The Basics:

* Genes are the instructions for building and maintaining an organism. They are encoded in DNA, a molecule that is passed down from parent to offspring.

* Mutations are changes in the DNA sequence. Mutations can occur spontaneously or be caused by environmental factors. These changes can alter the instructions encoded in a gene.

Why Genes Generally Don't Change:

* DNA is very stable: It's designed to resist changes over time.

* Cellular repair mechanisms: Cells have systems to correct errors in DNA replication.

* Germline vs. Somatic Cells:

* Germline cells are the cells that give rise to eggs and sperm. Mutations in germline cells can be passed down to offspring.

* Somatic cells are all other cells in the body. Mutations in somatic cells are not inherited.

Exceptions to the Rule:

* Somatic mutations: While mutations in somatic cells are not inherited, they can accumulate over time and contribute to aging and disease. For example, mutations in skin cells can lead to cancer.

* Epigenetic modifications: These are changes in gene expression that don't alter the underlying DNA sequence. They can be influenced by factors like diet, stress, and environment, and can sometimes be inherited.

* Viral Integration: Certain viruses can insert their genetic material into the host's genome, permanently altering the host's DNA.

In Summary:

While the underlying DNA sequence of genes typically remains stable throughout an organism's lifetime, there are some exceptions that can alter the expression and function of genes. These exceptions, such as somatic mutations and epigenetic modifications, contribute to the diversity of life and can play a role in disease development.