In the simple view of inheritance, it doesn't matter whether a gene comes from your mother or your father. But in real life, it does matter--especially in mammals. Genomic imprinting means that some genes are silenced if inherited from one parent but not the other.

From placenta to milk, baby mammals are especially good at extracting nutrients from their mothers. For a male fathering children with many mates, it's in the interest of his genes that the baby suck as much out of the mother as it can. Mothers, on the other hand, want to save resources for their future offspring. Genes are often imprinted according to these battle lines.

Take, for example, insulin-like growth factor 2 (Igf2) and its receptor (Igf2r) in mice. Igf2 promotes growth of the fetus, while Igf2r restricts growth by binding and inactivating Igf2. In order to limit how much the fetus can draw from its mother, the maternal version of Igf2 is silenced, while Igf2r is activated. For the father, who "wants" the fetus to grow as big as possible, the situation is exactly reversed (Igf2 activated; Igf2r silenced). Several other genes that control embryo growth and suckling behavior follow this same pattern.

Genomic imprinting is epigenetic, meaning that the underlying DNA sequence doesn't change. Instead, specially placed methyl groups--a carbon atom surrounded by three hydrogens--turn a gene on or off. Methylation and thus imprinting generally reset in each generation, so that an imprinted gene can be active in a man's daughter but then inactive in her offspring.