## **Core Concept**
The concept being tested here is related to genetics, specifically the phenomenon where the expression of a gene depends on its parental origin. This is a key aspect of epigenetics and genomic imprinting.

## **Why the Correct Answer is Right**
The correct answer, **Genomic Imprinting**, refers to the phenomenon where the expression of a gene depends on its parental origin. This means that if the gene is inherited from the mother, it may be expressed differently than if it were inherited from the father. This occurs due to epigenetic modifications, such as DNA methylation or histone modification, that are established in the germ cells (sperm or egg) and are resistant to reprogramming after fertilization. A classic example of genomic imprinting is the IGF2 and H19 genes, where IGF2 is paternally expressed and H19 is maternally expressed.

## **Why Each Wrong Option is Incorrect**
- **Option A: X-chromosome inactivation** is a process by which one of the two copies of the X chromosome in female mammals is inactivated. This is not directly related to the differential expression of genes based on parental origin but rather a mechanism to achieve dosage compensation between males (XY) and females (XX).
- **Option B: Epigenesis** is a broader term that refers to heritable changes in gene function that occur without a change in the underlying DNA sequence. While epigenesis encompasses the mechanisms involved in genomic imprinting, it is not the specific term used to describe the differential expression of genes based on parental origin.
- **Option C: Mosaicism** refers to the presence of two or more populations of cells with different genotypes in one individual. This can result from mutations occurring after fertilization or from the presence of cells from different zygotes. Mosaicism does not specifically relate to the differential expression of genes based on their parental origin.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl related to genomic imprinting is the understanding of **Prader-Willi Syndrome (PWS)** and **Angelman Syndrome (AS)**, both of which are associated with genomic imprinting. PWS and AS are caused by the loss of function of genes on chromosome 15 inherited from the other parent. For PWS, it is the loss of function of the paternal copy, and for AS, it is the loss of function of the maternal copy. This highlights the importance of parental origin in gene expression and disease.

## **Correct Answer:** . Genomic Imprinting