Medical Genetics - V. M. Zaporozhan 2005

Glossary of Genetic Terms

FISH method (fluorescent in situ Hybridization) — see Fluorescent in situ hybridization

Autosomes — non-sex Chromosomes; the human chromosomal Complement contains 44 autosomes, or 22 pairs.

Multiple alleles — a set of alternative states of a Gene (three or more) that arise through mutation at a single chromosome locus and differ in their phenotypic expression. Each Organism can possess a maximum of 2 alleles. A series of multiple alleles can be observed when studying a population.

Alleles, allelomorphs, allelic genes — alternative variants of a single gene locus, located at the same positions (loci) of homologous chromosomes and responsible for The Development of alternative trait expressions. One gene of the pair is inherited from the mother, and the other from the father.

Normal allele, wild-type allele — an allele that ensures the normal viability and functioning of individuals.

Allelic series — monogenic hereditary disorders caused by different Mutations within the same gene, yet belonging to different nosological groups according to their clinical manifestations.

Alternative trait expressions — traits determined by allelic genes.

Aminoacidemia — an elevated concentration of one or more Amino acids in Blood serum.

Aminoaciduria — an increased excretion of one or more amino acids in the urine.

Amniotic bands — conditions characterized by constricting rings around the limbs, sometimes leading to the amputation of fingers or limbs, as well as craniofacial and other dysmorphic features.

Amniocentesis — a prenatal diagnostic method; a Procedure involving the collection of a small amount of Amniotic Fluid containing embryonic Cells from the amniotic sac.

Amniocytes — fetal cells suspended in the amniotic fluid.

DNA amplifier (thermal cycler) — an instrument used to perform the Polymerase Chain Reaction.

Gene Amplification — The process of generating multiple copies of specific Regions of the genome (individual genes).

Base analogues — Purines and Pyrimidines that differ from the standard nitrogenous bases of Nucleic Acids (e.g., 5-bromouracil, 2-aminopurine); they can be incorporated into nucleic aci

ds during Replication and lead to mutation induction via incorporation errors or replication errors.

Anaphase Separation — the segregation of chromatids or chromosomes during the anaphase of Mitosis and Meiosis.

Aneuploidy (heteroploidy) — a type of mutation involving a decrease or increase in chromosome number that is not an exact multiple of the diploid set 2n (e.g., 2n+1, 2n-1, etc.).

Anomaly — a deviation from the norm characteristic of a given biological species.

Antigens — substances that, when introduced into an organism, trigger specific immunological reactions (the synthesis of Antibodies or the differentiation of a clone of sensitized lymphocytes).

Antimutagens — factors capable of reducing the frequency of Spontaneous and Induced mutations.

Antibodies (immune bodies, IMMUNOGLOBULINS) — globulins produced by plasma cells of lymphoid tissue following the Introduction of antigens into the organism. Antibodies are specific, meaning they react exclusively with the antigen that stimulated their synthesis.

Anticipation — the intensification of disease manifestations in successive generations (earlier age of onset, more severe clinical course). It is frequently observed in disorders associated with dynamic mutations (Trinucleotide Repeat Expansion).

Apoptosis — programmed Cell death.

Assortative mating — marriages in which partners select each other based on similar phenotypic traits (such as height, Skin color, intelligence, etc.), as well as consanguineous marriages; assortative mating increases population homozygosity for various genes.

Genetic association — the occurrence of two or more traits (with at least one being heritable) together more frequently than can be explained by chance alone.

Bacteriophages — a group of Viruses capable of replicating exclusively within bacterial cells; used as vectors in Genetic Engineering.

Adapter Proteins — cytoplasmic signaling proteins and protein Kinases (synonyms: adaptor proteins or Transduction factors) that transmit Cell Division signals from Membrane Receptors to The Nucleus.

Fetal tissue biopsy — a prenatal diagnostic method involving ultrasound-guided biopsy of fetal skin or Muscle.

Twin method — a method in human and medical genetics used to determine the relative roles of HEREDITY AND ENVIRONMENT in trait Variability through comparative analysis of concordance and discordance for specific traits or diseases in monozygotic and dizygotic twins.

