Genetics Exam 2: Complete Vocabulary Guide

Comprehensive glossary flashcards covering key terms and concepts for Genetics Exam 2, including gene interactions, environmental effects, linkage, recombination, epigenetics, mutations, DNA repair, and chromosome architecture.

Epistasis

When one gene masks or modifies the phenotypic expression of an independent gene locus.

Suppression

A second genetic mutation hides the phenotypic flaw or counteracts the first mutation, restoring the individual to a near-normal phenotype without altering the original mutant DNA sequence.

Sex-Limited Traits

Traits whose phenotypic expression is restricted to only one sex (e.g., horns in certain species of sheep where mutations only cause a phenotype in males).

Sex-Influenced Traits

Traits that appear in both sexes, but differ in severity, expressivity, or age of onset between males and females (e.g., pattern baldness showing earlier onset and greater severity in males).

Genetic Anticipation

The occurrence of a genetic disorder at progressively earlier ages of onset and with increased severity in subsequent generations.

Genomic Imprinting

A process where one copy of a gene is epigenetically silenced so that trait expression depends strictly on whether the allele was inherited from the mother or the father.

Conditional Mutation

A mutation whose mutant phenotype is expressed only under specific environmental or physical conditions.

Temperature-Sensitive Mutation

A conditional mutation where the mutant defect is expressed only at a restrictive temperature (e.g., coat color in Himalayan rabbits or yeast cell-cycle mutants).

Nutritional Effect

A mutant phenotype dependent on diet, typically involving mutations in enzymes required to synthesize an essential compound.

Acquired Characteristics

The disproven belief that phenotypic traits acquired or modified by the environment during an organism's life can be inherited by offspring.

Linkage

The tendency for genes located on the same physical chromosome to segregate together during meiosis.

Crossover

The physical exchange of corresponding chromosome segments between homologous chromatids during prophase I / metaphase I.

Recombination

Crossing-over events that generate new, nonparental combinations of alleles.

Cis vs. Trans Configuration

Describes dihybrid arrangement for linked genes; in cis (coupling), both mutant alleles reside on the same homologous chromosome; in trans (repulsion), mutant alleles reside on opposite homologous chromosomes.

Parental Combination

Allelic combinations in progeny that match those present on the original parental chromosomes.

Nonparental Combination

New combinations of alleles in progeny produced by meiotic recombination.

Locus

The specific physical location or chromosomal address of a gene or marker.

Mitotic Nondisjunction

The failure of sister chromatids to segregate properly to opposite poles during mitotic anaphase.

Mitotic Recombination

Rare genetic exchange between chromatids in somatic cells during or after DNA replication ($\sim 100\times$ less frequent than meiotic recombination).

Twin Spot

A genetic phenomenon where two adjacent, genetically distinct cell patches showing complementary mutant phenotypes appear side-by-side in tissue.

Epigenetics

A heritable state of gene function not encoded within the primary DNA sequence, often mediated by DNA methylation or histone modifications.

Prader-Willi Syndrome

A complex genetic disorder characterized by intellectual disability, obesity, and diabetes resulting from loss of active gene function on paternal chromosome $15$.

Angelman Syndrome

A neurodevelopmental disorder caused by maternal deletion or loss of the UBE3A gene where the paternal copy is imprinted and silenced.

Position Effect Variegation

Alteration of gene expression caused by relocating a gene (via translocation, inversion, or deletion) into a different chromosomal context such as heterochromatin.

X-Chromosome Inactivation

The dosage compensation mechanism in humans where one of the two X chromosomes in female somatic cells is condensed and silenced.

Prion

An infectious, misfolded protein capable of altering protein conformation and mediating heritable states.

Forward Mutation

A mutation that alters a wild-type allele into a mutant allele.

Reverse Mutation (Reversion)

A second mutation that restores a mutant allele back to the wild-type state.

Transition

A base substitution replacing a purine with another purine ($\text{A} \leftrightarrow \text{G}$) or a pyrimidine with another pyrimidine ($\text{C} \leftrightarrow \text{T}$).

Transversion

A base substitution replacing a purine with a pyrimidine or vice versa.

Base-Pair Substitution

The replacement of a single base pair in DNA with another.

Silent (Synonymous) Mutation

A nucleotide substitution that alters a codon without changing the encoded amino acid.

Conservative Mutation

A missense mutation replacing an amino acid with a biochemically similar amino acid.

Missense Mutation

A base change that alters a codon to specify a different amino acid.

