Three letter segments of mRNA code for specific

Sickle cells
Amino acids
Chromosomes
Disorders

mRNA segments for sickle cells: GAG (codes for glutamic acid)

mRNA segments for amino acids: AUG (codes for methionine)
mRNA segments for chromosomes: TGC (no amino acid, codes for a stop signal)
mRNA segments for disorders: ACG (codes for threonine)