JC Genetics Chapter 5

Lethal Effect Timing
- Lethal alleles may prevent cell division.
- Can cause early death in organisms.
- Example: Huntington disease
  - Progressive degeneration of the nervous system.
  - Symptoms include dementia and early death.
  - Onset typically between ages 30 and 50.

Definition: These alleles may kill an organism only under certain environmental conditions.
- Example: Temperature-sensitive lethal alleles
  - Drosophila larvae can be killed at 30°C but survive at 22°C (permissive temperature).
  - Result of mutations altering protein structure at nonpermissive temperatures.

Definition: Semi-lethal alleles kill some individuals within a population but not all.
- Environmental factors and other genes may mitigate the detrimental effects of semi-lethal alleles.

Mendelian Inheritance Rules:
1. Trait expression influenced by gene expression in the offspring.
2. Genes passed unaltered except in rare mutations.
3. Adhere to Mendel’s law of segregation.
4. Independent assortment during multi-gene crosses.
Deviation from Mendelian Patterns:
- Maternal Effect: Offspring phenotype is determined by the mother's genotype rather than their own.
- Epigenetic Inheritance: Changes in gene expression due to modifications (e.g., methylation) during gametogenesis or early embryonic development.
- Extranuclear Inheritance: Involves genes located outside the nucleus (e.g., mitochondrial or chloroplast genes).
- Linkage: Genes located close to each other on the same chromosome may not assort independently.

Definition: Inheritance pattern where female parent's genotype directly determines offspring phenotype.
- Male parent's genotype does not influence this phenotype, due to maternal gene products provided to developing eggs.

Example: A. E. Boycott discovered maternal effect genes while studying Limnaea peregra, observing right-handed (dextral) and left-handed (sinistral) shell morphologies.
Genetic Insights:
- Dextral (dominant) and sinistral (recessive) shell coiling determined by female genotype.

Maternal Gene Product Transfer: mRNA and proteins from nurse cells (containing D and d alleles) are transferred to the egg, affecting early cleavage patterns.
Cleavage Patterns:
- D allele leads to dextral cleavage; d allele produces sinistral cleavage.

Importance: Maternal effect genes encode products crucial for early embryonic development, influencing cell division and organismal structure.

Overview: Refers to modifications to nuclear genes or chromosomes affecting gene expression but are reversible and do not change the DNA sequence.
Dosage Compensation: Mechanisms that balance the differences in gene expression from sex chromosomes across genders, differently adapted in various species.

Examples:
- In placental mammals, one X chromosome in females is inactivated (Barr body).
- In Drosophila, gene expression on the X chromosome in males is doubled.
- Investigated across species including marsupials and nematodes.

Bird sex chromosomes (Z and W) display differing mechanisms of compensation, not universally compensating all genes.
In mammals, Barr body formation confirmed patterns of X chromosome inactivation first proposed by Mary Lyon.

Definition: A DNA segment marked and expressed differently depending on its parental origin; observed in monoallelic expression.
Examples: Igf2 gene - paternal allele expressed while maternal allele is silenced. Critical for normal development size.

Establishment: Imprints formed during gametogenesis.
Maintenance: Patterns preserved throughout embryonic and somatic cell development.
Erasure: Imprints are reset during germ-line formation before subsequent generations.

Prader-Willi Syndrome (PWS): Lacks paternal gene expression, leading to reduced motor function and obesity.
Angelman Syndrome (AS): Absence of maternal gene expression results in hyperactivity and cognitive impairment.

Overview: Refers to inheritance patterns involving genetic material found outside the nucleus, primarily in mitochondria and chloroplasts (cytoplasmic inheritance).
Genetic Composition: Each organelle contains its own circular DNA, differing in size and composition among species.
Transmission Mechanisms: Example: Maternal inheritance of mitochondria where sperm mitochondria are often destroyed or not transmitted.

Function: Mitochondrial DNA encodes essential proteins for oxidative phosphorylation and energy synthesis.
Human Mitochondrial Diseases: Chronic disorders stemming from mutations, often affecting energy-demanding tissues.

Technology: Emergence in mitochondrial diseases led to techniques ensuring healthy offspring via genetic contributions from three individuals.

Concept: Proposes mitochondrial and chloroplast origins from early eukaryotic cells incorporating bacteria.
Evidence and Implications: Supports evolutionary relationships by demonstrating genome similarities to bacteria.