Consanguineous marriage — a marriage between close biological relatives (such as first cousins, or an uncle and a niece).

Blot hybridization (blotting) — a technique for identifying macromolecules separated by gel Electrophoresis and immobilized on a solid matrix (nitrocellulose or nylon filters) by hybridizing the sample contents with labeled complementary probes.

Northern blot hybridization (Northern blotting) — a method for identifying RNA transcripts among electrophoretically separated mRNA molecules via hybridization with a labeled complementary DNA probe.

Southern blot hybridization (Southern blotting) — a method for identifying DNA sequences among electrophoretically separated restriction fragments immobilized on a solid matrix (nitrocellulose or nylon filters) via hybridization with a labeled complementary DNA probe.

Vector — a vehicle used to transfer and insert foreign DNA into a host genome in genetic engineering (such as a plasmid, bacteriophage, or other DNA form).

Insertion — a mutation resulting from the incorporation of an extra DNA sequence into The Genome.

Gamete — a mature reproductive cell containing a haploid set of chromosomes.

Haploid — an organism or cell with a single (haploid — n) set of chromosomes.

Haplotype — a specific combination of alleles at linked loci on a single chromosome.

Hemizygosity — a genotype state characterized by the absence of one gene from a pair of alleles, typically observed in males for genes located on the X chromosome. This phenomenon is also typical of certain mutations (such as deletions or monosomies).

Gene — the Structural and functional unit of heredity; a segment of DNA (or RNA in some viruses) containing a nucleotide sequence that encodes the Primary Structure of a polypeptide, tRNA, or rRNA, or is required to regulate the METABOLISM/31.html">Transcription of another gene.

Dominant gene — a gene that masks the expression of its allele (the recessive gene). Denoted by an uppercase Latin letter (A). Determines a dominant trait and is expressed in both homozygous and heterozygous states.

Lethal gene — a gene that severely reduces progeny viability, potentially leading to death.

Modifier gene — a gene that enhances or suppresses the phenotypic expression of a structural gene while having no independent Phenotypic effect of its own.

Recessive gene — a gene whose expression is suppressed by a dominant allele. Denoted by a lowercase Latin letter (a). Determines a recessive trait and is expressed exclusively in the homozygous state.

Genetics — the science of the fundamental principles of heredity and variation.

Clinical genetics — a branch of medical genetics focused on the Diagnosis, Treatment, and Prevention of Hereditary Disorders.

Medical genetics — a branch of Human Genetics that investigates The Role of heredity in human pathology, inheritance patterns, Etiology, and the Pathogenesis of genetic diseases; it develops Methods for the diagnosis, treatment, and prevention of hereditary conditions, including diseases with a genetic predisposition.

Genetic heterogeneity — the phenomenon where a specific trait is caused by different mutations within the same gene (allelic heterogeneity) or by mutations in different genes (locus heterogeneity).

Genetic map (chromosome map) — see Genetic map.

Genetic Code is a specific sequence of nucleic acid NUCLEOTIDES that encodes The sequence of amino acids in corresponding polypeptide chains.

Genetic monitoring is the tracking of mutation rates and spectra.

Genetic risk is the probability of a specific hereditary disease appearing in a counselee or their descendants.

Recurrent genetic risk is the probability of having an affected child in a family that already has children with the condition.

Genetic screening is the testing of a specific population to detect a hereditary disease or its underlying gene.

Housekeeping genes are genes whose products are necessary to maintain cellular Functions across all cell types of an organism; they are transcriptionally active in all cells.

Enhancer genes — see Enhancers.

Mutator Genes are genes whose functional impairment increases the mutation rate, thereby significantly raising the likelihood of various oncogenic mutations.

Polymeric genes are non-allelic genes that exhibit identical cumulative (additive) effects.

Tumor suppressor genes (anti-oncogenes) — see Tumor Suppressors.

