Gene duplication is the process of producing a copy of a gene. This could be by replication slippage, unequal crossing over, or retro-transcription of mRNA.
Replication slippage is caused by the dissociation of the DNA polymerase during DNA replication (S phase). The polymerase can then reattach to the DNA strand, potentially at a different location, resulting in either the duplication or omission of a section of DNA. When this occurs in satellite sequences, the number of repeats changes.
Unequal crossing over can lead to the duplication of gene, where the cross over occurs at the end of one copy (on DNA molecule 1) and at the start of the second copy (on DNA molecule 2). This will result in one DNA molecule having 2 (or more) copies of a gene, and the other having no (or fewer) copies of a gene.
Changing the number of copies of a gene contributes to genetic variation. For example, a gene duplication event could cause an increase in the expression of a gene, leading to more of an enzyme being produced (for instance). This could have unwanted impacts, resulting in the production of too much of a product. Where a gene has been lost from a DNA molecule, the protein product is not able to be produced. This could have fatal effects for the cell, due to this absence. The allele for Huntington’s disease undergoes expansion with each generation, leading to stronger and faster onset in offspring. This is caused by the duplication of the trinucleotide repeat known to cause the disease.