The answer is mutation.
Mutations are alterations in DNA sequences that result in changes in the structure of a gene. Both small and large DNA alterations can occur spontaneously. Treatment with ionizing radiation or various chemical agents increases the frequency of mutations.
Recessive mutations lead to a loss of function, which is masked if a normal copy of the gene is present. For the mutant phenotype to occur, both alleles must carry the mutation.
Dominant mutations lead to a mutant phenotype in the presence of a normal copy of the gene. The phenotypes associated with dominant mutations may represent either a loss or a gain of function.
In meiosis, a diploid cell undergoes one DNA replication and two cell divisions, yielding four haploid cells. The members of each pair of homologous chromosomes segregate independently during meiosis, leading to the random reassortment of maternal and paternal alleles in the gametes.
Dominant and recessive mutations exhibit characteristic segregation patterns in genetic crosses.
To learn more, see Molecular Cell Biology (4th edition) from NIH.gov.