Molecular Biology and Genetics: Principles, Variations, and Mechanisms

Comprehensive vocabulary flashcards covering classical Mendelian genetics, non-Mendelian extensions, chromosomal mechanics, ploidy variations, linkage mapping, population genetics, and bacterial/viral genetics.

Complete Dominance

A form of allelic interaction where the phenotype of the heterozygote is identical to that of one of the homozygotes; one functional allele is sufficient to produce the full wild-type phenotype.

Incomplete Dominance

A condition in which the phenotype of the heterozygote falls between the phenotypes of the two homozygotes and is intermediate, such that a monohybrid cross yields a $1:2:1$ phenotypic ratio.

Codominance

A pattern of inheritance in which the heterozygote simultaneously and distinctly expresses the phenotypic traits encoded by both alleles without producing an intermediate phenotype.

Gene I (ABO Locus)

The genetic locus encoding a transferase enzyme responsible for ABO blood type determination via three alleles: $I^A$ (adds acetylgalactosamine), $I^B$ (adds galactose), and $i$ (encodes a non-functional transferase).

Allelic Series

A dominance hierarchy established among multiple alternative forms (alleles) of a single gene within a population.

Null Allele

A mutant allele (amorphic loss-of-function allele) that produces a completely nonfunctional protein or results in no detectable protein product.

Hypomorphic Allele

A mutant loss-of-function allele that produces either a reduced level of gene product or a polypeptide with partial, weakened function, typically conferring a mild mutant phenotype.

Dominant-Negative Allele

A mutant allele producing an altered polypeptide that actively interferes with the function of the polypeptide produced by the normal wild-type allele.

Haplosufficiency

The condition where a single copy of a wild-type functional allele synthesizes roughly half the normal amount of protein, which is sufficient to produce a normal wild-type phenotype.

Haploinsufficiency

The state where a single functional copy of a gene in a heterozygote does not produce enough protein product to achieve a wild-type phenotype, causing the mutant loss-of-function allele to behave dominantly (e.g., Manx tailless cats).

Recessive Lethal Allele

An allele that causes lethality only when present in the homozygous state, altering the expected $3:1$ monohybrid phenotypic ratio among surviving progeny to a characteristic $2:1$ ratio (e.g., yellow coat color in mice).

Penetrance

The proportion or percentage of individual organisms possessing a specific genotype that actually express the expected phenotype.

Expressivity

The degree or severity with which a particular genotype is manifested or expressed phenotypically in an individual organism (ranging from mild to severe).

Norm of Reaction

The complete range of phenotypic variation expressed by a single genotype when exposed to different environmental conditions.

Temperature-Sensitive Allele

An allele encoding a protein whose functional activity depends on ambient temperature, such as the tyrosinase enzyme encoded by the Siamese allele ($c^s$) which is active only at cooler body extremities.

Phenocopy

A non-inherited, environmentally induced phenotype that mimics the phenotype typically produced by a specific genetic mutation (e.g., limb defects caused by thalidomide mimicking the rare genetic disorder phocomelia).

Complementation

The production of wild-type progeny when two organisms with different homozygous recessive mutations in different genes are crossed, each parent supplying the functional allele the other lacks.

Complementation Test

A genetic cross between two organisms homozygous for recessive mutations that produce the same phenotype, used to establish whether the mutations lie in the same gene or in different genes.

Epistasis

A genetic interaction where the allele of one gene at a specific locus masks or suppresses the phenotypic expression of alleles of a different gene at a distinct locus.

Recessive Epistasis

A form of gene interaction wherein homozygosity for a recessive allele at one gene locus masks the phenotypic expression of alleles at a second locus, resulting in an $F_2$ dihybrid phenotypic ratio of $9:3:4$.

Dominant Epistasis

A form of gene interaction wherein the presence of a single dominant allele at one locus completely masks the expression of alleles at a second locus, generating an $F_2$ dihybrid phenotypic ratio of $12:3:1$.

Pleiotropy

A genetic phenomenon in which a single gene influences multiple distinct and seemingly unrelated phenotypic traits or organ systems (e.g., Sickle Cell Disease and Cystic Fibrosis).

Inbreeding Depression

The reduced biological fitness, vigor, or performance observed in inbred populations resulting from increased homozygosity of deleterious recessive alleles.

Heterosis

Also known as hybrid vigor, the phenomenon where the progeny of crosses between diverse inbred lines are heterozygous across many loci and show superior performance or growth compared to either parental line.

Hardy-Weinberg Principle

A population genetics theorem stating that in a large, randomly mating population free from mutation, migration, and natural selection, allele frequencies remain constant and genotype frequencies stabilize at $p^2$, $2pq$, and $q^2$.

Dosage Compensation

The biological mechanism that equalizes the expression levels of X-linked genes between males and females despite their difference in sex chromosome copy number.

Lyon Hypothesis

The hypothesis proposed by Mary Lyon stating that early in mammalian embryonic development, one of the two X chromosomes in each somatic cell is randomly and permanently inactivated.

Barr Body

A densely staining, highly condensed heterochromatic mass representing an inactivated X chromosome found on the nuclear periphery of female mammalian somatic cells.

Genetic Mosaicism (X-Linked)

The presence of two genetically distinct cell lines within an individual female heterozygous for an X-linked trait, arising from random X-inactivation in different embryonic cell lineages (e.g., tortoiseshell or calico cats).

Z-W Sex Determination System

A chromosomal sex-determination mechanism common in birds, where females are the heterogametic sex ($76 + ZW$) and males are the homogametic sex ($76 + ZZ$).

