In the study of genetics, it is commonly understood that we inherit one set of genes from each parent, and both copies are active in our cells. The parent of origin effect is an exception to this rule. This effect describes a situation where a gene’s influence on a trait is dependent on which parent it was inherited from. The expression of the gene, and the resulting physical characteristic, differs depending on its maternal or paternal origin.
This is not a case of one allele being dominant over another, but a complete silencing of one parental copy. For certain genes, only the allele from the mother or the father is expressed, making its parental source a determining factor. This concept challenges the traditional understanding that both inherited copies of a gene contribute equally to an individual’s traits.
The Mechanism of Genomic Imprinting
The primary biological process for the parent of origin effect is called genomic imprinting. This process is a form of genetic “tagging,” where specific genes are marked or silenced in the parent’s reproductive cells—the sperm or the egg. Consequently, the inherited gene from one parent is switched off, while the copy from the other parent remains active in the offspring.
The most common method for this gene silencing is DNA methylation. During the formation of sperm and egg cells, small chemical tags called methyl groups are attached to specific parts of the DNA. These methyl groups act as “do not read” signals, preventing the cellular machinery from expressing that particular gene. This epigenetic mark does not change the DNA sequence itself but controls its activity.
These imprints are established in the germline and are specific to the sex of the parent. For example, a gene might be methylated and silenced during egg formation, meaning only the paternal copy will be active. Conversely, another gene might be silenced during sperm formation, leaving only the maternal copy active. These imprints are passed on through cell division as the embryo develops.
Human Conditions Linked to Imprinting
When the process of genomic imprinting is disrupted, it can lead to specific developmental and neurological conditions. The most well-understood examples involve a particular region on chromosome 15. Errors in the imprinting of this region cause two distinct syndromes, depending on which parent’s chromosome carries the defect.
If a child inherits a faulty or deleted copy of this chromosome region from their father, they develop Prader-Willi syndrome. The paternal genes that should be active are missing, and the maternal genes in that same region are naturally silenced by imprinting. This results in characteristics such as weak muscle tone in infancy, a constant feeling of hunger, and developmental delays.
Conversely, if the defective chromosome 15 is inherited from the mother, the result is Angelman syndrome. The maternal gene that should be active is missing or mutated, while the paternal copy is normally imprinted and silent. Children with Angelman syndrome often have a happy demeanor, severe intellectual disability, movement disorders, and seizures. These two conditions illustrate the parent of origin effect, as the outcome is determined by the parental source of the genetic error. Other conditions, such as Beckwith-Wiedemann syndrome, are also linked to imprinting errors on different chromosomes.
The Evolutionary Purpose
The existence of such a complex regulatory system has led scientists to question its evolutionary origins. The leading explanation is the “parental conflict hypothesis,” also known as the kinship theory. This theory proposes that genomic imprinting is a result of conflicting genetic interests between the mother and father regarding the allocation of resources to their offspring.
From an evolutionary perspective, a father’s genetic success is maximized by producing large, strong offspring. Therefore, paternally expressed genes tend to promote growth, even if it demands significant resources from the mother. This ensures the survival and propagation of his genetic line.
The mother’s genetic interests, however, involve balancing the needs of the current pregnancy with her ability to have future offspring. Her biological imperative is to conserve resources to ensure she can survive and reproduce again. As a result, maternally expressed genes often act to restrict or moderate fetal growth. Genomic imprinting is therefore viewed as a molecular battleground where these opposing evolutionary pressures are played out.
Distinctions from Other Inheritance Patterns
The parent of origin effect is fundamentally different from standard Mendelian genetics, which involves dominant and recessive alleles. In Mendelian inheritance, the expression of a trait depends on the combination of alleles an individual has, not on which parent they came from. A dominant allele will be expressed regardless of its parental source.
This effect should not be confused with mitochondrial inheritance. Mitochondria, the energy-producing structures in our cells, contain their own DNA. This mitochondrial DNA is passed down exclusively from the mother because the egg cell contributes all the cytoplasm, including mitochondria, to the developing embryo.