gene imprinting

Genetic phenomenon whereby a small number of genes function differently depending on whether they were inherited from the father or the mother. If two copies of an imprinted gene are inherited from one parent and none from the other, a genetic abnormality results, whereas no abnormality occurs if, as is normal, a copy is inherited from both parents. Gene imprinting is known to play a part in a number of genetic disorders and childhood diseases, for example, the Prader–Willi syndrome (characterized by mild mental retardation and compulsive eating).

Imprinted genes play a part in the development of the embryo and placenta. Paternal genes are responsible for placental growth, and maternal genes for embryo growth. If a normal sperm fertilizes an ovum without a nucleus the result is rampant placental growth without an embryo. If a sperm without a nucleus fertilizes a normal ovum a benign tumour forms that parallels embryonic tissue differentiation. There is no placenta. Consequently an imprinted gene from the mother will be expressed in embryo development and after, whereas one from the father will not.