Genetics Study Guide: Extensions of Mendelian Genetics, Chromosome Mutations, and Chromosome Mapping

Topic 3B: Extensions of Mendelian Genetics

Epistasis and Modified Ratios

Epistasis occurs when the expression of one gene is affected by one or more other genes. This can result in modified phenotypic ratios compared to classical Mendelian inheritance.

• Epistasis: Interaction between non-allelic genes where one gene masks or modifies the expression of another.

• Examples: Coat color in Labrador retrievers, flower color in sweet peas.

• Modified Ratios: Common ratios include 9:7, 12:3:1, 9:3:4, etc., depending on the type of epistatic interaction.

Pleiotropy

Pleiotropy refers to a single gene affecting multiple, seemingly unrelated phenotypic traits.

• Pleiotropy: One gene influences several distinct traits.

• Example: The gene responsible for sickle cell anemia affects both red blood cell shape and resistance to malaria.

Sex-Linkage and Sex-Influenced Traits

Sex-linked traits are associated with genes located on sex chromosomes, while sex-influenced traits are affected by the sex of the individual but are not located on sex chromosomes.

• Sex-linked inheritance: Traits determined by genes on the X or Y chromosome. Example: Color blindness in humans, white eyes in Drosophila.

• Sex-influenced traits: Traits whose expression differs between sexes due to hormonal differences. Example: Pattern baldness in humans.

Penetrance and Expressivity

Penetrance and expressivity describe the degree to which a genotype is expressed in the phenotype.

• Penetrance: The proportion of individuals with a particular genotype that actually displays the associated phenotype.

• Expressivity: The extent or degree to which a genotype is expressed in an individual.

• Position Effect: Changes in gene expression due to a gene's location within the genome.

Environmental Effects on Phenotypes

Phenotypes can be influenced by environmental factors such as temperature, nutrition, and chemicals.

• Examples: Coat color in Himalayan rabbits (temperature-sensitive), PKU in humans (nutritional intervention).

Genetic Anticipation

Genetic anticipation refers to the phenomenon where a genetic disorder is observed to increase in severity or appear at an earlier age in successive generations.

• Example: Huntington's disease, myotonic dystrophy.

Glossary/Definitions

• Epistasis: Interaction between genes affecting phenotype.

• Pleiotropy: One gene affects multiple traits.

• Penetrance: Frequency of phenotype expression among individuals with a genotype.

• Expressivity: Degree of phenotype expression.

• Sex-linked: Gene located on a sex chromosome.

• Sex-influenced: Trait affected by sex but not located on sex chromosome.

Topic 4: Mutations—Variations in Chromosome Number & Arrangement

Chromosomal Number Abnormalities

Chromosomal mutations can involve changes in the number or structure of chromosomes, leading to genetic disorders or variation.

• Aneuploidy: Variation in chromosome number (e.g., trisomy, monosomy).

• Polyploidy: More than two sets of chromosomes (e.g., triploidy, tetraploidy).

• Autopolyploidy: Multiple chromosome sets from the same species.

• Allopolyploidy: Multiple chromosome sets from different species.

• Examples: Wheat (hexaploid), strawberries (octoploid).

Chromosomal Mutations—Structural Changes

Structural changes in chromosomes can affect gene function and lead to genetic disorders.

• Deletion: Loss of a chromosome segment.

• Duplication: Repetition of a chromosome segment.

• Inversion: Reversal of a chromosome segment.

• Translocation: Movement of a segment to a non-homologous chromosome.

• Examples: Cri-du-chat syndrome (deletion), Charcot-Marie-Tooth disease (duplication).

Fragile Sites and Repeats

Fragile sites are specific areas on chromosomes prone to breakage, often associated with genetic disorders.

• Fragile X syndrome: Caused by CGG repeat expansion on the X chromosome.

Gene Mutations

Gene mutations can be beneficial, neutral, or detrimental, and may arise spontaneously or be induced by external factors.

