Genomic imprinting is caused by alterations in chromatin in certain locations of the genome that occur in the germline of one parent, but not of the other parent. These alterations include the covalent modification of DNA, such as methylation of cytosine to form 5-methylcytosine, or the modification or substitution in chromatin of specific histone types, which can influence gene expression within a chromosomal region. The allelic expression of an imprinted gene depends upon whether it resided in the male or female of the previous generation. Imprinted expression can also vary between tissues, developmental stages, and species.
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