Regulatory genes are DNA nucleotide sequences (functional regions) that control the reading of hereditary information from structural genes (i.e., regulate structural Gene Expression).

Silencer genes — see Silencers.

Predisposition genes are mutant alleles compatible with birth and postnatal survival that, under certain adverse conditions, can contribute to the development of a multifactorial disease.

Terminal differentiation genes (luxury genes) are genes that function only in specific cell types or at a particular stage of ontogenesis.

Gene Diagnostics (genodiagnostics) — see DNA diagnostics.

Genetic engineering is a set of methods and technologies (including recombinant DNA and RNA technologies) used to isolate genes from an organism, manipulate them, and introduce them into other organisms.

Gene Therapy is a method of etiotropic treatment for Hereditary diseases by introducing a normal gene into the genome or by altering a gene using genetic engineering techniques.

Germline gene therapy is gene therapy that alters the genome of Germ Cells and implies the Inheritance of the modified genome.

Somatic cell gene therapy is gene therapy aimed at introducing changes exclusively into the human somatic cell genetic apparatus, providing a therapeutic effect only for that individual.

Genocopy is a clinical syndrome manifesting under the guise of a known hereditary disease with a specific genetic defect, but caused by a mutation in a different gene.

Genome — 1) the haploid set of chromosomes with its localized genes; 2) broadly defined, the complete set of DNA (nuclear and mitochondrial) of an organism or cell, encompassing both genes and intergenic regions.

Genomic Imprinting is a process that ensures differential gene activity depending on whether the gene was inherited from the mother or the father.

Genotype — 1) in the narrow sense, an individual's allelic constitution at a specific gene locus; 2) in the broad sense, the total complement of genes in a somatic cell of an organism.

Gene pool is the complete set of genes within a single population, characterized by specific allele frequencies.

Heterogametic sex is the sex characterized by the presence of two different sex chromosomes (in humans, the male sex with XY chromosomes).

Genetic heterogeneity — see Genetic heterogeneity.

Heterozygote — an individual whose homologous chromosomes carry different alleles of the same gene (Aa), meaning one allele is dominant and the other is recessive.

Heteroplasmy — the presence of both normal and mutant Mitochondrial DNA molecules within the Cells and Tissues of an organism — see homoplasmy.

Heterochromatin — regions of Chromatin that are in a condensed state, stain intensely with Dyes, replicate later than euchromatic regions, and are not transcribed.

Hybrid — the offspring resulting from a cross between two genetically distinct organisms.

In situ hybridization — a hybridization reaction between labeled DNA or RNA probes and denatured cellular DNA on chromosome preparations specifically prepared for subsequent microscopic observation; this technique allows for the localization of sequences complementary to the probe.

Nucleic acid hybridization — the interaction between complementary strands (segments) of DNA (or DNA and RNA) isolated from different sources, resulting in The formation of a double-stranded molecule.

Fluorescence in situ hybridization (FISH method) — a molecular cytogenetic technique based on the identification of specific chromosomal regions using fluorochrome-labeled DNA probes (a modification of in situ hybridization).

Knudson's two-hit hypothesis of homozygotization — the hypothesis stating that both alleles of a suppressor gene must be inactivated to trigger The Mechanism of carcinogenesis (in hereditary tumor forms).

Histones — structural proteins of chromosomes; they possess basic properties and form structures known as nucleosomes, around which the DNA molecule is wrapped.

Holandric traits — traits that are inherited exclusively through the male line due to the localization of their controlling genes on the Y chromosome.

Homogametic sex — the sex characterized by the presence of identical sex chromosomes (in humans, this is the female sex — XX chromosomes).

Homozygote — an individual whose homologous chromosomes carry identical alleles of the same gene (AA or aa), meaning both are either dominant or recessive.

Homologues — homologous chromosomes.

Homoplasmy — the presence of a single type of mitochondrial DNA (either normal or mutant) in the cells and tissues (see heteroplasmy).

Risk group — members of the family being consulted who may be carriers of a mutant gene and have a high risk of developing the disease or transmitting the mutant gene to their offspring.