• Spontaneous mutations: Occur naturally due to errors in DNA replication or repair.

• Induced mutations: Caused by chemicals, radiation, or biological agents.

• Types of gene mutations: Point mutations, insertions, deletions.

• Examples: Sickle cell anemia (point mutation), cystic fibrosis (deletion).

DNA Repair Mechanisms

Cells possess multiple mechanisms to repair DNA damage and maintain genetic integrity.

• Mismatch repair (MMR): Corrects errors introduced during DNA replication.

• Base excision repair (BER): Removes damaged bases.

• Nucleotide excision repair (NER): Removes bulky DNA lesions.

• Double-strand break repair: Repairs breaks in both DNA strands via homologous recombination or non-homologous end joining.

Transposable Elements

Transposable elements are DNA sequences that can change their position within the genome, affecting gene function.

• Types: DNA transposons, retrotransposons.

• Autonomous elements: Can move independently.

• Non-autonomous elements: Require other elements for movement.

• Examples: Ac/Ds elements in maize, LINEs and SINEs in humans.

Glossary/Definitions

• Aneuploidy: Abnormal number of chromosomes.

• Polyploidy: More than two sets of chromosomes.

• Deletion: Loss of chromosome segment.

• Duplication: Repetition of chromosome segment.

• Inversion: Reversal of chromosome segment.

• Translocation: Movement of segment to another chromosome.

• Transposon: Mobile genetic element.

Topic 6A: Chromosome Mapping & Analysis (In Eukaryotes)

Linkage and Gene Mapping

Linked genes are located close together on the same chromosome and tend to be inherited together. Chromosome mapping determines the relative positions of genes.

• Linkage: Genes that do not assort independently due to proximity on the same chromosome.

• Crossing Over: Exchange of genetic material between homologous chromosomes during meiosis, leading to recombination.

• Genetic Maps: Diagrams showing the relative positions of genes based on recombination frequencies.

Mapping Techniques

Several techniques are used to map genes and estimate distances between them.

• Three-point mapping: Uses three genes to determine their order and distances.

• Recombination frequency: Used to estimate genetic distance; 1% recombination = 1 map unit (centimorgan).

• Physical mapping: Uses molecular markers such as RFLPs, microsatellites, and SNPs.

Genetic Analysis in Model Organisms

Model organisms such as Drosophila melanogaster are commonly used for genetic mapping and analysis.

• Example: Mapping eye color and wing shape genes in Drosophila.

Glossary/Definitions

• Linkage: Genes inherited together due to proximity.

• Crossing over: Exchange of genetic material during meiosis.

• Map unit (centimorgan): Unit of genetic distance.

• RFLP: Restriction Fragment Length Polymorphism.

• SNP: Single Nucleotide Polymorphism.

Table: Types of Chromosomal Mutations

Type	Description	Example
Aneuploidy	Abnormal number of chromosomes	Down syndrome (trisomy 21)
Polyploidy	More than two sets of chromosomes	Wheat (hexaploid)
Deletion	Loss of chromosome segment	Cri-du-chat syndrome
Duplication	Repetition of chromosome segment	Charcot-Marie-Tooth disease
Inversion	Reversal of chromosome segment	Hemophilia A (inversion in factor VIII gene)
Translocation	Segment moved to another chromosome	Chronic myelogenous leukemia (Philadelphia chromosome)

Key Equations

• Recombination Frequency:

$\text{Recombination Frequency} = \frac{\text{Number of recombinant offspring}}{\text{Total number of offspring}} \times 100$

• Map Distance (centimorgans):

$\text{Map Distance (cM)} = \text{Recombination Frequency (25)}$

• Gene Dosage Effect:

$\text{Gene Dosage} = \text{Number of gene copies} \times \text{Expression per copy}$

Additional info: Some definitions and examples have been expanded for clarity and completeness. The table includes common chromosomal mutations and their examples for reference.