Hi. I'm not quite as young as your daughter, but I was diagnosed with colon cancer August 7th. My doctors also think that my cancer is genetic. The surgeon said that a lot of the cases of colon cancer in younger patients turn out to be genetic (he used percentages but I was in shock and don't remember exactly what he said).
Getting the gene test is the only way that the doctor would be able to prove, or disprove that the cancer affecting your daughter is genetic. I personally am going to get myself tested, if I do carry the gene there is a 50% chance for each of my siblings that they also carry the gene... I want them to know so they can get the necessary screenings early enough.
Here's a link to the NCI talking about some of the different genetics of colorectal cancer. I don't know if reading that would help you or not, if you think not you can just ignore it. http://www.cancer.gov/cancertopics/pdq/genetics/colorectal/HealthProfessional/Page4#Section_89
Good luck. I'll be praying for you and your daughter.