmother.Will test my mother and one of my bros/cousins to confirm.

[Xuipa]

that chart is cute, and I know why that is what they want everyone to believe that , I mean they would have to rewrite all chapters on dna in school books just to name one... but it is not the truth .. you can't see one female on that chart ,you can see your mom's dad and your dad and that is it . everything else is just folks( mostly scientist) seeing what they want you to see.

[Dave-V]

Once you go back beyond your great-great-great-grandparents the odds get progressively greater that you didn't inherit any DNA from specific ancestors, the main reason being that the total number of recombining "chunks" of chromosomes is too few to carry DNA from the larger and larger numbers of genealogical ancestors.

Pedigree collapse (i.e. having the same ancestor(s) in multiple branches of your genealogical tree) can raise the odds of those specific ancestors passing you DNA, but it doesn't change the principle that past about 6-9 generations there are just too many "birth events" in your genealogy for a piece of everyone's DNA to have reached you.

Two of the best articles I've seen on that subject are

http://www.genetic-inference.co.uk/b...share-our-dna/ and
http://gcbias.org/2013/11/11/how-doe...ack-over-time/

(the second link is an update to the first).

But based on those links you'd be right that usually the odds are good (but never certain, as you pointed out) within great-great-great-grandparents on down that you inherited DNA from all of them.

[Deftextra]

I think I inherited less DNA from one side of my family than another, since I have disproportionately low African compared to my mother, and my great aunt (just one generation up from my mother) looks visibly mixed race. My "real" African is probably much higher than what I actually inherited.

There are so many ancestries at play in my genealogy, that me and my brothers and sisters all look like we could be from multiple different places. I guess we all inherited different ancestries to varying degrees.

[nee4speed111]

There are so many ancestries at play in my genealogy, that me and my brothers and sisters all look like we could be from multiple different places. I guess we all inherited different ancestries to varying degrees.

Yeah same here, the skin tone range among direct family is quite large, let along first cousins and the like, some who look very different. Guess that's just the magic of genetics at play.

[Piquerobi]

Other texts on the subject:

On Saturday while visiting the delightful Augusta Genealogical Society, besides giving three talks on NC research, I gave one talk on the basics of DNA testing for genealogy.

One point I made is that due to the nature of genetics, we don’t get exactly 25% from each grandparent and as you go back in time, you might find that you have large bits of genetic material from one branch of your family tree and practically none from another, regardless of what the basic math suggests on the surface (e.g. 50% from each parent who received 50% from each of their parents, etc)! It is very important to appreciate this when considering autosomal testing.

Well, Which Grandparent Are You Most Related to? pursues this concept in some depth based on extensive personal DNA testing in his own family along with the supporting genetics math about how we disproportionately inherit. Though I don’t necessarily agree with the author’s intended use of what he is learning as a geneticist, the point that we don’t inherit equally from grandparents and succeeding generations, is what I want to emphasize.

Related to this, Upfront with NGS recently blogged about In the future ... we might be able to reconstruct what our ancestors looked like! that is based on using DNA data to reconstruct what ancestors may have looked like. A component of this at the individual level is understanding the correlation of DNA to various features along with the recognition that we did not inherit precise percentages of genetic material from all the ancestors of any one generation in our tree.

Some of us look like the spitting image of maternal great aunt Lucy while a sibling might look like paternal uncle Alfred. We know we are siblings and yet genetic inheritance clearly has endowed us with dominant bits from different branches of our family tree. Something to keep in mind as you do DNA testing, start processing your test results and correlate them with test family members, photos of the deceased and more.

http://upfront.ngsgenealogy.org/2015...from-your.html

Which Grandparent Are You Most Related to?
Your family tree says you inherited 25 percent of your ancestry from each. Genetics says you didn’t.

What makes you who you are genetically? The easy answer is your family. The longer answer begins with the fact that all humans have two parents (at least for now), and usually four distinct grandparents (there are unfortunate exceptions). Genetically you are a recombination of four separate individuals. But that does not mean you have an equal contribution from four separate individuals. Humans normally carry 23 pairs of chromosomes: 22 autosomal pairs and one pair of sex chromosomes, either two copies of the X for a female or an X and a Y in the case of males. By Mendel’s law of segregation you receive one copy of each pair from your mother (via the egg), and one copy from your father (via the sperm). This means exactly half of your genome derives from each parent.
Things begin to get more complicated going back two generations. One might think that of the 44 autosomal chromosomes you would receive 11 from each of the four grandparents. (For simplicity we’ll leave the sex chromosomes out for now. If you are a female, you receive one X from each parent, while if you are a male you receive an X from your mother and a Y from your father, who got it from his father.) But while the proportion of one’s inheritance from parents is fixed by exact necessity, the fraction from grandparents is governed by chance. For each of the chromosomes you inherit from a given parent, you have a 50 percent chance of gaining a copy from your grandfather and a 50 percent chance of gaining a copy from your grandmother. The laws of independent probability imply that there is a 1 in 4 million chance that all of your maternal or paternal chromosomes could come from just one grandparent!* What’s more, genetic recombination means that chromosomes aren’t purely from one grandparent or the other; during the cell divisions that produce sperm and eggs, chromosomes exchange segments and become hybrids. You almost certainly have different genetic contributions from your four grandparents.

