Chp. 16 - Exam 3 Bio 190

Chapter 16: Transmission of Genetic Information from Parents to Offspring II

• Covers epigenetics, linkage, and extranuclear inheritance.

16.1 Overview of Epigenetics

• Epigenetics studies changes in gene expression that can be passed from cell to cell.
• These changes are usually reversible and do not alter the DNA sequence.
• Learning outcomes include defining epigenetics and epigenetic inheritance and outlining molecular changes that affect gene expression.
• Honeybee example: Queen and worker bees differ due to epigenetic modifications related to differences in diet, not allelic differences.
• Some genes follow Mendelian inheritance, while others follow non-Mendelian patterns.
• Mechanisms identified: DNA methylation, chromatin remodeling, covalent histone modification, localization of histone variants.

16.1 Overview of Epigenetics (continued)

• Experiment: Determining how royal jelly affects worker and queen bees explores epigenetic differences in DNA methylation.

16.2 Epigenetics: Genomic Imprinting

• Learning outcomes include predicting the outcome of crosses for imprinted genes and explaining the molecular basis of genomic imprinting.
• Genomic imprinting involves marking a segment of DNA that remains throughout the organism's life. Marking can occur during egg formation or sperm production but not both.
• Some human genes are imprinted.
• Imprinted genes do not follow the Mendelian inheritance pattern.
• Process involves epigenetic modifications, influences gene expression and the offspring expresses either a maternal or paternal allele, but not both.
• DNA methylation is the marking process involved in imprinting, for most genes, it silences gene expression, but sometimes it may enhance gene expression.
• Somatic cell divisions maintain the methylation state, and it may be altered when creating gametes. Enzymes catalyze additions or removals of methyl groups.

16.2 Epigenetics: Genomic Imprinting (continued)

• Example of imprinted gene: Igf2, which encodes insulin-like growth factor 2, a growth hormone.
• Mutations in Igf2 can produce different phenotypes in offspring.

16.3 Epigenetics: X-Chromosome Inactivation

• Learning outcomes include explaining how X-chromosome inactivation affects phenotypes of female mammals and describing the process at the cellular level.
• Female mammals have two X chromosomes, while males have one X and a Y chromosome.
• During embryonic development in female mammals, one X chromosome is inactivated, which is called X-chromosome inactivation. This inactivation occurs in each somatic cell and is called XCI.
• The inactivated X chromosome is highly compacted and forms a Barr body, which silences the genes carried by the inactivated chromosome.
• Calico cats are an example of X-chromosome inactivation influencing coat color. Coat color is an X-linked gene. Orange (XO), black (XB) and heterozygous XOXB females are calico.
• XCI is maintained during subsequent cell divisions.
• Female mammals are mosaics due to the presence of two cell types. XCI achieves dosage compensation, equalizing X-linked gene expression in males and females. Cells in humans and other mammals can "count" X chromosomes and allow only one X to be active. Extra X chromosomes form Barr bodies.

16.3 Epigenetics: X-Chromosome Inactivation (continued)

• A short region of the X chromosome called X inactivation center (Xic) plays a critical role in XCI. This leads to gene expression of Xist (X-inactive specific transcript).
• Xist RNA coats one of the two X chromosomes, and promotes chromosome compaction into a Barr body.
• Role of a short region of the X chromosome (Xic) and Xist gene's product (RNA molecule) in influencing X-chromosome inactivation.

16.4 Epigenetics: Effects of Environmental Agents

• Learning outcomes include explaining how dietary chemicals affect phenotype and listing examples of chemicals that cause epigenetic changes, contributing to cancer.
• Environmental chemicals in the diet can promote epigenetic changes.
• Studies of the Agouti gene in mice demonstrate how diet-related chemicals can affect phenotype. Agouti gene encodes a protein that controls yellow pigment deposition in hairs.
• Variations in the expression of Agouti has been identified in mice as a gain of function mutation due to the insertion of a new promoter.
• Overexpression of Agouti causes mice to be yellow, but there is variation in the phenotype of mice carrying the Avy allele.
• Nutrients that inhibit DNA methylation can affect coat color in the Avy mice
• Environmental factors (chemicals in diet) can lead to epigenetic changes contributing to cancer
• Researchers have identified many diseases associated with epigenetic changes: Alzheimer's disease, diabetes, multiple sclerosis, asthma, cardiovascular disease.

16.5 Extranuclear Inheritance: Organelle Genomes

• Learning outcomes include describing mitochondrial and chloroplast genome features, explaining maternal inheritance of chloroplast and mitochondrial genes, and listing human diseases associated with mitochondrial gene mutations.
• Genes outside the nucleus (extranuclear) are transmitted via mitochondria and chloroplasts, this is called extranuclear inheritance.
• Mitochondria and chloroplasts contain their own genome.
• Endosymbiosis theory describes the origin of these semi-autonomous organelles.
• Mitochondrial genome in mammals has 37 genes, 24 encode tRNAs and rRNAs needed for translation, and 13 encode proteins for oxidative phosphorylation. Chloroplast genomes in flowering plants have 100-200 genes that encode proteins involved in photosynthesis.
• Mitochondrial and chloroplast inheritance patterns are usually maternal in most organisms.

16.5 Extranuclear Inheritance: Organelle Genomes (continued)

• Leaf pigmentation in 4-o'clock plants shows maternal inheritance (only maternal parent's pigmentation determines the offspring's pigmentation).

16.5 Extranuclear Inheritance: Organelle Genomes (continued)

• In seed-bearing plants, the maternal inheritance of chloroplasts is common, while some species exhibit biparental (both egg and pollen contribute) or paternal (pollen only contributes) inheritance.

16.6 Linkage of Genes on the Same Chromosome

• Learning outcomes include describing how linkage violates the Law of Independent Assortment and explaining how experimental crosses demonstrate linkage relationships.
• Two genes located closely on the same chromosome tend to be transmitted as a unit (linkage) and are not independent of each other.
• Linkage violates the Law of Independent Assortment.
• Experimental crosses can demonstrate linkage through recombination frequencies.
• P generation: two flies (one grey, straight wings and one black, curved wings)
• F1 generation (offspring of P generation): all gray, straight wings (homozygous dominant)
• Testcross: crossing F1 generation with recessive traits and observing offspring phenotypes to determine linkage.