This is a great question! Genes themselves are too small to see, even with a really powerful microscope they just look like a bunch of molecules. DNA is also really really long so it would be hard to “see” one particular gene on it.

Because of this, geneticists use “molecular techniques” to get information about gene variants (called alleles) that are associated with diseases.

Some tests on a panel look for size differences compared to the standard variant. Maybe the disease-associated allele is longer or shorter or missing entirely. In this case the researcher makes many many copies of the part of the DNA that contains the gene of interest and then uses a molecular sieve to find out its size.

Some tests look directly at sequence differences. They can either use microarray technology to look at many genes at once or sequencing technology to find out the actual specific genetic sequence a person has. In this case the researchers again make many copies of the part of the DNA with the gene of interest and then use biochemistry and powerful computers to glean its sequence.

All of these techniques are a little bit like getting just one or two pages from a book. You can read the pages you got, but they don’t help you figure out what’s on the pages around them.

To find out of your deletion is larger, the geneticists will need to specifically hunt down that one region of DNA.