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  • Thanks to Skillshare for supporting this episode of SciShow.

  • [ ♪ Intro ]

  • 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 youyou.

  • 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 singleheight 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.

  • [ ♪ Outro ]

Thanks to Skillshare for supporting this episode of SciShow.

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B1 US gene genetic mutation genome test height

What Can You Actually Learn from Your Genome?

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    joey joey posted on 2021/05/11
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