I need to make a chi2 test to see if the frequencies of specific genotypes are associated with BMI (body mass index). I believe that I need to perform a Chi2 test, where BMI is divided into groups (25-30 = overweight, 30< = obese). Is that true?

My problem is that I don't know how to convert my nummerical BMI data into these two groups and I can't figure out how to do this.

If someone could help I would be really grateful, preferably in R Commander, but regular R is also okay

Thank you!