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When it comes to your health, it seems like genetic testing promises a lot.

And it doesn't just make promises about big health conditions, either:

These tests even give advice about what lifestyle, diet, and level of exercise

are best for you based on your genes.

But generally speaking, it might be worth taking some of that information with some grains of salt.

Because while we have gotten pretty good at reading the human genome,

we are not experts at it yet.

And when it comes to our bodies, our genes aren't so much an open book,

as the world's biggest crossword puzzle.

Your genome is the complete set of DNA hidden inside your cells.

It consists of about 3 billion base pairs, which are molecular building blocks represented

by four letters: A, G, C, and T.

For most people, those letters are spread across 23 pairs of chromosomes and your mitochondria.

And they contain pretty much all the instructions that make you… you.

In the last couple of decades, we've gotten much better at analyzing those instructions

and understanding what those strings of letters correspond to.

And today, genetic testing can help us solve crimes, discover our ancestors,

and make important health decisions.

At least some important health decisions.

One thing genetic tests are great at, is telling us about things caused by one, well-understood variable.

Huntington's disease, for example, is a disorder that kills neurons in the brain.

And it's caused by a mutation to a single gene known as HTT, which sits on chromosome four.

In this mutation, a section of the gene that normally repeats 10-35 times

is instead copied much more, turning its normally helpful product into toxic fragments.

So if that mutation comes up in a test, doctors can pretty confidently say what's going on.

This is also true with sickle-cell disease.

That story happens on chromosome 11, where a mutation in the gene HBB

ultimately causes red blood cells to change shape.

That can cause anemia and vascular problems.

And again, the mutation is pretty easy to identify.

There are even some smaller things genetic tests can confidently say,

like how you'll respond to certain medications.

For example, people with a certain variant in the gene VKORC1, which is involved in making blood clots,

can be especially sensitive to the blood thinner warfarin.

Essentially, warfarin works by grabbing onto the VKORC1 enzyme and blocking it from doing its job.

But some mutations can make a person produce less of this enzyme than normal.

So a typical dose of warfarin becomes an overdose, putting the patient at risk of abnormal bleeding.

There are also other mutations that make it harder for warfarin to bind to this enzyme,

meaning those patients might be underdosed by a typical prescription.

So a doctor can use a genetic test to figure out what dose to give them

or if this drug is even worth trying.

The key, though, is that each of these examples is caused by a specific mutation in a well-known gene.

That means a genetic test can be fairly accurate and pretty useful,

because we know exactly what we're looking for and what it does.

But when you move beyond these cases, the situation gets a lot more complicated.

Because for most things, there isn't just one gene to blame.

Instead, there can be hundreds or thousands of them.

Let's take something simple, like height.

Tall parents tend to have tall children, right?

That's something we've noticed for generations.

But as much as we've looked, there doesn't seem to be a single “height gene.”

Instead, one study from 2014 found almost 700 variants and more than 400 locations in our genome

that could all play a part in height, usually only about a millimeter each,

according to one of the authors.

Later on, some of those same authors did find a few mutations that had bigger effects,

but it was still maybe only a couple of centimeters at most.

The reason so many genes affect height is that, well, a lot of things go into determining how tall someone is,

like how long their leg bones are,

or how much growth hormone their body makes.

And each of those little additions may come with its own gene or suite of genes.

But even if we could perfectly calculate all those effects,

we still might not be able to really predict how tall someone will become.

That's because, as far as we can tell, genes only make up about 60-80% of someone's height.

The other 20%-40% comes down to a person's environment and life history, like their nutrition.

This is a really common example, but the general principles are true for all kinds of things,

including more serious conditions.

A big one being cancer.

Genetic tests can identify if someone has inherited a rare gene variant that can increase their risk of cancer,

including BRCA1 and 2, which are associated with breast and ovarian cancer.

But since this disease doesn't just have one associated mutation,

there's no guarantee someone will develop it even if their BRCA1 gene is mutated.

There are also plenty of other things at work, including other genes and environmental factors

and also random chance.

The same is true for something like type-2 diabetes, or obesity.

In 2018, a cardiologist speaking to NPR

noted that the genes only account for about 5-10% of the risk associated with diet-related conditions.