Nonsense Mutation

A mutation converting an amino acid codon into a premature stop codon.

Frameshift Mutation

Nucleotide insertions or deletions (not in multiples of $3$) that shift the translational reading frame.

Somatic Mutation

A mutation occurring in body cells; produces a clone of mutant cells but is not transmitted to offspring.

Germ-Line Mutation

A mutation occurring in reproductive germ cells that can be inherited by progeny.

Loss-of-Function Mutation

A mutation that reduces or eliminates normal gene function (usually recessive).

Null Mutation

A loss-of-function mutation resulting in a complete loss of gene function.

Gain-of-Function Mutation

A mutation conferring a novel or constitutive gene activity (usually dominant).

Dominant Negative Mutation

A mutant allele producing a protein product that interferes with the function of the wild-type protein.

Spontaneous Mutation

Mutations arising naturally from replication errors, depurination, deamination, or oxidative damage without exposure to exogenous mutagens.

Depurination

Spontaneous loss of a purine base (A or G) from DNA, leaving an apurinic site.

Deamination

Loss of an amino group from a base (e.g., cytosine deamination yields uracil, leading to $\text{C-G} \rightarrow \text{T-A}$ transitions).

Oxidative Damage

Base modifications caused by reactive oxygen species generated during aerobic respiration or radiation.

Trinucleotide Repeats

Unstable repetitive $3$-base sequences that can expand during replication, causing disorders like Fragile X or Huntington's disease.

Base Analogs

Chemical compounds structurally resembling natural bases that incorporate into DNA during replication and cause mispairing (e.g., 5-bromouracil, 2-aminopurine).

Intercalators

Flat planar molecules (e.g., acridine orange) that slide between adjacent base pairs, distorting the double helix and inducing frameshifts during replication.

Ames Test

A biological assay using Salmonella typhimurium His- strains to assess whether a chemical compound is a mutagen/carcinogen.

Photolyase

A light-dependent enzyme that directly breaks covalent bonds in UV-induced pyrimidine dimers.

Alkyltransferase

An enzyme that directly removes alkyl groups from damaged DNA bases (e.g., O6-ethylguanine).

Excision Repair

Repair pathways (nucleotide excision repair for bulky lesions; base excision repair via glycosylases) that excise damaged bases and resynthesize the DNA strand.

Mismatch Repair

An enzymatic system that recognizes and corrects base mismatches and small insertion/deletion loops on the newly replicated strand.

Error-Prone Repair (SOS Response)

An emergency bypass repair pathway that inserts random nucleotides opposite damaged DNA, often causing mutations.

Recombinational Repair

A repair mechanism using a homologous chromosome/sister chromatid as a template to repair double-strand DNA breaks.

Chromatin

The nucleoprotein complex of DNA, histones, and non-histone proteins forming eukaryotic chromosomes.

Genome

The complete set of genetic instructions in an organism or cell.

Nucleosome

The fundamental structural repeating unit of chromatin, consisting of DNA wrapped around an octamer of histone proteins (H2A, H2B, H3, H4).

Solenoid (30 nm Fiber)

A higher-order chromatin structure formed by coiling nucleosomes into a helix with $\sim 6$ nucleosomes per turn.

Chromomeres

Dark, bead-like local swellings along chromosomes visible during prophase.

Centromere

The constrained chromosomal region where kinetochores assemble and spindle fibers attach during division.

Metacentric

A chromosome with its centromere in the middle, creating equal-length arms.

Submetacentric

A chromosome with an off-center centromere, forming distinct p (short) and q (long) arms.

Acrocentric

A chromosome with its centromere located near one end.

Telocentric

A chromosome with its centromere at the very end.

Heterochromatin

Densely packed, highly condensed chromatin that is transcriptionally inactive.

Euchromatin

Less condensed, lightly staining chromatin that is transcriptionally active.

Polytene Chromosomes

Giant chromosomes produced by repeated rounds of DNA replication without cell division (endoreplication) in Drosophila larval salivary glands.

Kinetochore

A multiprotein complex assembled on centromeric DNA that binds spindle microtubules.

Holocentric Centromere

A structural organization where spindle fibers attach along the entire length of the chromosome.

Localized Centromere

A centromere where spindle attachment is restricted to a single discrete region.

Telomere

Specialized tandemly repeated DNA sequences at the terminal ends of eukaryotic chromosomes that maintain structural integrity.

Telomerase

A reverse transcriptase enzyme carrying an internal RNA guide template that extends telomeres to prevent chromosome shortening.