Mutations and gene duplications are the only sources of completely new alleles in a population. Transcription errors at the gene level and the chromosome level are the ultimate source of genetic variation.
Gene mutations arise when there are alterations in the DNA nucleotide sequence. These altered sequences are called alleles. Organisms that reproduce sexually produce gametes containing recombinations of the original inherited genetic information. This recombination process occurs during the crossover phase of meiosis. Genetic recombinations can also result during the fertilization process or due to the random segregation of chromosomes during meiotic division.
Chromosomal mutations result in changes in the number of chromosomes inherited, resulting in missing, extra or irregular portions of chromosomal DNA. These can be divided into two general types: numerical disorders and structural disorders. Numerical disorders arise when the organism is missing a chromosome or has more than the usual number of chromosomes. Down Syndrome arises in humans having three copies of chromosome 21 instead of the usual two. Turner syndrome results when a person is born with only one sex chromosome, an X chromosome, instead of the usual XX or XY. Mutations are randomly occurring and may be harmful, neutral or, occasionally, beneficial to the inheriting offspring.