I've downloaded my raw data from 23andMe. I'm a male. For most loci on the X chromosome, there is a single base in the "genotype" column. However, at the beginning and end of the chromosome, for a few hundred loci, there are two bases in the "genotype" column.
I've looked at my wife's 23andMe data, and her X chromosome has two bases for every loci on the X chromosome.
Where can I learn more about which loci in the X chromosome have one base versus two bases in a male? In particular, can I determine from the position number whether there will be one or two bases? Clearly I can come up with a decision rule based on manual inspection of the 23andMe data, but I'm curious if there's a rule that covers genomic loci that 23andMe does not yet measure.