Update: Feature was always there. Just hard to find.
23andMe did a site redesign. Most of it is user interface clean up, but there one particular cool function: if you have an individual’s pedigree up to grandparents you can see which allele they inherited. Just select “Family Traits” under “Family & Friends.”
You then get a pedigree you can fill out. A few weeks ago my wife inferred that my daughter did not carry the allele for an autosomal dominant disease which I exhibit (treatable, don’t worry!) by looking at the segments inherited. On the chromosome in question there were no recombination events and the whole chromosome was from my father. Since I received my copy of the gene which results in the disease from my mother logically my wife was able to conclude my daughter does not carry it.
The new feature makes it much easier. You can population the tree with individuals who have been typed, and enter in the gene code. It tells you which maternal and paternal grandparent the alleles come from. This is useful because many times you might be carrying a high penetrance Mendelian disease allele which happens to not be on a SNP chip. Or, there haven’t been many GWAS studies done on this disease in any case, so though the likely genetic region is known, the candidate variants are still a mystery. But if you have phenotype information you don’t need that research (I have it in my mother’s disease state).
Another reason to get your whole family typed.