DNA shared by siblings

-A curious adult from the U.S.

August 2, 2017

In this case you are almost certainly full siblings. Because of how this (and many other companies) calculate shared DNA, that 38% number is actually equivalent to 50%. In other words, the two of you do share around 50% of your DNA.

The reason they report 38% has to do with how they figure out shared DNA. And how DNA is passed from parent to child.

The first thing to notice is that each parent has two rectangles. This is because all of us essentially have two copies of our DNA. Each copy is a bit different which is why one rectangle is a solid color and the other has stripes. (And also a big reason why we aren’t all exactly alike!)

When we have kids, we pass down one rectangle’s worth of DNA. That is how generation after generation people keep two copies of their DNA—they get a complete copy or rectangle from each parent.

Let’s see what these parents’ first kid might look like:

So for example, they share a chunk of mom’s blue striped DNA (the first box in the middle on the left) and a chunk of mom’s solid DNA (in the bottom box). Notice also that in some areas, like the top of the blue rectangles, they do not share DNA.

If we tally this up, we will probably get around 50% shared DNA. Here it might be a bit more or less than that but keep in mind that this is just an example. Real life is more complicated.

Genetics of skin color

-A curious adult from North Carolina

July 25, 2017

The short answer is, yes! A couple can have a baby with a skin color that isn’t between their own. The long answer, though, is much more interesting.

The long answer has to do with the parts of your DNA that give specific instructions for one small part of you. In other words, your genes.

As you can see, if you have two babies, they might end up with very different decks! And so very different skin colors.

This random dealing actually happens. Some mixed race parents have twins that look very different (click herehere and here for some great pictures of real-life examples).  

It turns out this switch doesn’t just change skin color. Hair color is also affected!

The rest of the person’s genes are saying, “make the hair colored!” so the person isn’t going to be blonde. But the person can’t flick the switch for brown, so their hair turns out red!

Genes Are Important, But You Still Have Control.

Your DNA contains all the information to make you. But that doesn’t mean it controls all of your future!

Curing sickle cell anemia

-A middle school student from New Hampshire

July 18, 2017

Not very easily! Right now a cure involves a bone marrow transplant and these things are pretty dangerous.

According to this site, 5-10% of young sickle cell patients don’t survive the transplant. The numbers are so bad for older people that it isn’t usually even offered to them.

Some new research suggests that there may be a way around this last problem. In this new approach they fix a patient’s bone marrow cells and put them back in the patient. The cells match because they are the patient’s own cells.

Back in March, 2017, a group of scientists reported that this new approach seems to have worked for a teenager with sickle cell. We will have to wait and see if this can become a new way to cure lots of people of their sickle cell.

Fixing the Typo in Sickle Cell

Fixing the typo in sickle cell is a tricky process, although it can be done. To make the process easier, scientists just add a copy of the correct instructions. They add a copy of the gene that does not lead to sickle cell.

Scientists use viruses to add a copy of the gene with the right instructions. These viruses naturally insert DNA into human cells and so are the perfect tools for this “gene therapy.” 

Resurrecting Neanderthals

-A curious adult from Texas

July 7, 2017

Just barely. It would take a lot of work and take a long time but if we wanted to, we could just about recreate a Neanderthal.

Since we know what Neanderthal DNA looked like, you might think it shouldn’t be too hard to resurrect one. After all, knowing Neanderthal DNA means we have all the instructions for making a Neanderthal.

Inserting Lab-Made DNA into a Cell

Another way to get Neanderthal DNA into a cell that might be possible in the near future is to make the DNA in a lab and get a cell to take it up. Scientists have been able to do this with very simple beasts like bacteria but anything much more complicated is much trickier.

Here the tricky part would be growing one of these iPS cells all the way to a baby Neanderthal. No one has yet done this with people but it should be possible. Scientists have been able to do something similar with mice for years.

Scientists coax the mouse iPS cell into an embryo and then put that embryo into a surrogate mouse. The embryo grows and develops in the womb and in the end you have a mouse pup.  

Odds for a child with sickle cell trait

-An undergraduate from Nigeria

June 28, 2017

To quickly answer your question, your third child still has a chance to end up with sickle cell trait. In fact, assuming your husband is not a carrier, your next child has the same 50% chance your other two kids did.

Think of it like flipping a coin. When you flip a coin, there is a 50% chance that you will get heads and a 50% chance that you will get tails. Do the chances change with each flip? Nope. 

The gene involved in sickle cell trait is found on chromosome number 11. It is called HBB and makes hemoglobin, the part of our blood that carries oxygen.

The HBB gene can come in at least two versions (or alleles): HbA and HbS. The HbA allele causes no problems but the HbS version can lead to either sickle cell trait or to the more severe sickle cell disease.

This means that each child has a 50% chance for having sickle cell trait. What it doesn’t mean is that half your kids will have sickle cell trait and half won’t.

This is a point that is often confusing about Punnett Squares—they do NOT mean that if you have four kids, two will have sickle cell trait and two will not. It is only figuring out the odds for each child. Like figuring out the chances for heads or tails in a coin flip.

Fragile X premutation

-A curious adult from India

June 22, 2017

Fragile X is a tricky genetic disease. Given that you don’t have the disease you are either not a carrier or a sort of carrier.

