The genotyping technology used by 23andMe and similar efforts is a genotyping array chip. These are only capable of detecting common variants. This is great for

The genotyping technology used by 23andMe and similar efforts is a genotyping array chip. These are only capable of detecting common variants. This is great for calculating ethnicity and common traits, and even maybe some moderately rare conditions (such as the most common BRCA1 variants), but is completely incapable of detecting variants that cause rare conditions. Rare conditions can only be caused by rare variants. Whole genome sequencing can detect most of these rare variants.