Deletion — the loss of a chromosomal segment or a gene.

Denaturation (melting) — the separation of double-stranded DNA into two single strands upon heating or alteration of the chemical environment.

Diheterozygote — a cell or organism that is heterozygous for two pairs of genes (AaBb).

Diploid — an organism with two homologous sets of chromosomes (a diploid set — 2n) in its somatic cells.

Discordance — a situation where relatives (typically sibs or twins) exhibit different phenotypic variants of a trait (e.g., one is affected and the other is healthy). Discordance is the opposite of concordance. In twin studies, discordance refers to the percentage of twin pairs that do not share a trait.

DNA diagnostics — a set of molecular methods used to detect alterations in the genome.

DNA probe — see Probe.

Repetitive DNA — nucleotide sequences of DNA that appear in the genome in A large number of copies. These copies can be dispersed throughout the genome or organized as tandem repeats.

DNA polymerase — the enzyme responsible for the synthesis (replication) of the DNA molecule.

Recombinant DNA — a chimeric DNA molecule constructed from fragments of different origins.

Genetic Drift — A change in gene frequency within a population driven by random factors.

Duplication — the doubling of a specific chromosome region or gene.

Exons — the protein-coding regions of eukaryotic genes (DNA) that carry the Amino Acid Sequence information and are fully represented in the mRNA molecule.

Exonuclease — an enzyme that hydrolyzes phosphodiester bonds at the 3' or 5' end of a polynucleotide, cleaving nucleotides from the respective terminus.

Trinucleotide repeat expansion — see Dynamic mutation.

Expressivity — the degree of phenotypic expression of a trait controlled by a given gene.

Gene expression — the Selection/27.html">Realization of Genetic information encoded in DNA via transcription (into mRNA) and Translation.

Endonuclease — an enzyme that hydrolyzes internal phosphodiester bonds of a polynucleotide, cleaving the strand internally.

Enhancers — regulatory DNA nucleotide sequences that interact with specific transcription factors to enhance the transcription of a particular gene.

Epistasis — the suppression of the expression of one gene by another non-allelic gene (a suppressor gene).

Eukaryotes — organisms whose cells possess a well-defined nucleus; eukaryotes include all animals, plants, and Fungi.

Euchromatin — chromosomal regions that are maximally despiralized (uncondensed) during interphase, stain poorly with dyes, and contain the majority of actively transcribed genes.

Founder effect — genetic drift resulting from the founding population consisting of a very small number of individuals, leading to the dissemination of a rare gene.

Position effect — the phenomenon whereby a gene's action changes depending on its chromosomal Location, typically manifested during various chromosomal rearrangements.

Eugenics — The Study of hereditary human health and its improvement through the preferential reproduction of individuals possessing desirable traits (positive eugenics) and the prevention of reproduction among the ill (negative eugenics).

Reverse transcription — see Reverse transcription.

Probe — a small single-stranded DNA (or RNA) fragment labeled with radioisotopes or fluorophores (fluorescent dyes). It is used to screen for complementary sequences among diverse DNA or RNA molecules in molecular or molecular-cytogenetic research.

Linkage group — the set of all genes localized within a single chromosome, resulting in their linked (joint) inheritance.

Sex linkage — the Inheritance of Traits determined by genes localized on the sex chromosomes.

Idiogram — a schematic representation of a karyotype graphically depicting individual chromosomes of a set along with all their structural characteristics (centromere placement, absolute arm lengths).

Isochromosomes — chromosomes with two genetically

identical arms (either both short or both long) that arise due to aberrant centromere division; a type of chromosomal aberration.

Imprinting — see Genomic imprinting.

Inbreeding — mating between closely related individuals.

Inversion — a type of chromosomal structural rearrangement involving the rotation of a chromosome or chromatid segment by 180°.

Paracentric inversion — an inversion of a chromosomal segment that does not include the centromere.

Pericentric inversion — an inversion of a chromosomal segment that includes the centromere.

Insertion — see Insertion.

Introns — non-coding regions of a eukaryotic gene (DNA) that are excised during the maturation of mRNA.

