Gene imprinting is a genetic phenomenon in which alleles (i.e., specific versions) of certain genes function differently depending on whether they are inherited from the paternal or maternal parent. In other words, the same exact gene may act differently in a child depending on whether it was donated by the mother or donated by the father.
In standard Mendelian genetics, for all non sex-linked genes (i.e., autosomal genes, or those not on the chromosome pair that determines sex), each parent contribute equally to the offspring by contributing one allele. However, sometimes this isn't the case; only one of the offspring's alleles is expressed and the other is inactivated. Oddly enough, the inactivated allele is always from one parent or the other; for some genes, the father's contribution is inactivated, and for others, the mother's contribution is inactivated.
This parent-specific inactivation is believed to be the result of the chemical effect known as methylation. Methylation of components of DNA change the ability of that piece to function properly, as additional carbons and hydrogens are attached to the molecule. This enables proteins to distinguish between the two, and they may reject components which are methylated.
Here's the situation: on some genes in both the mother and the father, a large number of methyl (one carbon and three hydrogen atoms) groups are added in a variety of places. When these genetic elements are contributed to the offspring, the proteins, when deciding which of the parents' genetic material to use, will choose not to use the ones with heavy methylation. Thus, the genes of one parent can be the sole influence over a child. Some genes have only the mother provide non-methylated genes, others have only the father provide; this makes sure that both parents do not contribute methylated genes.
The most well known example of gene imprinting in humans is the odd case of Huntington's disease. The symptoms of Huntington's disease, which is an inherited neuromuscular disorder, first occur during adolescence if the mutant allele that causes the disease is inherited from the male parent but symptoms generally do not arise until middle age if the allele derives from the female parent.
Another example of this phenomenon comes from the tendency for the development of glomus tumors, which are benign tumors that develop in the inner ear. The disease is known to be autosomal (meaning not sex-linked in the classical sense), but it is also shown to be passed from father to offspring. This unusual effect is the result of this phenomenon, which experiments involving methylation and this disease have shown.
Gene imprinting is one of many genetic phenomena that help to explain the complex procedure of reproduction.