This is an autosomal* recessive genetic disorder.
It is caused by a point mutation in the β-globin chain of haemoglobin where the hydrophilic amino acid glutamic acid (GAG) is substituted by the hydrophobic amino acid valine (GUG). As you can see, it is caused by a single mutated nucleotide in the codon.
This is expressed in the phenotype as red blood cells having an abnormal, rigid, sickle shape.
This can cause many complications:
Anaemia - may cause fatigue, pallor and tachycardia (low BP)
Vaso-occlusive crisis - obstruction of capillaries, may cause ischaemia, pain and necrosis.
Splenic sequestration crisis - painful enlargements of the spleen, may lead to infarction, patients may die in 1-2 hours if left untreated.
Haemolytic crisis - acute accelerated drops in haemoglobin level (as red blood cells degrade more rapidly).
*Autosomes are non-sex chromosomes. Humans have 22 pairs of autosomes and 1 pair of sex chromosomes.