trisomy-21 isn’t present in sperm or eggs, it’s a mistake in cell division early in fetal development

This is incorrect. Non-disjunction (where chromosomes incorrectly separate) typically happens during the formation of egg or sperm. (We know this because most everyone with down syndrome has three different chromosome 21 instead of two identical copies of one chromosome.)

A related more general mnemonic works for the running lights too. Red has fewer letters than green, so it goes with left which has fewer letters than right and port which has fewer letters than starboard.