Epigenetic inheritance describes the ability of different states, which may have different phenotypic consequences to be inherited without any change in the sequence of the DNA (Figure 10.3). Epigenetic mechanisms can be divided into two classes.
- Covalent modification of the DNA by the attachment of a moiety and this is perpetuated. Two alleles with the same sequence may have different states of methylation and hence have different properties.
- A self-perpetuating protein state may be established involving the assembly of protein complex, modification of specific proteins and establishment of an alternative conformation.
Figure 10.3 (a) Types of epigenetic information (b) Inheritance of DNA methylation in somatic cells
Methylation establishes the epigenetic inheritance as long as the maintenance methyltransferase acts constitutively to restore the methylated state after each cycle of replication. In mammalian cells, epigenetic effects are first erased in primordial germ cells and then created latter by resetting the state of methylation differently in male and female meioses during gametogenesis.
The Epigenetic Inheritance of X Chromosomes
Variation in the number of X chromosomes in mammals poses a problem for gene regulation. If X-linked genes are expressed equally in each sex, females will have twice as much of the X genetic content as males. However, this does not happen and is prevented by dosage compensation, which equalizes the level of expression of X-linked genes in both male and female sexes. In mammals, one of the X chromosomes is inactivated completely. This inactivation of X chromosome is mediated by a gene called ‘Xic—X chromosome inactivation centre’. Inactivation spreads from Xic along the entire X chromosome. As the result, females have only one active X chromosome just like the males. This active X chromosomes of females and the single X chromosome of males are expressed equally. The inactive X chromosome is perpetuated in a heterochromatic state whereas the active X chromosome is euchromatic. Once the inactive state is established, it is inherited by descendant cells. This is an example of epigenetic inheritance as it is not dependant on the DNA sequence.