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A small change in your DNA can have life-changing implications. This is the case with mutations that make the difference between having and not having a medical condition.

Some traits and diseases are determined by many different genes working together. Others are determined by a combination of genes and the environment.

But some conditions come down to a single gene. For each of these conditions, whether someone will have the condition is as straightforward as what version of a given gene they get — something that can be easily identified with a genetic test.

Thanks to the direct-to-consumer genetic testing revolution, it’s now possible to test your DNA for the fingerprints of some genetic conditions. Examples of conditions you can look for with home DNA kits include:

  • Cystic Fibrosis
  • Phenylketonuria
  • Sickle Cell Anemia
  • Tay-Sachs Disease

A common reason people get genetic tests done for these inherited conditions is to find out not whether they have the condition themselves, but whether they might have a copy of the gene that could affect their children — that is, whether they’re carriers for the disease.

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To see how this works, recall that you have two copies of each gene, one from each of your parents. Some diseases are dominant, meaning you’ll get the disease if you have only one copy of the mutation.

Many diseases, however, are recessive, meaning you need to have two copies of the mutation to get the disease. If you only have one copy of the mutation, you won’t have the disease — but your children could still inherit two copies of the mutation (and therefore have the disease) if your partner also has a copy.

Therefore, some people use online DNA tests to see what genetic conditions their children could be at risk for and whether further genetic counseling is necessary.

If you’re interested in a particular genetic condition, it’s essential that you choose a test that actually covers the genes associated with that condition. Of the “big three” online DNA tests (23andme, AncestryDNA, FamilyTreeDNA), only 23andme explicitly looks at genetic conditions as part of their service — you can find a list of conditions they test for on their website.

However, you can still use other direct-to-consumer DNA tests to see whether you’re a carrier for some conditions, even if the tests don’t advertise that service. The way you would do this would be to order the test, then run your raw genetic data from the test through a third-party application like Promethease that analyzes the health implications of your results.

Whatever DNA testing service you’re leaning toward, it’s important to check whether that service looks at genes associated with the conditions you’re interested in. With a little detective work, there are a few different resources you can use to piece this together, including the testing service’s site, SNPedia and (of course) Google.

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One thing to keep in mind is that there are some diseases that aren’t pure genetic conditions that still have genes associated with them that can dramatically alter your risk of getting the disease. For example, there are genes that drastically raise your odds of Alzheimer’s disease and breast cancer. Angela Jolie famously tested positive for the latter and wrote eloquently about her experience in the New York Times.

With genetic conditions, though, it’s not just a question of being at higher risk. Rather, the genes you have completely determine whether you’ll have the condition.

Moreover, the genes you and your partner have determine the odds that your children will have the disorder. If you and your partner each carry one gene for a recessive condition, each child has a 1 in 4 chance of inheriting the disease-causing mutation from both you and your partner.

Knowing what’s hidden in your genes can help you make more informed decisions and be ready for the possibilities that lie ahead. Online genetic testing has made it easier to find out what your DNA holds than ever before, and more and more people are deciding they’d rather know than not know.