Interpreting microarray results

So, if you are one of the chosen few to get a microarray genetics test to diagnose Williams syndrome, you will receive a result that will look something like this:

"A 1.55 Mb deletion was observed at 7q11.23 from linear location 72,337,897 - 73,837,643"

Size of the deletion

So what does this mean? Let's start with the 1.55 Mb. Mb stands for million basepairs. DNA is made up of a string of base pairs (adenine or A, thymine or T, guanine or G and cytosine or C). A stretch of base pairs or gene is basically the blueprint for one functional protein. So, if you get results that there is a 1.55 Mb deletion then your child is missing 1, 550,000 total base pairs from their DNA. The classic or average deletion in a person with WS is 1.5-1.8 Mb.

Chromosome maps

Chromosome labels give specific information about the location of the gene so that scientists can communicate information easily. The WS deletion is at 7q11.23. The first number on the location marker represents the chromosome the gene is found on. There are 23 total chromosomes in the human genome and Williams syndrome is a deletion on the 7th chromosome.

The next letter tells the scientists which half of the chromosome to find the gene. Chromosomes look like two threads tied together near the center by something called the centromere. Think of the centromere like a belt. The belt usually fits off-center so one side of the chromosome will have longer sections (called arms) than the opposite side of the centromere. This is where the q comes in. The q (short for queue) means that the deletion sits on the long arm of the chromosome. If the deletion was on the short arm it would say p (short for petite).

When scientists study a chromosome, they stain it and take a photograph called a karyotype. The stain will create a banding pattern on the chromosome. This band is how genes are grouped and labeled. The numbers at the end of the chromosome map indicate how far that band sits from the centromere. The 11 indicates that the WS region is the 11th band away from the centromere and sits at a sub-band 23.

So what genes are missing?

The final section of the microarray results will tell you what gene locations are missing. The base pairs are numbered throughout the chromosome. So, this hypothetical person is missing base pairs #72,337,897-73,837,643. At the end of this blog post, I've listed the genes in the WS region with their base pair ranges. You can see what region of the chromosome your child is missing and then look up the functions of the genes, some of which I've described on the genetics page of this blog. You'll find that there are some numbers missing from the list. These areas are considered "genetic junk" Most genes have long strands of base pairs that do not code for any useful protein and are largely ignored by the body.