Statistics of modification and mutational variation

A mutation is a sudden and lasting hereditary change of the genetic material (a gene or a chromosome); an organism that has undergone a mutation is a mutant. In a gene (point) mutation one or several nucleotides are replaced, so the amino-acid sequence in a protein may change (for example in sickle-cell anaemia one nucleotide in the haemoglobin gene is changed); in a chromosome mutation a piece of a chromosome is lost, doubled or moved to another chromosome; in a genome mutation the chromosome number changes: one more or fewer, or a whole set multiplied — polyploidy, which is common in plants and can raise yield. A mutation in a sex cell (generative) is passed to offspring, while one in a body cell (somatic) stays in that organism but may be passed on in vegetative (asexual) reproduction. Mutations may be dominant or recessive, beneficial, harmful or neutral; most are recessive and neutral or harmful, but when the environment changes some become beneficial and provide material for selection. Mutagens are factors that raise the mutation rate: X-rays and other ionising radiation, ultraviolet light, some chemicals; H. Muller showed in 1927 that X-rays greatly increase mutation.