Comprehensive Genetics: Cellular Reproduction to Molecular Genetics

Essential vocabulary flashcards covering classical Mendelian genetics, meiotic and mitotic processes, chromosomal abnormalities, pedigree analysis, linkage mapping, sex determination, and molecular genetic principles.

Germ cell

A reproductive cell capable, when mature, of being fertilized and reproducing an entire organism.

Somatic cell

A cell that serves as a non-reproductive structural or functional component of the body.

Gamete

A mature male or female reproductive cell that contains a haploid set of chromosomes.

Synaptonemal complex

A tripartite protein structure consisting of lateral elements, a central element, and transverse fibers that mediates the synapsis of homologous chromosomes during prophase I of meiosis.

Pachynema

The sub-stage of prophase I in meiosis where homologous chromosomes are fully condensed and paired into bivalents (tetrads) and crossing over occurs.

Diplonema

The sub-stage of prophase I in meiosis during which homologous chromosomes begin to repel and separate slightly, remaining in contact at chiasmata.

Terminalization

The movement of chiasmata away from the centromeres and toward the telomeres of paired chromosomes during metaphase I of meiosis.

Principle of Segregation

Mendel's law stating that in a heterozygote, the two members of a gene pair (alleles) separate from each other during gamete formation, each gamete receiving one allele.

Principle of Independent Assortment

Mendel's law stating that alleles of different genes assort independently of each other into gametes, provided the genes reside on different chromosomes.

Testcross

A cross between an individual of unknown genotype showing a dominant phenotype and an individual homozygous recessive for the trait(s) of interest.

Multiplicative (product) rule

A probability principle stating that if events $A$ and $B$ are independent, the probability that they will occur together is the product of their individual probabilities: $P(A \text{ and } B) = P(A) \times P(B)$.

Additive (sum) rule

A probability principle stating that the probability that either of two mutually exclusive events occurs is the sum of their individual probabilities: $P(A \text{ or } B) = P(A) + P(B)$.

Chi-square ($\chi^2$) test

A statistical test used to evaluate whether observed genetic data deviate significantly from expected Mendelian ratios, calculated as $\chi^2 = \sum \frac{(\text{Observed} - \text{Expected})^2}{\text{Expected}}$.

Incomplete dominance

A condition in which the phenotype of a heterozygote is midway between the phenotypes of the two homozygotes because one allele is partially dominant over the other.

Codominance

A mode of inheritance in which a heterozygote simultaneously expresses the full phenotypes of both homozygotes, as seen with the $L^M$ and $L^N$ alleles of the MN blood group.

Amorphic allele (null allele)

A mutant loss-of-function allele that produces no functional polypeptide, resulting in a severe mutant phenotype when homozygous.

Hypomorphic allele

A recessive mutant allele that produces a partially functional polypeptide, resulting in a milder mutant phenotype compared to an amorphic allele.

Dominant-negative mutation

A dominant mutation in which the mutant allele produces an abnormal polypeptide that interferes with or antagonizes the function of the wild-type polypeptide.

Penetrance

The percentage of individuals carrying a specific genotype who actually exhibit the associated phenotype.

Expressivity

The degree or severity with which a particular genotype is manifested at the phenotypic level among individuals exhibiting the trait.

Epistasis

A form of gene interaction wherein an allele of one gene overrides or masks the phenotypic expression of alleles at a different gene locus.

Pleiotropy

The phenomenon where a single gene or mutation influences multiple, apparently unrelated phenotypic characteristics.

Inbreeding coefficient

A statistic measuring the probability that two gene copies in an individual are identical by descent from a common ancestor.

Heterosis

The phenomenon, also called hybrid vigour, in which the progeny resulting from a cross between two different inbred lines display superior physiological vigor and heterozygosity.

Linkage

The tendency of two or more genes located close together on the same chromosome to be inherited together during meiosis.

Coupling (cis conformation)

The linkage phase in a double heterozygote where two dominant alleles reside on one homologue and two recessive mutant alleles reside on the other homologue ($AB/ab$).

Repulsion (trans conformation)

The linkage phase in a double heterozygote where each homologue carries one dominant allele and one recessive mutant allele ($Ab/aB$).

Recombinant frequency

The proportion of testcross progeny exhibiting nonparental combinations of alleles, calculated as $\frac{\text{Number of recombinants}}{\text{Total number of testcross progeny}} \times 100$.

Map unit (centiMorgan, cM)

A unit of measure for genetic distance along a chromosome equal to a crossover frequency of $1\%$ between two gene loci.

Coefficient of coincidence ($c$)

The ratio of the observed frequency of double crossovers to the expected frequency of double crossovers, calculated as $c = \frac{\text{Observed DCO}}{\text{Expected DCO}}$.

