In genomic imprinting the ability of a gene to be expressed depends upon the sex of the parent who passed on the gene. In some cases imprinted genes are expressed when the are inherited from the mother. in other cases they are expressed when inherited from the father. Unlike genomic mutations that can affect the ability of inherited genes to be expressed, genomic imprinting does not affect the DNA sequence itself. Genomic imprinting affects gene expression by chemically modifying DNA and/or altering the chromatin structure. Often, genomic imprinting results in a gene being expressed only in the chromosome inherited from one or the other parent. While this is a normal process, when combined with genomic mutations, disease can result. For example, Prader-Willi syndrome and Angelman syndrome are two distinct diseases caused by a deletion in the same part of chromosome 15. When this deletion occurs on the chromosome 15 that came from the father, the child will have Prader-Willi syndrome. However, when the deletion occurs on the chromosome 15 that came from the mother, the child will develop Angelman syndrome. This occurs because genes located in this region undergo genomic imprinting.
Genetic imprinting is a rather mysterious phenomenon which has become somewhat better understood in the last few years. Essentially, what it refers to is the chemical modification of a DNA sequence. Keep in mind here that the DNA sequence itself is not changing. These are modifications to the DNA sequence itself that occur in a cell--usually refers to a germ cell, either an egg cell or a sperm cell--and that change is passed on from one generation to another. The reason it confused scientists for many years is that it is a non-sequenced-based mechanism of inheritance. Initially, it was thought that all inheritance is based on changes in sequence; this turns out not to be true. In one of those mechanisms, which is not involved in change of sequence, but rather an inherited chemical change to a DNA sequence, is referred to as imprinting. And that imprinting, the reason it's important is that chemical modification, which is passed on from the mother or the father to the offspring, changes the function of the gene or the gene product, whether it's expression or actually the function of the gene product itself.- Christopher P. Austin, M.D.