But this is not just abstract theorizing. Imagine that you could know that 22 percent of the genome of your child derives from your mother, and 28 percent from your father. Also imagine that you know that 23 percent of the genome of your child derives from your partner’s mother, and 27 percent derives from your partner’s father. And you could know exactly how closely your child is related to each of its uncles and aunts. This isn’t imaginary science fiction, it is science fact.

Last year, I sent a sample of my then 2-month-old daughter’s genetic material to the firm 23andMe and received her results. As it happens I already had the genotype of my daughter’s mother, father, all her uncles and aunts, and all four of her grandparents. In other words, her full pedigree was already available when her results came back, and she was easily slotted into the bigger genomic family photo album. Not only do I know which proportions of her ancestry derived from each grandparent, I know which regions of her genome derive from each of her grandparents. For example, one grandmother is half Norwegian, so genealogically my daughter is one-eighth Norwegian. But I quickly calculated using diverse data sets of various nationalities that genomically she is somewhat more than one-eighth Norwegian. This is reasonable as 28 percent of her ancestry, according to how her DNA analysis matches up with the DNA of the rest of the family, happens to come from her half Norwegian grandmother.

One might think these sorts of facts are useful only for the sake of satisfying curiosity, but sometimes theoretical knowledge can be put to practical use. Last spring my wife asked our pediatrician about testing my daughter for a treatable autosomal dominant condition which I happen to exhibit. The physician’s reaction was straightforwardly paternalistic. She would not authorize the test because she believed our daughter was too young. This did not sit well with my wife. The mutated gene which causes my condition has been well characterized. My wife went home and quickly used one of 23andMe’s features to find out if my daughter inherited a copy of the gene through me from my mother or my father. My mother is affected by the same ailment as I am, while my father is not. The happy ending is that my daughter almost certainly does not have the condition, because she inherited that genetic region from my father. The bigger moral of the story is that decentralized genetic information can allow persistence to pay off.

All of this is a consequence of the fact that I have an obsession with genetics. But it is also contingent on the fact that for less than $500, you can send in a kit and receive back a record of 1 million genetic variants in short order. This would have been unfathomable just 10 years ago. Out of 3 billion base pairs in the entire human genome, 1 million may not seem like much, but the ones tested were chosen because they vary across the population, and they represent a substantial proportion of the variable genome. Much of the information is banal, trivial, and redundant. My eyes are dark brown, and those of my wife are blue. Therefore, you will not be surprised to learn that a quick check on variants that code for eye color predicts that my daughter will shake out to have a light brown shade (as a matter of fact her eyes are light brown or hazel).

Other results are more relevant. I am a “PTC nontaster.” This means that I lack the ability to perceive a particular chemical called phenylthiocarbamide, which is often applied to paper one can touch to the tongue in high school biology classes to illustrate genetic inheritance of recessive traits. My wife has a copy of the tasting variant, as well as the nontasting one. (Her father, a notoriously finicky eater, has two copies of the tasting variant, and so he is a “super-taster.”) In real world terms it means that my wife has a greater sensitivity to bitter foods than I do, something I can attest to as a real phenomenon. For me, salad dressing is optional. There are studies which suggest that small children who are tasters tend to be fussy eaters, as opposed to those who are nontasters. You can guess which result I was rooting for. And in this case the die landed exactly as I would have preferred: My daughter is a nontaster, just like her father!

At this point you may wonder if my daughter’s future, in my mind, is predicted down to minute details by the character of her genotype. No. The reality is that most heritable traits are difficult to predict from specific genes because they are influenced by so many genes. (This is why I was so interested in which grandparents she received more of her genes from overall, by the way.) When it comes to individual prediction, most of the strongly predictive genes influence traits we already can see with our eyes. (My daughter will have wet earwax!) But even facts which might seem silly can loom large for grandparents. To my mother’s chagrin, my father is quite proud of his disproportionate contribution to his granddaughter. Similarly, it is clear my mother-in-law has no complaints about being overrepresented in the genetic constitution of her first grandchild.

My daughter is a nontaster, just like her father!

But the story does not end here. Let’s shift from science fact to science fiction. A survey of 1 million variants of a 2-month-old are to a great extent for entertainment purposes only, as the promotional material for 23andMe cautions. But in the very near future, parents will be able to avail themselves of precise and accurate genomes of their future child in utero. Potential sperm donor clients will be able to simulate thousands of potential offspring outcomes from different male contributors. Currently, only primitive assays are available in utero, such as whether your child has a chromosomal abnormality which might indicate Down syndrome. But imagine a scenario in which parents are given a summary from their genetic analyst of their future child at the sixth week of pregnancy. That summary will include superficial characteristics, perhaps even predictions of facial morphology. But it will also have a number that indicates mutational load for large-impact genes. This number will vary from fetus to fetus. Mutation is after all somewhat random.

