Sickle-cell disease is caused by a single point mutation (a missense mutation) in the beta-hemoglobin gene that converts a GAG codon into GUG, which encodes the amino acid valine rather than glutamic acid. Transversion, refers to a point mutation in DNA in which a single (two ring) purine (A or G) is changed for a (one ring) pyrimidine (T or C), or vice versa. A transversion can be spontaneous, or it can be caused by ionizing radiation or alkylating agents.

Transition, refers to a point mutation that changes a purine nucleotide to another purine , or a pyrimidine nucleotide to another pyrimidine . Approximately two out of three single nucleotide polymorphisms (SNPs) are transitions.
A substitution mutation occurs when specific bases (A, T, C or G) in a gene are swapped for different ones. This type of mutation doesn't cause a difference in the number of bases like insertion or deletion mutations do. Substitution mutations just switch out one or more bases for different ones.
Polyploidy, the condition in which a normally diploid cell or organism acquires one or more additional sets of chromosomes. In other words, the polyploid cell ororganism has three or more times the haploid chromosome number.