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Edit Article: Genomic Imprinting

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Genomic imprinting is an epigenetic phenomenon that causes genes to be expressed in a parent-of-origin-specific manner, thus influencing the conditions at which genes are inherited. A maternal and paternal gene is inherited to every offspring, but the origin and methylation of the gene will effect what gene is active. Forms of genomic imprinting have been demonstrated in fungi, plants, and animals. As of 2014, there are about 150 imprinted genes known in the mouse and about half that in humans.

Genomic imprinting is an inheritance process independent of the classical Mendelian inheritance. It is an epigenetic process that involves DNA methylation and histone methylation without altering the genetic sequence. Methylation happens when methyl groups attach to DNA segments during production of egg and sperm cells and actively determine which genes are inherited. These epigenetic marks are established ("imprinted") in the germline (sperm or egg cells) of the parents and are maintained through mitotic cell divisions in the somatic cells of an organism. The methylation and demethylation of the DNA segments can also ultimately control the activity of these genes.

Only a small portion of genes are imprinted however, appropriate imprinting of certain genes is important for normal development. Human diseases involving genomic imprinting include Angelman syndrome, Prader–Willi syndrome and male infertility.

* 1 thus influencing the conditions at which genes are inherited. A maternal and paternal gene is inherited to every offspring, but the origin and methylation of the gene will effect what gene is active.

* 2 Methylation happens when methyl groups attach to DNA segments during production of egg and sperm cells and actively determine which genes are inherited.

* 3 The methylation and demethylation of the DNA segments can also ultimately control the activity of these genes.

* 4 Only a small portion of genes are imprinted however,

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Section Title: Imprinted Loci Phenotypic Signatures

Unfortunately, the relationship between the phenotype and genotype of imprinted genes is solely conceptual. The idea is frame worked using two alleles on a single loci and hosts three different possible classes of genotypes (Lawson, et al., 2013). The reciprocal heterozygotes genotype class contributes to understanding how imprinting will impact genotype to phenotype relationship. Reciprocal heterozygotes have a genetically equivalent, but they are phenotypically nonequivalent (de Koning, et al., 2000). Their phenotype may not be dependent on the equivalence of the genotype. This can ultimately increase diversity in genetic classes, expanding flexibility of imprinted genes. (Knott, et al., 1998). This increase will also force a higher degree in testing capabilities and assortment of tests to determine the presences of imprinting.

When a locus is identified as imprinted, two different classes express different alleles (Lawson, et al., 2013). Inherited imprinted genes of offspring are believed to be monoallelic expressions. A single locus will entirely produce one's phenotype although two alleles are inherited. This genotype class is called parental imprinting, as well as dominant imprinting (Wolf, et al., 2008). Phenotypic patterns are variant to possible expressions from paternal and maternal genotypes. Different alleles inherited from different parents will host different phenotypic qualities. One allele will have a larger phenotypic value and the other allele will be silenced (Lawson, et al., 2013). Underdominance of the locus is another possibility of phenotypic expression. Both maternal and paternal phenotypes will have a small value rather than one hosting a large value and silencing the other.

Statistical frameworks and mapping models are used to identify imprinting effects on genes and complex traits. Allelic parent-of -origin influences the vary in phenotype that derive from the imprinting of genotype classes (Lawson, et al., 2013). These models of mapping and identifying imprinting effects include using unordered genotypes to build mapping models (Knott, et al., 1998). These models will show classic quantitative genetics and the effects of dominance of the imprinted genes.

References

de Koning DJ, et al. Genome-wide scan for body composition in pigs reveals important role of imprinting. Proceedings of the National Academy of Sciences, USA. 2000;97:7947–7950.

Knott SA, et al. Multiple marker mapping of quantitative trait loci in a cross between outbred wild boar and large white pigs. Genetics. 1998;149:1069–1080. This study developed the first model for identifying imprinted loci in QTL analyses. It is also the first application of the line-cross design to identify parent-of-origin effects

Lawson, H. A., Cheverud, J. M., & Wolf, J. B. (2013). Genomic imprinting and parent-of-origin effects on complex traits. ''Nature reviews. Genetics, 14''(9), 609–617. https://doi.org/10.1038/nrg3543

Wolf JBM, CJ, Roseman C, Hager R. Genome-wide analysis reveals a complex pattern of genomic imprinting in mice. PLoS Genetics. 2008;4:e1000091. This study developed the framework in Box 1 to describe the complex patterns of imprinting identified, including first describing the patterns of polar underdominance and bipolar dominance imprinting.