The rest comes down to behavior and diet.

In other words, for most conditions, your DNA is not your destiny.

Genetic testing is still useful, though, because it can help someone know to be vigilant

or take precautions, or it could help doctors plan treatments for them.

And really, it only gets more complicated from there,

especially once you jump into the realm of mental health or lifestyle choices.

Just look at major depression.

We know that it can run in families, which suggests it has something to do with a person's genetics.

And we've even found genes that appear to be linked to it.

But, if you thought 700 variants for height was a high number, brace yourself.

Because for depression, there are literally thousands of genes or gene variants involved.

One 2014 review estimated that as many as one in every five genes expressed in the brain

might play a role in major depression.

And even then, the estimated heritability was less than 40%.

That means more than half of a person's risk of experiencing depression

might come down to things totally unrelated to their genetics.

So sure, there are some conditions genetic tests can confidently identify.

But in the vast majority of cases, deciphering which genes link to which conditions

and how strongly is very difficult.

And actually, this applies to doing research on these traits, too.

Because while the number of possible variants is definitely a reason understanding a genome is so hard,

it's not the only reason.

Sometimes, it's just hard to figure out what all those letters mean.

Say you test a thousand people with a certain condition and find one mutation that really sticks out.

But then a different scientist wants to replicate your study with another group,

which, admittedly, is always a good idea.

How can that researcher be sure their group had the same life history as yours?

Or that there's not another, unknown gene that could influence the results?

Also, what if the condition you're studying is actually multiple conditions that look similar?

How do you control for that?

To make things even harder, these studies often need a ton of samples,

which can be hard to obtain for rare conditions.

Like, that height study needed data from more than a quarter of a million people.

And the follow-up used DNA from 700,000 people.

All of this can make answering even basic questions about our genes and our bodies complicated.

Nevertheless, we've made a ton of advancement in the last few years,

which really makes you realize how impressive research is these days.

According to the National Institutes of Health in the U.S.,

there's testing available for over 2000 rare and common conditions.

We can also use genetic testing to help plan cancer treatments

or tell if we're silent carriers for any serious conditions.

It's even commonplace to gene test newborn babies,

just in case they have a condition that requires immediate care.

But as genetic testing becomes more common,

it's also become more important to make sure people understand what it can and can't do.

Because while it is powerful, there are plenty of limitations.

Also, if you are considering genetic testing, there is one important thing worth noting:

Even if they can all be helpful, not all tests are created equal.

There are actually a lot of different ways to test a person's genes.

For example, some tests look for what are called single nucleotide polymorphisms, also called SNPs.

These are places where a single letter of DNA can vary from person to person.

So some might have a G, and another person might have a T.

Many direct-to-consumer tests work by, essentially, scanning your DNA for known, potentially dangerous SNPs.

But while that might help catch the most important cases,

these kinds of tests might also ignore less important or less-studied ones.

Also, while SNPs are some of the most common mutations in our genome, there are other ones, too.

So a test that is only looking for SNPs might miss duplicated, deleted, or shifted genes

that another test might catch.

That's not necessarily a flaw, since the test is working as designed.

But especially if you have a family history of disease,

it might be worth keeping in mind or talking with a doctor to get the test that's right for you.

Studying genetics can sometimes seem overwhelming, because there are just so many pieces involved.

But by breaking it down into small parts, you can usually figure out something.

And the same is true for learning any new skill.

Like, if you want to start a YouTube channel, there's a long list of things you'll need to figure out,

from your name to your camera to your intro music.

But if you take it one step at a time, it will probably be a lot less stressful.

And that's where Skillshare can help you out.

They have a bunch of video-related classes, including one called DIY Cinematography,

taught by filmmaker Ryan Booth.

And he goes over your basic vocab and tools,

along with things like how you can make the most of your location.

Skillshare has more than 25,000 classes besides this one, and if you are a premium member,

you can get unlimited access to all of them.

Also, an annual subscription fee is less than $10 a month, which is a pretty good deal.

So whether you want to pick up new skills for school, for your career, or even your own YouTube channel,

Skillshare has you covered.

Also, the first 500 SciShow subscribers to use the link in the description will get a 2-month free trial.

So check it out, and let us know if you found any classes that you really liked.

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