One of the ways a gene can be disrupted or turned off is through in vitro mutagenesis. In this technique, a cloned gene is mutated in the lab and is then put into the organism. This engineered DNA is able to knock out the normal gene so only the mutated gene is expressed. The scientist can see what happens and determine a likely function.
Many knock-out organisms have been produced (not including Channing Tatum), but the process takes a lot of time, and is not guaranteed to work. Over the past decade, RNA interference (RNAi) has been exploited to temporarily silence gene expression.
In RNAi, double-stranded RNA is cut in pieces that are 19-21 nucleotides in length by an enzyme called Dicer. These little pieces are called siRNAs or short interfering RNAs.
siRNAs are incorporated into a protein complex called RISC (RNA-induced silencing complex) and then hunt for the mRNA in the cell with the matching sequence for degradation. The anti-sense, or complementary, siRNA strand binds to the mRNA sequence. This sends red flags to the cells and calls for the mRNA to be destroyed. If no mRNA for a particular gene exists, then no protein can be made, and the effects can be observed.
The effects of RNAi are only temporary. The mRNA is considered to be "knocked down" rather than "knocked out." Using this boxing analogy, if someone is knocked down they can get back up. Eventually, expression of the mRNA continues. However, if a gene is knocked out, no mRNA will ever be made.
Note that both in vitro mutagenesis and RNA interference are usually done in model organisms such as mice and fruit flies, as well as cultured cell lines. It would be unethical to do these tests in humans. Who would ever volunteer for something like that, anyway?
If you want to search for the genetic cause of a disease, you can look for genetic markers in the sequences of normal individuals vs. those with the disease. Genetic markers are what give rise to alleles. They are sequences in the DNA that vary in the population.
A single nucleotide polymorphism (SNP and pronounced snip; snip, snip, snip) is a variation in one nucleotide that occurs in at least 1% of the population. A SNP is usually located every 100-300 bases. They can affect how a person reacts to the environment, medication, or so on.
Scientists have been creating a map of SNPs. These maps allow conditions that are due to multiple genes to be more easily identified. Also, the human genome is so large that mapping SNPs can aid in the organization of all that data. They can serve as mile markers on the extensive roadmap that is the human genome.
Image from here.
Brain SnackQ: What do you get when you cross purple petunias with a little serendipity?
1In 1990, Rich Jorgensen, a molecular geneticist was trying to create a vibrant purple petunia by inserting an extra copy of the gene that codes for the purple color. He thought an additional copy of the gene would make the flower even more purple. Makes sense, right? Well, the flower turned out white! It took almost ten years to figure out the mechanism that caused this. Today we know that the mRNA was targeted by RNA interference.