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^ Imprinted genes: For most genes, both copies, the one inherited from father and the 
one inherited from mother, are expressed. In contrast, only one of the two copies of an 
imprinted gene, either the maternal one or the paternal one, is active. The two copies 
are distinguished by methylation marks (see endnote 4) that are imposed on imprinted 
genes either during oogenesis (maternally imprinted genes) or during 
spermatogenesis (paternally imprinted genes). Thus, the two copies of imprinted 
genes are epigenetically different in the zygote and remain so in all somatic cells. 
These epigenetic marks distinguish the two copies and cause only one copy to be 
expressed whereas the other copy remains silent. It is estimated that between 100 
and 200 genes (of the total of 30,000 genes) are imprinted. Disturbances of normal 
imprinted gene expression lead to growth abnormalities during fetal life and can be 
the cause of major diseases such as Beckwith-Wiedeman or Prader-Willi syndrome. 
^ Epigenetic changes : Cells of a multicellular organism are genetically identical but 
express, depending on the particular cell type, different sets of genes (“tissue specific 
genes"). These differences in gene expression arise during development and must be 
retained through subsequent cell divisions. Stable alterations of this kind are said to 
be "epigenetic", as they are heritable in the short term (during cell divisions) but do 
not involve mutations of the DNA itself. 
^ DNA methylation: Reversible modification of DNA (methylation of the base cytosine) 
that afreets the “readability" of genes: usually, methylated genes are silent and 
unmethylated genes are expressed. DNA methylation represents an important 
determinant of the “epigenetic state" of genes and affects the state of the chromatin: 
methylated regions of the genome are in a “silent" state and unmethylated regions are 
in an “open" configuration that causes genes to be active. 
References 
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Buehr, M., Nichols, J., Stenhouse, F., Mountford, P., Greenhalgh, C. J., 
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Byme, J. A., and Gurdon, J. B. (2002). Commentary on human cloning. 
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