Chromosome interference ($I$)

The measure of the degree to which a crossover in one chromosomal region inhibits additional crossovers in an adjacent region, defined as $I = 1 - c$.

Deletion mapping

A cytogenetic mapping method that localizes a gene to a specific chromosomal segment by observing whether a recessive mutation is unmasked when paired with a chromosome deficiency.

Proband

The affected individual through whom a family pedigree is initially brought to the attention of geneticists or clinicians.

Heteroplasmy

The presence of more than one organellar genome type (e.g., wild-type and mutant mitochondrial DNA) within a single cell or individual.

Paracentric inversion

A structural chromosomal inversion in which the segment that breaks, rotates $180^\circ$, and reinserts excludes the centromere.

Pericentric inversion

A structural chromosomal rearrangement in which an excised segment containing the centromere is inverted $180^\circ$ and reinserted into the chromosome.

Reciprocal translocation

A structural chromosomal rearrangement in which segments from two nonhomologous chromosomes break and are reciprocally exchanged without a net loss of genetic material.

Robertsonian translocation

A chromosomal rearrangement resulting from the fusion of the long arms of two acrocentric chromosomes at their centromeres, with the concomitant loss of the tiny short-arm heterochromatic fragments.

Attached-X chromosome

A compound chromosome formed by the stable fusion of two homologous X chromosomes in Drosophila that is inherited as a single maternal unit.

Cri-du-chat syndrome

A human clinical condition caused by an interstitial or terminal deletion in the short arm of chromosome 5 ($46, 5\text{p}-$), characterized by mental deficits, facial and laryngeal abnormalities, and a cat-like cry.

Autopolyploid

A polyploid organism whose multiple complete chromosome sets are derived from the duplication of genomes within a single species.

Allopolyploid

A polyploid organism produced through interspecific hybridization followed by chromosome doubling, creating a genome containing distinct sets from two or more ancestral species.

Endomitosis

A process involving chromosome replication and sister-chromatid separation without nuclear or cell division, producing polyploid tissues.

Polytene chromosome

A giant chromosome consisting of hundreds of sister chromatids produced by repeated endoreplication without separation, aligned side by side in a visible banding pattern.

Nondisjunction

An error during meiosis or mitosis in which homologous chromosomes or sister chromatids fail to disjoin properly, generating aneuploid gametes or daughter cells.

Turner syndrome

The only viable monosomy condition in humans ($45, \text{X}$ or $45, \text{XO}$), characterized by female phenotype, short stature, webbing of the neck, and absence of secondary sexual characteristics.

Klinefelter syndrome

A sex chromosome aneuploidy ($47, \text{XXY}$) resulting in subfertile males with underdeveloped testes, tall stature, long limbs, and variable breast development.

SRY gene

A gene located on the human Y chromosome that encodes testis-determining factor (TDF), which induces the medulla of embryonic indifferent gonads to differentiate into testes.

Testicular feminization (androgen insensitivity syndrome)

An X-linked condition in XY individuals caused by a mutation in the $Tfm$ gene encoding the testosterone receptor, preventing responsiveness to androgens and leading to female external secondary sexual characteristics.

Genic balance system (Drosophila)

The mechanism of sex determination in Drosophila where sex is dictated by the ratio of X chromosomes to sets of autosomes ($X:A$ ratio), where $X:A = 1.0$ specifies female and $X:A = 0.5$ specifies male.

Hemizygous

The genetic condition describing genes present in only a single copy in an otherwise diploid organism, such as X-linked genes in XY males.

Barr body

A densely staining, highly condensed heterochromatic mass in the somatic cell nuclei of female mammals, representing an inactivated X chromosome.

Sex-influenced trait

An autosomal genetic trait whose expression or dominance relationship varies depending on the sex of the organism, such as pattern baldness in humans.

Sex-limited trait

An inherited autosomal trait that is expressed in only one sex due to anatomical or physiological limitations, such as milk production in female mammals.

Chargaff's rules

The biochemical observation that double-stranded DNA contains equimolar quantities of adenine and thymine ($A = T$) and of guanine and cytosine ($G = C$), yielding a purine-to-pyrimidine ratio of $\frac{A + G}{T + C} = 1.00$.

Central Dogma of Molecular Biology

The core framework governing the directional transfer of genetic information from DNA to messenger RNA via transcription, and from RNA to functional protein via translation.

Colinearity

The direct point-by-point linear correspondence between the sequential nucleotide triplets of a gene's coding region and the resulting amino acid sequence of its encoded polypeptide.

Wobble hypothesis

Francis Crick's proposal that structural flexibility at the $5'$ base of a tRNA anticodon allows non-standard base pairing with the $3'$ base of an mRNA codon, explaining codon degeneracy.

Quaternary structure of protein

The highest level of protein organization, formed by the noncovalent association and assembly of multiple polypeptide subunits into a single multi-subunit functional complex.