Chapter 18

Chapter 18: Epigenetics, Extranuclear Inheritance, and Linkage

Definition of Epigenetics: The study of mechanisms leading to changes in gene expression that can be passed from cell to cell and are reversible, without altering the DNA sequence. An epimutation refers to a heritable alteration in gene expression not caused by a change in DNA sequence.

Epigenetic changes may be stable or reversible across generations. Some changes can last a lifetime while others may revert.

Vernalization: The process where plants require cold exposure to flower, illustrating a form of epigenetic response to environmental change.

Key mechanisms of epigenetic change include:
- DNA Methylation: Addition of methyl groups to cytosine bases, typically resulting in transcription repression when near promoters.
- Chromatin Remodeling: Movement or eviction of nucleosomes can alter transcription levels.
- Covalent Histone Modification: Chemical modifications of histones (like acetylation or phosphorylation) that can either activate or repress transcription.
- Localization of Histone Variants: Specific histone variants can impact transcription by being localized to gene promoter areas.

Environmental agents can induce epigenetic changes, which may be involved in disease adaptation and manifestation. Findings from studies, such as those involving Darwin's finches, indicate distinct epigenetic differences due to environmental conditions, influenced by DNA methylation patterns.

Genomic Imprinting Definitions:
- An imprinting process marks segments of DNA affecting gene expression throughout the individual's life. Depending on the gene, it may be marked by the mother or father (but not both).

Imprinting involves specific epigenetic modifications that differentiate alleles based on parental origin, leading to expression of either maternal or paternal alleles, not both.
Example of Genomic Imprinting: The Igf2 gene, which encodes insulin-like growth factor 2, demonstrates imprinting, as its expression depends solely on the paternal allele.

DNA methylation is critical for genomic imprinting, typically silencing the unexpressed allele through methyl group addition at specific genomic locations, influencing gene transcription.

Mechanism: In female mammals, one X chromosome in each somatic cell is randomly inactivated during embryonic development, a process leading to the formation of compacted X (Barr bodies), effectively silencing most of its genes.

Microscopic evidence includes observing Barr bodies in female but not male cats. The calico coat color in cats arises due to random inactivation of X-linked color alleles.

X-chromosome inactivation equalizes gene dosage of X-linked genes in males and females, ensuring that expressed alleles maintain balance despite differing X chromosome counts.

Studies link certain environmental factors to epigenetic changes associated with diseases such as cancer, diabetes, and neurological disorders. Toxic substances may lead to gene modification without direct causation of disease symptoms.

Definition: The transmission of genes located in organelles outside the cell nucleus (primarily mitochondria and chloroplasts).
Examples:
- Mitochondrial and Chloroplast Genomes: Typically consist of circular DNA, with specific genes related to critical biochemical processes, like oxidative phosphorylation and photosynthesis.

Genes located in chloroplasts exhibit maternal inheritance, evident in plant traits such as leaf pigmentation. In a four-o'clock plant, chloroplast type determines offspring color solely based on maternal lineage.

When genes are located on the same chromosome, they can exhibit linkage, wherein they are inherited together, violating Mendel’s law of independent assortment.
Morgan's Findings: Thomas Hunt Morgan demonstrated that genes on linked chromosomes are transmitted as linked units and that crossing over during meiosis can lead to recombination events, affecting inheritance patterns.