The implications are obvious. Go with a child with a higher mutational load than you prefer? Or try again? This will be especially important for the children of older fathers, who are already contributing more than their share to the mutations in their offspring. Accuracy is of the essence for what comes next. The personal genetics tests available today look for common variations within the population. One of the possible “killer apps” of genomics in the near future may be to assess the regions of the genome where an individual has unique mutations, ones that differ from the parents’ DNA. These variants are informative in a way that family history simply can’t be.

When it comes to personal genomics, we currently live in the age of the hobbyist. There are a plethora of websites that can do genealogical analysis for you if you don’t have computational skills. Off the shelf applications give you updates on the latest research on a particular variant of personal interest. I hope in the near future I can install a smartphone app that feeds me a stream of actionable results. Personal genomics is truly leveraged once you integrate it holistically into the quantified-self framework. Not only would you know the genome you were born with, but you could have hourly updates on biomarker results, as well your continuous pace of physical activity. Disease risk in many cases is a combination of both genes and environment, and integrating information from both domains opens up many possibilities. Naturally there are people in Silicon Valley working on projects which aim to take this framework to the mass market.

The goal here is not to foresee my daughter’s destiny, it is to understand it and shape it so that she can flourish. I take a mild interest in my own genomics, but I am surprised at how much more interested I am in my daughter’s results. For her the future will be. Much of my future has been determined by choices made in the pre-genomic era. By the time she is entering elementary school I expect to have a full and accurate copy of her whole genome. At that point we’ll start a new discussion about who she is, and who she wants to be. I’m a geneticist, not a developmental psychologist, so this is going to be an experiment where I am totally at a loss for forecasting the outcome. But someone has to step out of the cave of personal genomic ignorance first, and see if the coast is clear.

http://www.slate.com/articles/health...related.2.html

[Piquerobi]

Understanding Patterns of Inheritance (And Why It Matters)

Your DNA contains a record of your ancestors, but you aren’t a carbon copy of any one of them. The particular mix of DNA you inherit is unique to you. You receive 50% of your DNA from each of your parents, who received 50% of theirs from each of their parents, and so on. In the chart below you can see how the amount of DNA you receive from a particular ancestor decreases over generations. If you go back far enough, there is a chance that you inherited no DNA from a particular ancestor.

The chart below helps illustrate how different segments of DNA might have been passed down from your grandparents to make your unique DNA. Assume each letter represents a segment of DNA. Things to notice are:



Which letters get passed down to each generation is random (the fact that the letters spell names in this example is simply to help with the illustration). Not all of the letters get passed down. Just because a child doesn’t have a letter doesn’t mean that an earlier ancestor didn’t have that letter. Siblings can have different combinations of letters.

In the example on the chart, your paternal grandfather has the unique DNA of ANDREW. He can pass down only 50% of his DNA to each child. In your father’s case, the “pieces of DNA” randomly selected to be passed on to him are represented by the letters DEW. At the same time, grandmother SANDRA provides the randomly selected segments ADR, which combine with her husband’s DEW to create your father’s unique genetic signature: EDWARD. Notice that not all of the letters from ANDREW and SANDRA get passed down to EDWARD.

Your father, EDWARD, has three children with your mother, whose genetic signature is ANGELA. EDWARD and ANGELA each pass 50% of their DNA, randomly selected, to each of their children, who end up with the genetic signatures GLENDA, GERALD, and REAGAN.

Again, the parents don’t get to choose which segments (letters) go to each child. And while having more children increases the chances of passing on more of your DNA, if you look closely, you’ll see that even with three children, not all of EDWARD and ANGELA’s DNA segments made it to the next generation.

This is a simplified example of how genetic inheritance works in all of us. By understanding how DNA is inherited, you can see how and why you have some DNA segments that match your relatives, and others that do not, why you may or may not have inherited DNA segments associated with a certain ethnicity, and why getting multiple people in your family tested can help discover more of your family’s genetic tree.

Each of us carry unique pieces of DNA that can unlock our family’s story. If I had just used my DNA results to infer my genetic story, I would have missed out on a few pieces. So it’s important to get parents, siblings, aunts, uncles and even 1st cousins tested to help you do more with your DNA results.

https://blogs.ancestry.com/ancestry/...hy-it-matters/

[Pappy]

While we receive 100% of each parents DNA in the fertilization stage, I think it's wrong to say we get 50% from each parent to make you. Study and logic rejects that.
And nature isn't that tidy.

I guess we'll find out the answer when they understand the recombination issues at hand.