How do scientists learn the location of a gene for a genetic disorder?

As a geneticist, you've noticed that a particular trait runs in families. You identify a family with a large pedigree. Some members of this family have the trait, but others don't. You have enlisted the support of all family members and have gathered DNA samples from the blood of each individual.

What do you do now? How do you find the gene?

It is kind of like a jigsaw puzzle.

Say the genetic information in each of us were like a jigsaw puzzle. The SHAPES of our puzzle pieces (akin to our genes) would all be the same, since all humans have the same genes. So if we depicted generic human DNA as a jigsaw puzzle, it might look like this:

The exact information conveyed by our genes differs somewhat between individuals, however. As a result, for each of us, the COLORS of our puzzle pieces would be unique. We all share puzzle piece colors with other people, but unless you're an identical twin, your exact arrangement of colors is different from everyone else's.

Compare the puzzles of the parents and child below to see how a child inherits different genetic information from each parent:

parent 1

parent 2

child

Can you see how you might use genetics to relate a child with a potential father, as often happens in paternity suits? About half of the child's puzzle pieces would be the same as the father's.

So, exactly how do you find the gene?

To try and find genes responsible for genetic disorders, scientists first find large families that have the disorder. Then they study the DNA from many members of the family to try and identify the one puzzle piece responsible for the disorder.

How do they do this?

They compare the DNA, or jigsaw puzzles, of family members who have the disorder ("affected" members) with the DNA of those who don't ("unaffected" members). They're looking to find a specific color puzzle piece shared by all affected members of the family. If it's the right one, then none of the unaffected members would have this color puzzle piece. The correct puzzle piece would contain the gene responsible for the disorder. This process involves hard work -- a lot of DNA sequencing and a lot of math. But imagine the reward when you find the gene!