Incestuous marriage — a prohibited marriage between close relatives of the First degree of kinship (brother-sister, father-daughter, mother-son).

Carcinogenesis — the process by which a malignant tumor develops.

Carcinogenic Factors — environmental factors (physical, chemical, biological) that induce tumor formation; most carcinogens are mutagens and cause mutations in genes responsible for carcinogenesis.

Karyotype — the diploid set of chromosomes of an organism, characterized by a specific combination of chromosomal features (number, size, shape, and structural details).

Genetic map (chromosome map) — a graphical representation of the linear arrangement of genes on a chromosome, indicating the distances between them. Gene distances are determined by the recombination frequency between them. The unit of distance

— centimorgan, which corresponds to one percent recombination.

Mapping — the Determination of the localization of genetic elements on a chromosome.

Kilobase (kb) — a unit of DNA length equal to one thousand Base Pairs.

Clinical and Genealogical analysis — the analysis of a disease inheritance pattern using a pedigree.

Cell Cycle — the life of a cell from one division to the next division or death; in continuously dividing cells, The Cell cycle coincides with the mitotic cycle.

Clone — a population of genetically identical cells produced by successive mitotic divisions of a single ancestral cell.

Codominance — the expression of both alleles of a gene simultaneously in a heterozygote.

Codon — a sequence of three adjacent nucleotides in DNA or mRNA that encodes a single amino acid or serves as a Translation termination signal.

Termination codon (stop codon, nonsense codon) — a codon that does not encode an amino acid, but rather signals the termination of Polypeptide chain synthesis during translation.

Colchicine — an alkaloid found in certain plants of the lily family (e.g., autumn crocus — Colchicum autumnale); a mitotic poison that disrupts the spindle fibers, preventing sister chromatids from segregating to opposite poles and arresting cell division at metaphase; used in karyotyping.

Compound heterozygote — a heterozygote carrying two different mutant alleles of the same gene on homologous chromosomes. Typically, compound heterozygotes for an autosomal recessive disorder manifest the disease.

Conjugation — the temporary pairing of homologous chromosomes during meiosis.

Cordocentesis — a prenatal diagnostic procedure involving the retrieval of fetal blood from the umbilical cord under ultrasound guidance via the anterior abdominal wall.

Crossing-over — the reciprocal exchange of equivalent homologous segments between paired homologous chromosomes during prophase I of meiosis.

Unequal crossing-over — crossing-over between misaligned paired homologous chromosomes, resulting in duplication and deletion.

Lyon hypothesis — a confirmed hypothesis stating that in the somatic cells of a normal female embryo, random inactivation of one of the X chromosomes occurs (lyonization).

Leader sequence — an untranslated region at the 5'-end of mRNA that precedes the Translation initiation codon.

Liposomes — lipid vesicles that can be used as vectors in somatic cell gene therapy.

Locus — the physical position of a specific gene (or its allele) on a genetic or cytological chromosome map.

Genetic markers — various types of protein and DNA polymorphisms used in population studies, association studies, and gene mapping.

Maternal inheritance — the inheritance of traits encoded by Mitochondrial Genes (a trait is inherited from the mother by all children). This should be distinguished from maternal-line inheritance in X-linked disorders.

MEDICAL Genetic Counseling — a form of specialized medical care that involves counseling families (or individuals) to identify genetically determined conditions, determine the prognosis for future offspring, and provide preventive and therapeutic measures. Based on this information, the family must make a decision regarding their reproductive behavior.

Mendelian traits — traits that are inherited according to Mendel's Laws.

Methylation — The addition of a methyl group. In Molecular Genetics, a methyl group is typically added to cytosine, leading to the formation of methylcytosine. Methylation results in a decrease in the transcriptional activity of a gene.

Microdeletion — a deletion of chromosomal material too small to be seen under a Microscope; usually detected using Molecular Cytogenetic Methods.

