Aneuploidy often alters gene dosage in ways that are detrimental to the organism therefore, it is unlikely to spread through populations. Some aneuploid individuals are viable, for example trisomy 21 in humans, which leads to Down syndrome. Aneuploidy is often harmful and in mammals regularly leads to spontaneous abortions (miscarriages). Aneuploidy Īneuploidy occurs when nondisjunction at a single chromosome results in an abnormal number of chromosomes. Retrogenes can move between different chromosomes to shape chromosomal evolution. Many retrogenes display changes in gene regulation in comparison to their parental gene sequences, which sometimes results in novel functions. Resulting sequence usually lack introns and often contain poly, sequences that are also integrated into the genome. Transcripts are reverse transcribed to DNA and inserted into random place in the genome, creating retrogenes. Retrotransposons, mainly L1, can occasionally act on cellular mRNA. Replication slippage is also often facilitated by repetitive sequences, but requires only a few bases of similarity. When the polymerase reattaches to the DNA strand, it aligns the replicating strand to an incorrect position and incidentally copies the same section more than once. At some point during the replication process, the polymerase dissociates from the DNA and replication stalls. During replication DNA polymerase begins to copy the DNA. Replication slippage is an error in DNA replication that can produce duplications of short genetic sequences. Schematic of a region of a chromosome before and after a duplication event Replication slippage Repetitive genetic elements such as transposable elements offer one source of repetitive DNA that can facilitate recombination, and they are often found at duplication breakpoints in plants and mammals. Ectopic recombination is typically mediated by sequence similarity at the duplicate breakpoints, which form direct repeats. The products of this recombination are a duplication at the site of the exchange and a reciprocal deletion. The chance of it happening is a function of the degree of sharing of repetitive elements between two chromosomes. Mechanisms of duplication Ectopic recombination ĭuplications arise from an event termed unequal crossing-over that occurs during meiosis between misaligned homologous chromosomes. Common sources of gene duplications include ectopic recombination, retrotransposition event, aneuploidy, polyploidy, and replication slippage. Gene duplications can arise as products of several types of errors in DNA replication and repair machinery as well as through fortuitous capture by selfish genetic elements. It can be defined as any duplication of a region of DNA that contains a gene. Gene duplication (or chromosomal duplication or gene amplification) is a major mechanism through which new genetic material is generated during molecular evolution. Duplication of a gene sequence within a genome
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