Mutations are changes is the sequence of bases in DNA. Sometimes they are not dangerous, sometimes error in gene is invisible thanks to the second functional copy, sometimes they cause diseases. Mutation is transmitted to the cell's offspring.

Mutations can affect little pieces of DNA or larger ones. Basing on the extent of the alternation they can be described as chromosomal mutations (concerning structure and number of chromosomes) or gene mutations. The first one often results in stillbirth or natural abortion of fetus, the latter are less serious more common.

As a result of deletion or addition reading frame is shifted and sequence of amino acids from that point forward is completely different. If both mutations occur near to each other effect may be similar to that of substitution. Frame remains the same and one (if it's point mutation) or several amino acids are altered. Such change results in only slight change of protein. However it can be still serious. Sickle cell anemia is effect of point mutation in gene coding for hemoglobin.

Mutations that affect somatic cells influence body function, for example they can cause cancer development, but they are not inherited. Only mutation in germ cells (eggs and sperm) are transmitted to the next generation.

Mutations appear spontaneously. Considering vast number of DNA replications that take place in human body mistakes are likely to occur. Mutation can also be induced by environmental agents, such as ionic radiation, X-rays, toxic chemicals. There are certain mechanisms that repair damaged DNA.

Mutations are material for evolution. If they are harmful (most of cases) they result in less adaptability and organisms survival rate. If they bring some benefit mutated organisms have greater chance to reproduce, their offspring is better adapted and have greater chance to survive. The share of mutant gene in the pool gene increases and the traits determined by it becomes norm in the population.