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I am a newbie in biology and I am simply trying to discover the field of genomics.

Consider the question - Are genetic tests accurate?

I don't believe they are as the genes may not provide the complete information or maybe there are other genes responsible for a particular trait but they may not be known at the point when the test is conducted. This may not be the right answer though — I simply wish to understand from people here as to what they can say about the accuracy of these tests. If they are not accurate, can you explain why that is the case?

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There are a lot of "genetics tests" you can do. Please describe more detailed, what kind of testing you mean, otherwise this question is too broad. (Accurate as in "Now we know 100% sure that...?" Of course not. Like all other tests you do on organisms. They are just too complex to be understood 100%) – skymningen Sep 13 '13 at 9:31

Like the comment said this question is very broad but Im going to try and clear some things up for you. A "genetic test" is just simply a test that tells you whether or not you have a certain genotype. Yes, it is often times used to tell if a person has a disease or not, but that doesnt always have to be the case. I may just want to know, for example, if a mouse I have carriers a certain transgene. The reason why genetic tests are good at determining if someone has a disease is because many diseases are simple, or at least have a well-defined genotype. Sure, you can make a genetic test for a complex disease like obesity if you want, but no one does this because as you pointed out in your question there are sometimes more than one gene involved, so a genetic test for that would not be very good.

But for genetic tests of diseases of a known genotype, they are very accurate. Of course you have some false positives and false negatives. Look up the terms selectivity and sensitivity. If you have a sensitive test you will catch a lot of people with the disease but also many false positives. If you have a selective test you will not have many false positives but you may miss people who actually have the disease. Its a trade-off. Testing someone twice will clear things up usually.

And then there are cases where you have a disease with a known genotype but still have an inaccurate genetic test on rare occasions. Heres a paper about a disease known as Spinal Muscular Atrophy. There is a genetic test that works 95% of the time but sometimes it does not match up with the phenotype. In the paper they find a mutation that explains why the genetic test does not match the phenotype. Two things are interesting about the paper. One, the genetic test is not wrong, its just that the phenotype is not what you'd expect. And two, not all people who have a discordant genetic test have this mutation. So whats going on in these patients and why their test doesnt match their phenotype is anyone's guess.

The first point is especially important so I will repeat it. The genetic test was not wrong, the phenotype just did not match it as expected. Genotype and phenotype are not always correlated, and mistakes can happen when you assume they are.

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@von-mises summed it up nicely enough, but you'll want to read the Wikipedia articles on heredity and hereitability, in particular the latter. Our genes can tell us so much about ourselves, but it's not always that simple. As you mention, there are lots of other factors that could and usually do come into play. Often times there isn't just one gene, but rather many genes playing a role (these are called polygenic traits). For example, see my answer to a question on hair color; there are dozens of simple changes that can affect the color of hair and eyes. These complex traits are really hard to tease out!

Additionally, not everything is 100% from genes; the environment can also play a huge role. Often we talk about how heritable a gene or a trait is by giving a percent, meaning how much of is due to genes you inherit from your parents. The heritability of IQ is an interesting read for an introduction to the concept.

So, what is the use of the genetic tests? Well, for starters, ancestry is really interesting and useful for people. Not just paternity or maternity (which is obviously accurate and widespread) but knowing where you are from. 23andMe is a great service (I'm 2.9% Neanderthal!) for finding out where you came from genetically.

There are, additionally, a number of traits that we've studied, or that do have strong genetic factors. Sickle-cell disease is a classic example of something just from your genotype, whereas other things are less determinate; for example, the heritability of baldness is around 81%, and I know from my genome that I have a slightly lower chance of becoming bald. There are other, more serious health risks that people would want to be tested for (BRCA for breast cancer, LRRK2 for Parkinson's, APOE for Alzheimer's) as well as inherited traits that are very genetically determined (like sickle-cell or Tay-Sachs).

In short, the genetic tests are very good at determining what the genotype is, which can be very useful for certain traits but completely useless for others. The more we learn about what our genes do the more we learn how complicated the whole system is. We are more than just our genes.

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