Monoallelic gene expression refers to the allele-specific expression of genes. Only one allele of the gene is actively transcribed. It can occur in different ways such as genomic imprinting, random choice of one allele, and X-inactivation. Other allele expressions are balanced and unbalanced expressions. Balanced expressions have both alleles equally expressed whereas unbalanced ones do not express each allele equally. Goldilocks principle states that everything should fit in the certain margin; neither too much nor too less. When the amount of gene product changes, it no longer follows the “just right” principle of Goldilocks.
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Barr Body

Image: “The interphase inactive X in normal and mutant cells: histone modification and macroH2A1 association. Photomicrograph examples of normal, ICF, and Rett fibroblasts that were FITC-labeled using antisera to various modified histones. Arrows point to sex chromatin on DAPI-stained cells, and to the corresponding sex chromatin site in the FITC-labeled photo. A. Normal, ICF, and Rett fibroblasts FITC-labeled using antisera to acetylated histone H4 (acH4), acetylated histone H3 (acH3), and dimethylated K4 histone H3 (meK4H3).” by Stanley M Gartler, Kartik R Varadarajan, Ping Luo, Theresa K Canfield, Jeff Traynor, Uta Francke and R Scott Hansen – BMC Biology 2004, 2:21 doi:10.1186/1741-7007-2-21. License: CC BY 2.0


Monoallelic Expression

Monoallelic gene expression is the type of gene expression in which one copy of the gene is active, and the other copy is silent. Once it is initiated early in the development of an organism, the monoallelic expression is stably maintained after that. The more prevalent form of gene expression is biallelic expression. It is the transcription of both alleles of a gene.

Unbalanced expression

Each allele is not expressed equally. Allelic imbalance is exhibited in 5-20 % of the autosomal genes.

Balanced expression

Equal amount of each allele is expressed.

The monoallelic expression can occur in a number of ways.

  • Genomic imprinting
  • Random choice of one allele
  • X-inactivation

Genomic Imprinting

  • Different epigenetic marks are placed in the male and female germ line during gametogenesis.
  • Genomic imprinting is a type of monoallelic expression that is determined by these epigenetic marks. Thus the copies of imprinted genes of the fertilized egg that came from the paternal and maternal contributions have different marks.
  • It is only from the paternal allele that some imprinted genes are expressed, while others are expressed only from the maternal allele.
  • Same active allele is present in all cells in which a given gene is imprinted. This active allele is determined by the parent of origin of the allele. Genomic imprinting is also known as parent of origin imprinting.
  • All imprinted genes encode a protein. Different proteins have different and a wide variety of functions within the cells.
  • The difference between genomic imprinting and random forms of monoallelic expression is that in genomic imprinting the choice is nonrandom for the allele which is to be expressed and is determined totally by the parental origin.
  • During the process of imprinting, there is an introduction of epigenetic marks at specific locations in the genome. This occurs in the germline of one parent and results in monoallelic expression of one or multiple genes that are present within the imprinted region.
  • More than 100 genes which have a function in development are affected by genomic imprinting.
  • Imprinting marks certain genes to have come from the mother or father. This happens during gametogenesis and before fertilization.
  • Imprinting is maintained throughout development and adulthood.

Random Choice of One Allele (Random Monoallelic Expression)

In this type, differential epigenetic regulation of two alleles results in monoallelic expression. This leads to unique cell identity for each cell. Some of the genes that are subjected to random monoallelic expression include:

  • Pheromone receptor genes
  • Interleukin genes
  • Genes encoding receptors on natural killer cells

Allelic silencing: In this process, only one allele of a gene is expressed while the other allele is silenced.

Somatic rearrangement: In this type, there is a change in DNA organization to produce functional gene at one allele, but not at the other. Genes affected by this type of random monoallelic expression are:

  • T-cell receptor genes
  • Immunoglobulin genes

Developmental origin of somatic rearrangement is B and T cell lineage.

X-inactivation

  • It is the epigenetic silencing of one or more alleles in the imprinted region.
  • There are cis-regulatory elements and non-coding RNAs interactions in a region called the X-inactivation center.
  • Most X-linked genes in females are affected by X-inactivation.
  • Developmental origin of X-inactivation is early embryogenesis.
  • There is a random choice of one X chromosome, however, sometimes there is a skewing of X inactivation.

Some of the epigenetic marks that are associated with the X-inactivation are as follows:

Barr Body

Image: “The interphase inactive X chromosome: macroH2A1 association. Photomicrograph example of normal fibroblast that was FITC-labeled using antisera to histone macroH2A1. Arrow points to sex chromatin in DAPI-stained cell nucleus, and to the corresponding sex chromatin site in the FITC-labeled photo.” by Stanley M Gartler, Kartik R Varadarajan, Ping Luo, Theresa K Canfield, Jeff Traynor, Uta Francke and R Scott Hansen – BMC Biology 2004, 2:21 doi:10.1186/1741-7007-2-21. License: CC BY 2.0

  • Late replication
  • DNA methylation at CpG islands
  • Hypoacetylation of histones
  • Unusual histone subunit deposition
  • Dosage compensation

Barr body: a heterochromatic mass seen during interphase is known as barr body.

Dosage compensation

Dosage compensation is a term that describes the processes by which organisms equalize the expression of genes between members of different biological sexes. It results in random epigenetic silencing of one X chromosome.

Often changes in the level of gene expression go unnoticed, however, sometimes even a small change can have severe clinical consequences.

Goldilocks Principle

Goldilocks principle states that something must fall within certain margins, as opposed to reaching extremes. It has to be just right, neither too much, nor too less. Many genetic principles involve changes in the amount of gene product. When the amount of gene product changes, it no longer follows the “just right” principle of Goldilocks.

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