Gene mutations are sudden alterations in the DNA sequence, and its effects are seen immediately.
Gene mutation can be caused by chemical means such as by aniline dyes etc. or by physical means such as Xrays, UV rays etc.
Gene mutation can be:
1) point mutations, where alteration is produced only in one nucleotide or a single codon eg: in sickle cell anaemia ( in the 6th position of the beta globin chain glutamic acid is changed to valine)
2) Frameshift mutations which are caused by deletion or addition of nucleotide/s leading to change in the reading of the sequence.