Epigenetics, Extranuclear Inheritance, and Linkage

The role of epigenetics in determining physiological outcomes.
Example: Female honeybee larvae fed royal jelly develop into queen bees, while those not fed become worker bees.
- Queens are larger, live longer, and can produce up to 2,000 eggs/year.
- Workers are sterile, shorter-lived, and have specialized tasks.
- Diet induces significant developmental changes through epigenetic modifications rather than allelic differences.

Explanation of Mendelian inheritance:
- Unaltered gene transmission across generations (excluding rare mutations).
- Laws:
1. Law of Segregation: Formation of gametes where alleles segregate.
2. Law of Independent Assortment: Alleles segregate independently when multiple genes are involved.
Intricacies of Non-Mendelian inheritance include:
- Epigenetics: Contradicts rule 1.
- Organellar inheritance: Contradicts rule 2.
- Linkage: Genes do not assort independently if located on the same chromosome, contradicting rule 3.

Epigenetics: The study of changes in gene expression that do not involve alterations in DNA sequence but can be inherited. Changes can be reversible.
Epimutation: Heritable changes in gene expression without changes to sequence.
Epigenetic inheritance: Transfer of epigenetic changes from parents to offspring via gametes, not all changes are inherited.
Mechanisms include repression or activation of gene transcription.

Key forms of molecular changes include:
- DNA Methylation: Attachment of methyl groups to DNA, typically repressing transcription.
- Chromatin Remodeling: Movement of nucleosomes, altering the level of transcription.
- Covalent Histone Modifications: Changes to histone amino terminals, affecting transcription.
- Localization of Histone Variants: Specific histones affecting transcription instruction.

Epigenetic changes influenced by environmental factors:
- Study (2017) indicated adaptation of species, like Darwin’s finches, to urban vs. rural areas through DNA methylation variations.

Genomic Imprinting: Specific DNA segments imprinted affecting gene expression; distinct depending on maternal or paternal origin (e.g. Igf-2 gene).
- Does not conform to Mendelian patterns; offspring express either maternal or paternal allele exclusively.
Example: Igf2 Gene (insulin-like growth factor 2)
- Functional expression required; homozygous recessive results in dwarfism.
- Paternal allele is expressed while the maternal allele remains silent, illustrated through crosses.

DNA methylation serves as the marking process during imprinting, silencing most genes within its influence.
The pattern of methylation established post-fertilization dictates gene transcription behavior in offspring.

In female mammals, one of the two X chromosomes is inactivated (becomes Barr body), which affects gene expression.
Evidence includes visual patterns in calico cats:
- Heterozygous females express varied colors due to random inactivation of X chromosomes in different cells.
Dosage compensation ensures equal expression of X-linked genes between genders; excess X chromosomes are converted to Barr bodies.

Various chemicals in diets and environmental factors can lead to epigenetic alterations linked to diseases:
- Diseases include cancer, diabetes, and cardiovascular conditions.
- Some evidence shows that certain toxins directly induce epigenetic changes leading to conditions like lung cancer.
Table 18.3 lists environmental agents associated with various cancers and their effects on gene expression.

Genes can reside outside of nuclear chromosomes in organelles like mitochondria and chloroplasts, highlighting the concept of extranuclear inheritance.
Mitochondrial and chloroplast genomes are critical for cellular functions, each containing essential genes.
Example: Chloroplast inheritance in plants displays maternal inheritance through the ovule, influencing traits such as leaf pigmentation.

Genes located near one another on the same chromosome are often inherited together, showing that independent assortment applies only to genes on different chromosomes.
Bateson and Punnett's sweet pea experiments demonstrated deviations from expected ratios, validating this theory.
Thomas Hunt Morgan extended these ideas through fruit fly cross-examining gene linkage, observing parental and recombinant types in offspring.

The intricate pathways of epigenetic mechanisms and gene inheritance inform modern genetics, offering profound insights into how traits are expressed, inherited, and potentially altered by environmental factors.