Recombinational Loss of Heterozygosity (recLOH) is a process by which one copy of genetic code is copied over others. The result is identical values. In genetic genealogy, this is most significant for the Y chromosome. Palindromic STR (short tandem repeat) markers may be copied over each other.
For example, DYS385 may have a,b values of 12,19 for a father. His son may have values of 12,12. This is a single recLOH event.