Microsatellite DNA — a type of satellite DNA represented by sequences up to several hundred nucleotides long, with a repeat unit size of 1-13 bp (most commonly 2-4 bp). The number of repeats in microsatellites is highly individual, allowing their analysis to be used in forensic practice for personal identification and paternity testing.

Minisatellite DNA — a type of satellite DNA that forms blocks ranging from 100 to 20,000 bp in length. The repeat unit size ranges from 14 to 500 bp. The number of repeats in minisatellites exhibits high individuality, making their analysis useful in forensic practice for personal identification and paternity testing.

Variability — a phenomenon characteristic of All living organisms, opposite to heredity, which consists in changes in hereditary determinants as well as the variability of their phenotypic expression during organism development under the Influence of Environmental interactions.

Missense mutation — a type of gene mutation that causes the substitution of a single amino acid in a polypeptide.

Mitotic cycle — the set of cellular processes from one division to the next; it includes interphase and mitosis.

Mitochondrial Diseases — in a broad sense, disorders caused by inherited metabolic defects in Mitochondria; in a strict sense, disorders caused by mutations in mitochondrial genes.

Mobile Genetic Elements — DNA sequences capable of moving around the genome.

Mosaicism — the presence in an organism's cells of two or more cell clones with different genotypes that developed from a single zygote (i.e., share the same genetic origin); a consequence of somatic mutation.

Gonadal Mosaicism — the presence in an individual of two genetically distinct clones of germ cells (normal and mutant), resulting from a mutation occurring during the Development of the Gonads.

Monogenic traits — see Monogenic traits.

Monosomic — a cell or organism lacking a single chromosome in its diploid set.

Monosomy — a type of genomic mutation (aneuploidy) characterized by the absence of one of the homologous chromosomes in the diploid set (2n-1).

Multifactorial inheritance — a type of inheritance in which traits or diseases are determined by the interaction of several independent genetic and environmental factors.

Multifactorial Diseases — see Multifactorial diseases.

Mutability — the ability of a gene to mutate spontaneously or under The Influence of endogenous or exogenous Mutagenic Factors.

Mutagenesis is the process of mutation formation.

Mutagens are a general term for chemical, physical, and biological agents capable of inducing mutations.

A mutant is an organism in which a mutation (gene, chromosomal, or genomic) has resulted in an altered trait or characteristic.

Mutations are abrupt, sudden changes in genetic material that lead to alterations in various traits of an organism.

Germline mutations are mutations that occur in sex cells or in the cells from which sex cells are derived.

Gene Mutations are mutations that alter The structure of a gene.

Genomic mutations are mutations involving a change in the chromosome number of an individual.

Induced mutations are mutations caused by the action of a mutagen.

Somatic mutations are mutations that occur in somatic cells.

Chromosomal mutations (Chromosomal aberrations) are

mutations associated with changes in Chromosome structure.

Cytoplasmic mutations are mutations that occur in cytoplasmic Organelles containing DNA (in humans, in mitochondria); they are inherited exclusively through the maternal line.

Dynamic mutation (trinucleotide repeat expansion) is an increase in the number of trinucleotide repeats located in the coding or regulatory regions of a gene, leading to the disruption of its function.

Frameshift mutation is a type of gene mutation involving insertions or deletions of nucleotides that result in the alteration of all subsequent triplets.

Major mutation is a mutation that occurs with high frequency in a specific population.

Neutral mutation is a mutation that has no phenotypic manifestation.

Regulatory mutation is a mutation in the 5'- or 3'-untranslated regions of a gene that disrupts The regulation of its expression.

Splicing mutation is a type of gene mutation affecting splice sites or leading to the creation of new splice sites within gene introns, thereby disrupting splicing.

Spontaneous mutation is a mutation whose occurrence is not associated with the action of exogenous factors.

Point mutation is a mutation affecting one or a few nucleotides.

Chromosome nondisjunction is the failure of chromosomes to separate properly during mitosis or meiosis, leading to a genomic mutation.

Nonsense codon — see Termination codon.

Nonsense mutation is a type of gene mutation that leads to the formation of a stop codon within a gene and premature termination of translation.

