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.

still much to learn about human genome

-A curious adult from India

May 2, 2017

While we have had the human genome mapped for over a decade, we still don’t know the genes that are needed to make a human heart. Heck, we don’t know what a whole lot of our genes are doing.

Mapping the human genome is like having a globe of the Earth. We can see the big picture but we can’t see the location of individual countries and cities.

How Bodies Make Organs

You probably know we all start out as a fertilized egg. This single cell then goes on to become the trillions of cells that make up you.

What this means is that first cell can become any other kind of cell. It can become a heart cell, a lung cell, and so on.

It can do this because the genome has all the instructions for making every kind of cell. What makes a lung cell a lung cell is the subset of genes that were used to make it and the subset of genes that keep it a lung cell.

How Scientists Mimic the Process

You might think given everything I’ve said so far that scientists wouldn’t be able to grow an organ. And you’d be partially right.

Scientists have managed to grow crude organs called organoids. They can’t do everything a normal organ can, but they are a start. And as we learn more, they are getting better all the time.

Basically we know how to get certain cells to start down the right path. We know the gate that pushes the ball in the right direction. Once it heads down the path, it can’t go back.

Celebrity Punnett squares

-A middle school student from South Carolina

April 27, 2017

This sounds like a fun project that really dives into genetics!  And may not be as easy as it sounds.

Traits like hair and eye color can be tricky. Two parents without red hair might have a child with red hair. Or a brown eyed parent and a blue eyed parent might have a green eyed child.

This is where that dominant and recessive you talked about comes into play.

Ed Sheeran and You

The gene that controls whether you’ll have red hair or not is called MC1R.  As I said, there are two alleles, the dominant not-red one that we’ll call “R” and the recessive red one we’ll call “r.”

This is what geneticists typically do—they use uppercase letters for dominant alleles and lowercase for recessive ones. 

Remember, this does not mean that half your children will have red hair.  It all depends on which copy of the MC1R gene that you pass on!  All of your kids could have red hair, or none of them could have red hair. (Click here to learn more about this.)

Do you need to partner with a redhead to have kids with a chance of red hair? Nope.

Decoding mutation nomenclature

-A curious adult from Oklahoma

April 18, 2017

It is a very specific way of saying that his person has a different gene from somebody else.

What it doesn’t tell us is which gene is affected. It also does not tell us if the difference has any effect on the person.

What it specifically means is that at position 113 of a protein (p.), there is an asparagine (Asn) instead of an aspartic acid (Asp). This is probably not very helpful yet!

A Simple Genetic Code

Our genes are made up of DNA which is made up of 4 nucleic acids represented by letters. We call these nucleic acids A, T, G, and C.                                    

These four letters are repeated many times and in different combinations in order to make up a gene. They form the “alphabet” of our instructions.

When your body reads a gene, it reads the nucleic acid letters in groups of three. These are the “words” in our genetic language (called codons). There are 64 possible “words” with 4 “letters.”

Finally, p.Asp113Asn

So now let’s put all of this information together to answer your question. As I mentioned earlier, “p.” means that we are talking about a protein. “113” means that we are talking about the 113th codon in this gene.

Asp stands for aspartic acid. Because Asp comes before 113, we know that in this gene, the three letters that make up this codon should lead to the amino acid Asp being added to the protein.

Gedmatch half vs full sibling

-A curious adult from Texas

April 13, 2017

From the ancestry data alone, you can’t really tell if the two of you are half or full siblings. As we talk about in our previous answer here, the two of you could have much more different ancestries than this and still be full siblings.

Full vs. Half Siblings

Here is part of a typical result for full siblings (from Kitty Cooper’s blog):

And here is part of a typical result from a half sibling (from the same blog):

Yellow vs. Green

OK so the key difference here is green vs. yellow. If you have lots of big blocks of green you are almost certainly full siblings.

To get into what these different colors mean, we need to take a step back and talk a little bit about your DNA.

DNA is stored in long stretches called chromosomes. People typically have 23 pairs of chromosomes for a total of 46.

We get one chromosome in each pair from mom and one from dad. That is why we are half related to mom and half related to dad.

Rh negative blood and sickle cell trait unlinked

-A curious adult from North Carolina

March 29, 2017

First, thank you for your question! I think what you might be really getting at is whether someone can inherit sickle cell trait from a white mother who has sickle cell trait and Rh- blood.

And the short answer to this question is yes they can! There are almost certainly people with sickle cell trait who inherited it from Caucasian mothers with Rh- blood and sickle cell trait.

Now this doesn’t mean it is necessarily common.

Now let’s say you want a lemon lollipop and blueberry bubblegum. And you have to either pick a piece of bubblegum and a lollipop from the first two bowls or a blowpop from the third. Blindfolded.

In this case your best bet is bowl 3. Because the two candies are linked in that bowl, you’ll get the right combination each time.

If you choose to pick from bowls 1 and 2 instead, then you have a 1 in 2 chance of getting the right lollipop and a 1 in 2 chance of getting the right bubblegum. Here, your chances of getting both drop to 1 in 4.

What Causes Sickle Cell Trait and Rh- Blood?

Earlier we said that sickle cell trait and Rh- blood are both traits, and that DNA is the molecule our bodies use to spell out the instructions needed to make each of these traits.

So, our bodies have a specific set of instructions that code for sickle cell trait and a specific set of instructions that code for Rh blood type. Spelling mistakes in these trait instructions are called mutations.

How GWAS works

-A curious student from California

March 21, 2017

I think this is a question that a lot of people have after doing tests like 23andMe. These markers are just places in the DNA that in some studies were found in more people with a particular illness.

Most of these markers in the “health” section—especially with 23andMe health results in the past—are not for sure things. Sometimes, they aren’t even found in people with the illness that much more often than in people without the illness.

For ALS and this SNP, the odds ratio looks at how often people with rs3849942 (A;A) (the specific SNP) get ALS. Then we compare that to the number of people with the same SNP who don’t get ALS.

Notice that there are people who have rs3849942 (A;A) who don’t have ALS. So we can say that having this SNP definitely does not mean someone will get ALS 100% of the time! That’s why the overall risk of someone who has this SNP having ALS is so low.

Lots of Limitations

Odds ratio like the one above for rs3849942 and ALS tell us that this SNP probably isn’t the best predictor of ALS. In fact, one might even ask if it is a SNP worth knowing: it only increases someone’s risk by a small amount. It could also cause someone to worry about getting the disease in the future.

The small odds ratio makes sense with what we know about the causes of ALS. Most cases aren’t inherited from our families.

Born lactose intolerant

-A high school student from Indiana

March 7, 2017

Yes, it is possible but it’s rare. Only something like 1 in 60,000 newborns are born lactose intolerant.

The most common situation for people (and most mammals) is that they are born able to digest lactose, a sugar in milk, and they lose the ability when they grow up. Pigs and people are pretty much the only mammals we know of where lots of adults can still drink milk.

Lactose is a Sugar Found in Milk

Did you know milk has sugar? Well it does, and it’s called lactose!

Milk is made up of water (mostly) and has a small amount of lactose. Milk also has other stuff like proteins, fat, vitamins, and minerals.

Because a lot of food items contain milk, lactose is found in many different foods like ice cream, cheese, and creamy soups and sauces, just to name a few.

Developing Lactose Intolerance Over Time

While lactose intolerant babies are rare, lactose intolerant adults are pretty common. Around 65% of the human population loses the ability to digest lactose as they grow and develop.

It’s actually the “normal” trait. When you can drink milk as an adult, you have a condition called lactase persistence.

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