Lab Work - Sickle Cell Disease - Electrophoresis of Different Genotypes

Soundtrack excerpt:
12th grade STL: An individual's phenotype is linked to the expression of their genotype.
1st grade S: The macroscopic phenotype depends on the cellular phenotype itself induced by the molecular phenotype (1A).

Scientific problem statement:
Sickle cell disease is the most common genetic disease in France. Particularly prevalent in Mediterranean populations, it is estimated that 50 million individuals worldwide are affected.
How can a single codon mutation severely impact a specific type of cell (for sickle cell disease, the red blood cells) and generate symptoms of varying severity at the whole organism level?