Imprinting is the deactivation of a locus in gametes of only one sex. A maternally imprinted locus has inactive maternal alleles; a paternally imprinted locus has inactive paternal alleles. Nonimprinted genes express both maternal and paternal alleles. Some imprinted regions contain a single gene, while other imprinted regions contain multiple genes. Imprinting deactivates via chromatin alteration: covalent modification of DNA, such as cytosine methylation to form 5-methyl-cytosine; and/or modification or substitution in chromatin of specific histone types.
Imprinting occurs during gametogenesis and is a reversible process; it does not change the DNA sequence nor is it a mutation. Thus, a female carrying a paternal imprint will switch the imprint when passing it on to her offspring. Similarly, a male carrying a maternal imprint will switch the imprint when passing it on to his offspring. This conversion is controlled by imprinting centers within imprinted regions. Imprinting centers initiate the epigenetic (non-mutation) change in the genome that eventually leads to condensation of the entire imprinted region.
Androgenetic embryos have an overlarge placenta but diminished fetal growth. Gynogenetic embryos have a diminished placenta and fetal growth thus arrests. This pattern is likely due to the male's evolutionary drive to have powerful progeny, and the female's need to conserve her own bodily resources. Genetic diseases involving imprinting include Prader-Willi Syndrome and Angelman Syndrome. In these cases, the patient inherits two imprinted chromosomes (or parts) from the same parent. An entire chromosome segment is thus unexpressed.
Evidence for imprinting in the early embryo
|Biparental||Maternal and paternal genomes.||Normal development.|
|Gynogenetic||Two maternal genomes.||Small placenta causes halt of embryonic development.|
|Androgenetic||Two paternal genomes.||Embryonic growth retarded.|