In the first case your kids aren’t at any risk for getting Fragile X. You are not a carrier and so you can’t pass it on to them.

In the second case your daughters are at a very, very, very low risk of having Fragile X (your sons are at no risk). However, in this second case, your daughter’s sons have a higher risk of getting Fragile X.

This isn’t just important for gender because the cause of Fragile X is on the X chromosome (that is where the “X” comes from). Your brother has Fragile X, which means it came from your mom because he got a Y from dad. Mom is a carrier which means she has the chromosome that can lead to the disease but does not have the disease because of her other X.

Now we know your mom is carrier, but we don’t know what kind of carrier she is. There are two kinds of carriers with Fragile X—premutation and full.

Your Daughters’ Children

If a daughter ends up with the full blown mutation, then each of her sons has a 50% chance for ending up with Fragile X. (Each daughter has a 50% chance too but remember that if they have any symptoms, they would be less severe.)

It gets a little trickier if a daughter has a premutation because now there is a higher chance it can transform into a form that can cause Fragile X. Not 100% but significant. (We can’t give exact numbers because it depends on the specific premutation a person has.)

Identical twin great aunt grandmother DNA

-A curious adult from Vermont

May 31, 2017

Yes it would. At the DNA level, you are related to the baby more like your identical twin sister is. You are actually closer to being the child’s grandmother instead of his/her great aunt.

This is because you and your identical twin sister essentially share 100% of your DNA. So when you look at her kids and their kids it is like looking at your kids and your grandkids. Well, only at the DNA level.

So dad has a dark blue and a light blue chromosome and mom has a dark orange and a light orange one. Here dad passed his dark blue one and mom passed her dark orange one.

This child got half his/her DNA from dad (dark blue) and half from mom (dark orange).

But this turns out to be much too simple! Chromosomes are rarely inherited whole. 

Before getting packaged into the sperm or egg, the two chromosomes in a pair swap DNA.  You end up inheriting a chromosome that is a mix of the two from your parent. 

Exploring Your Case

Let’s apply our new knowledge to your case. Here is a summary of what we learned about how much DNA is shared in different family relationships:

Now let’s take a look at your family tree and fill in some of the relationships: 

Chromosome number not correlated with complexity

-A middle school student from Florida

May 25, 2017

Just the luck of the draw during evolution. Over millions of years, potatoes happened to end up with 48 and we ended up with 46. And there are even bigger differences out there

A carp (a kind of fish) has 104 and a rattlesnake fern has 184. Most likely neither of these is as complicated as we are (especially the fern).

It Isn’t the Amount of DNA…

Imagine the car’s instructions are written really small and the bicycle’s are written really large. The car is still more complicated even though it is written over fewer pages.

The same thing is true with your chromosomes except instead of pages, they are made up of DNA. More DNA does not mean more complexity.

Number of Chromosomes Not Constant

And finally, the number of chromosomes can and does change over time. A million or two years ago, people had the same number of chromosomes as today’s potatoes—48.

Then two chromosomes ended up stuck together. Now most every human has 46 instead of 48.

Our ancestors with more chromosomes weren’t more advanced than us. They just had their DNA packaged a bit differently.

Same DNA Different Ancestry Results

-A curious adult from New Jersey

I can see why you’re confused. The same DNA should give the same ancestry results. And yet they haven’t.

This is pretty common with DNA ancestry tests and it isn’t just a 23andMe thing. Companies like Ancestry.com or MyHeritage will give these sorts of results too.

The companies then compare your DNA to the DNA landmarks they found in these families. The parts that match that group of Germans is German, the parts that match that group of people from the British Isles is English and so on.

Sounds easy enough but most people are not like the reference group. They have lots of different ancestral DNA scattered in chunks throughout their DNA. This is where things can get tricky.

One Piece of the Puzzle

What I just described is certainly part of the reason for why the same DNA can end up with different results. But it is by no means the only way it can happen.

Analyzing DNA for ancestry is very complicated for lots of reasons (check out this outstanding blog from 23andMe to get a feel for what they are up against). It is technically very challenging.

H63D ALS neurodegenerative iron hemochromatosis

-An undergraduate from Illinois

May 9, 2017

Thank you for your question! I’ll try to answer it up front for you, but you can find a full explanation below.

People like you with one copy of H63D (“heterozygotes”) are at a higher risk for certain neurodegenerative diseases. But not by much.

Usually you need to also have another mutation, like C282Y, to be at a higher risk for hereditary hemochromatosis.

And even these people don’t get it for sure. They are at a much higher risk, but not everyone with both H63D and C282Y ends up with the disease.

It might be a bit surprising that H63D turned up as a risk factor for neurodegenerative diseases.

How can a somewhat wonky iron system make getting a neurodegenerative disease like ALS more likely?

Scientists don’t know the answer to this question just yet. But they have a few theories…

And there are actually a bunch of genes that makes sure we have just the right amount of iron in our bodies. This last set of genes includes an important one called HFE.

The HFE gene helps humans control iron levels by detecting the amount of iron in our bodies. It uses this information to control another gene called HAMP.

Very fascinating, but what does this have to do with H63D? Well, H63D describes a very specific mistake, a mutation, in the HFE gene.

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