Principle of Segregation (Mendel's First Law)

The principle stating that each individual diploid organism possesses two alleles encoding a trait, and these alleles separate in equal proportions during gamete formation in Anaphase I of meiosis.

Principle of Independent Assortment (Mendel's Second Law)

The principle stating that alleles at different loci separate independently of one another during gamete formation in Anaphase I of meiosis, producing a $9:3:3:1$ dihybrid phenotypic ratio when traits are unlinked.

Synapsis

The close pairing of duplicated homologous chromosomes along their entire lengths during Prophase I of meiosis, forming a tetrad or bivalent.

Chiasma

The physical junction and cytologically visible crossover site formed between non-sister chromatids of homologous chromosomes during Prophase I of meiosis.

Nondisjunction

The failure of homologous chromosomes to separate during Meiosis I, or of sister chromatids to separate during Meiosis II or mitosis, leading to an unequal distribution of chromosomes in daughter cells.

Aneuploidy

An abnormal chromosomal condition in which a cell or organism gains or loses one or more individual chromosomes rather than an entire chromosome set (e.g., $2n+1$ trisomy or $2n-1$ monosomy).

Autopolyploidy

A form of polyploidy where all additional chromosome sets originate from within the same single species, often caused by meiotic nondisjunction or uncoupling of mitosis.

Monoploidy

The condition of possessing only one single complete set of chromosomes ($N$), characteristic of male ants, bees, and wasps.

Allopolyploidy

A condition in which an organism possesses multiple complete sets of chromosomes derived from hybridization between two or more different, usually related, species followed by chromosome doubling.

Amphidiploid

An allotetraploid produced by the hybridization of two diploid species followed by doubling of the chromosome complement, allowing each chromosome to pair with a homologous partner during meiosis (functioning cytologically as a diploid, e.g., $2N = 52$ in Upland cotton).

Triticale

A synthetic cereal grain produced as an allopolyploid hybrid between wheat and rye, combining the high protein content of wheat with the high lysine content and soil hardiness of rye.

Triticum aestivum

Common bread wheat; an allohexaploid plant ($6N = 42$) that originated in Asia approximately $8,000$ years ago from a cross between Triticum turgidum ($4N = 28$) and Triticum tauschii ($2N = 14$).

Linked Genes

Genes located close together on the same chromosome that tend to be inherited together during meiosis and do not assort independently.

Recombination Frequency

The proportion of recombinant gametes or progeny produced in a cross, calculated as: $\frac{\text{Number of recombinants}}{\text{Total number of offspring}} \times 100\%$.

Map Unit (Centimorgan)

A unit for measuring genetic linkage distance on a chromosome, where one map unit ($\text{cM}$) is defined as equivalent to a $1\%$ recombination frequency.

Cis Configuration (Coupling)

The chromosomal arrangement in a double heterozygote where both dominant alleles are located on one homolog and both recessive alleles are located on the other homolog ($AB/ab$).

Trans Configuration (Repulsion)

The chromosomal arrangement in a double heterozygote where each homolog carries one dominant allele and one recessive allele ($Ab/aB$).

Bacteriophage T4

A complex double-stranded DNA virulent bacterial virus with a contractile tail sheath and baseplate that reproduces exclusively through the lytic cycle, causing host cell lysis.

Bacteriophage Lambda ($\lambda$)

A temperate bacteriophage capable of either executing a lytic reproductive cycle or establishing a lysogenic cycle by integrating its viral DNA into the host bacterium's genome.

Prophage

The dormant, integrated form of a temperate bacteriophage's genome residing within the circular bacterial chromosome, with its lytic genes kept repressed.

Plaque

A clear, distinct zone of lysis on a lawn of cultured bacterial cells produced by the localized infection, replication, and destruction of bacteria by bacteriophages.

Transformation (Bacterial)

A parasexual mechanism of horizontal gene transfer in which a competent bacterium directly takes up free, naked extracellular DNA from its surrounding environment, which is sensitive to DNase and requires no cell contact.

Competence (Bacterial)

The physiological state of a bacterial cell that enables it to bind exogenous DNA fragments and transport them across its cell wall and membrane into the cytoplasm.

Conjugation (Bacterial)

A parasexual process involving unidirectional transfer of genetic material from a donor bacterium to a recipient bacterium through direct physical contact via a cytoplasmic bridge or conjugation channel.

F Factor (Fertility Factor)

A specialized bacterial episome in E. coli that directs the synthesis of the F pilus and conjugation bridge, mediating horizontal DNA transfer via rolling-circle replication.

Hfr (High Frequency of Recombination) Cell

A donor bacterial cell in which the fertility (F) factor has integrated into the circular bacterial chromosome, promoting high-frequency transfer of chromosomal genes during conjugation.

Transduction (Bacterial)

A parasexual mechanism in bacteria where bacteriophages inadvertently package host bacterial DNA fragments during viral assembly and transfer those genes to a new recipient bacterial cell.

Episome

A genetic element, such as certain plasmids, that is capable of replicating autonomously in the cytoplasm or integrating directly into the host bacterial chromosome.

Replica Plating

A microbiological technique where colonies from a primary master plate are transferred onto replica plates using a sterile velvet surface to identify nutritional auxotrophs and antibiotic-resistant mutants.

Bacterial Parasexual Processes

The three non-meiotic mechanisms of horizontal gene transfer in bacteria—transformation, conjugation, and transduction—distinguished primarily by their requirement for direct cell contact and their sensitivity to DNase digestion.