Please see the [upcoming] Fall 2021 Bulletin article for a change of position on this issue.
In short, a series of letters that is not an abbreviation should be in Nemeth Code.
So, non-abbreviation series like those in “blood type AB” or “genotype Yy” should be transcribed in Nemeth Code.
This is in line with the spirit of #6 under Additional Guidelines in the Guidance for Transcription Using the Nemeth Code within UEB Contexts, which says, "A series of two or more letters (not abbreviations) identifying geometric shapes or figures is transcribed in Nemeth Code and capitalized individually if needed. (e.g. Triangle ABC, line EF). The name of the figure is not included within the switches. A single letter identifying a shape or figure may be transcribed in UEB. (e.g. Circle O)."