Genomic Imprinting

Understanding genomic imprinting

Normally, humans inherit two copies of their genes; one from the father and one from the mother. The two copies of genes are active in body cells. In other cases, only two of the copies are turned on or active. A copy is often active based on the parent of origin. There are genes that are only active when inherited from an individual’s father and others are active when inherited from the mother. This condition is known as genomic imprinting.

Genes that undergo imprinting process are marked or stamped during formation of sperm and egg cells. The process of stamping is popularly known as methylation. It is a chemical reaction that attaches to other small molecules or methyl groups and then to other DNA segments. The molecules identify the type of copy of a gene that was inherited from father and the gene inherited from the father. The removal or addition of methyl groups can further be used to control gene activities.

It is however imperative to note that only a small percentage of genes undergo the imprinting process. Researchers are yet to establish why some genes undergo the imprinting process and others do not. Over the past years, it has been established that genomic imprinted genes often cluster together in the same area of chromosomes.

The two major genes that have been established or identified in humans include long q arm of chromosome 15 and short p arm of chromosome 11.

It is also good to note that in mammals, many imprinted genes are involved in metabolism process and growth. Paternal imprinting in this case often favors production of larger offspring while maternal genomic imprinting favors smaller offspring. Often, paternally and maternally imprinted genes work through the same growth channels or pathways. This is a conflict of interest that sets up epigenetic battle between many parents.

Imprinting in some animal species

Imprinting in mammals often lead to competition between males for maternal resources. More than one male can father offspring from the same litter in some species. For instance, a house cat can mate more than once while on heat and litter kittens of more than two fathers. If the kitten of one father grows bigger than the rest, the offspring will easily survive to adulthood thus, passing the genes along.

Therefore, in mammals, it is often the interest of the father to produce larger offspring. This is because they will easily and comfortably compete for maternal resources at the expense of offspring from the other father.

Why is genomic imprinting important?

The process of genomic imprinting is important in humans because it allows for normal development. Normally, a person has one active copy of an imprinted gene. However, improper imprinting can lead to presence of two inactive or two active copies of the genes. As a result, severe developmental abnormalities occur, for example cases of cancer and other related issues.

Angelman syndrome and Prader-Willi syndrome are some of the disorders that occur due to abnormal imprinting. They are two very different disorders but closely linked to the same imprinting process around chromosome 15 region.  Some of the DNAs in the region are silenced in the eggs and one gene is silenced in the sperm. Therefore, a person who inherits a defect of the chromosome misses various active genes based on whether the chromosome came from the dad or the mum.

Beckwith-Wiedemann syndrome is also another condition that can occur as a result of poor or abnormal imprinting. It is characterized by increased risk of cancerous tumors and accelerated growth. The abnormalities often occur on the short arm of chromosome 11.

Genomic imprinted genes and the environment

With an idea of what imprinting is all about and why it is needed, it is essential to note that genes that are imprinted are very sensitive to the environment. This is because they have only one active copy and no backup. Therefore, any epigenetic changes have a significant impact on gene expression.

Similarly, environmental genes can affect the process of imprinting in itself. This is because it occurs during the formation of an egg and a sperm when epigenetic tags are included to silence specific genes. Hormones, diets and toxins can also affect the process thus affecting the gene expression in the next generation.

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Sources

http://learn.genetics.utah.edu/content/epigenetics/imprinting/

http://www.geneimprint.com/site/what-is-imprinting

http://www.nature.com/scitable/topicpage/Genetic-Imprinting-and-X-Inactivation-1066