The seminal work of Ohno in 1970 popularized the idea of gene duplication and divergence. DNA sequence comparison studies reveal that a large portion of the genes in bacteria, archaebacteria, and eukaryotes was generated by gene duplication and divergence, indicating its critical role in evolution.
The duplicated copies of the gene are called Paralogs. Paralogs with similar sequences and functions form a gene family. Across several species, a large number of gene families are characterized. For example, the trypsin gene family in D. melanogaster has over 111 members; the olfactory receptor gene family in mammals has around 1000 member genes.
Gene duplication can arise due to the following four reasons. First, the unequal crossing over during meiosis can give rise to duplicated DNA segments containing a part of a gene or several genes.
The second is replication slippage. In rare instances, during DNA replication, the polymerase enzyme can dissociate from DNA and get realigned at an incorrect position, and copy the already replicated sequences again. This process can create duplicate copies of the DNA over several hundreds of bases.
The third is the retrotransposition. Here, cellular mRNA may get reverse transcribed into DNA copies called retrogenes. These retrogenes can then insert themselves back into the genome resulting in gene duplication. Since the inserted copy lacks promoters and other regulatory elements for transcription, most of these duplicates lose their function and become pseudogenes.
In addition to gene duplications, large-scale chromosome duplications or whole-genome duplications also occur. Some chromosomes may fail to segregate into daughter cells during meiosis, resulting in haploid cells with an abnormal number of chromosomes. For example, patients with Down syndrome have an additional copy of chromosome 21. In plants such as wheat, the entire genome is duplicated over six times, creating a hexaploid.