A carrier is an individual who carries one copy of a mutant gene without developing the disease. The term is typically used to denote heterozygotes for recessive disease genes or female heterozygotes for X-linked recessive disease genes.

Nucleases are a Class of Enzymes that cleave internucleotide bonds in DNA or RNA.

Nullisomy is a type of genomic mutation (aneuploidy) characterized by the absence of a pair of homologous chromosomes (2n-2) in a diploid set.

Single nucleotide polymorphism (SNP) is a variation at a single position in a DNA sequence among individuals.

Trait is a morphological or physiological characteristic whose development depends on a specific gene and environmental influences.

Monogenic traits are traits encoded in an organism by a pair of allelic genes.

Sex-limited traits are genetically determined traits that phenotypically manifest in individuals of only one sex.

Polygenic traits are traits encoded in an organism (cell) by many genes, each having a minor individual effect on the degree of trait expression.

Oligonucleotide is a short single-stranded DNA or RNA molecule.

Oncogenes are cellular or viral genes (introduced into a cell by a virus) that cause the transformation of normal Eukaryotic cells into malignant ones.

Palindrome is a DNA sequence that reads the same in both the 5' to 3' and 3' to 5' directions.

Panmixia is the random mating of individuals of opposite sexes with different genotypes within a population.

Penetrance is the frequency of phenotypic expression of an allele of a specific gene in a population of individuals carrying it. It is expressed as the percentage of individuals in whom a given trait is observed relative to all carriers of the gene determining the trait.

Primary transcript is an mRNA molecule after the completion of transcription and prior to maturation (Processing).

Chromosome crossing — see Crossing-over.

Plasmids are very short, extrachromosomal, circular DNA molecules in Bacteria that contain one or more genes. They replicate autonomously, independently of the rest of the genetic material, and are frequently transferred from one cell to another.

Placentocentesis is a method of prenatal diagnosis involving the retrieval of placental tissue.

Pleiotropy is the phenomenon where a single gene influences multiple traits.

Generation is a group of individuals within a population that are at the same degree of genealogical distance from common ancestors.

Polygenic inheritance is the inheritance of traits encoded by a large number of genes with additive effects.

Polygenic traits — see Polygenic traits.

Polymerase chain reaction (PCR) is a method for the in vitro cyclic synthesis of a large number of copies of a specific DNA region ranging from tens to several thousand base pairs in length; used in DNA diagnostics.

Polymorphism is the presence of two or more alleles of the same gene (or genetic markers) in a population, with a frequency of the rarer allele of 1% or greater.

Restriction fragment length polymorphism (RFLP) is a variation in DNA sequence that can be detected using restriction enzymes.

Polyploidy is a genomic mutation resulting in an increase in the number of chromosomes by multiples of the haploid set (3n — triploidy, 4n — tetraploidy).

Primers are short single-stranded DNA molecules complementary to the 3'-ends of the sense and antisense strands of DNA fragments. They are used in the polymerase chain reaction for the amplification of a specific DNA fragment.

Proband is an affected individual or carrier of a pathological trait; in genealogical studies, it is the person who seeks genetic counseling and from whom the pedigree chart is started.

Promoter — a DNA nucleotide sequence at the 5'-end of a gene to which the RNA polymerase enzyme binds to initiate the transcription of a structural gene.

Protein kinases — enzymes that phosphorylate specific proteins, thereby altering their functions.

Proto-oncogenes — non-mutant forms of an oncogene; normal cellular genes that normally promote cell mitosis. Mutations that enhance their function convert proto-oncogenes into oncogenes.

RNA Processing — the maturation of mRNA in eukaryotes; a series of reactions in which the primary RNA transcript undergoes various modifications (splicing, capping, polyadenylation) to become a mature mRNA molecule.

Tumor suppressors (antioncogenes) — genes that inhibit cell proliferation. The inactivation of these genes increases the likelihood of neoplasm formation, whereas the restoration of their function, conversely, inhibits tumor cell division.

Reading frame — a nucleotide sequence consisting of a series of consecutive triplets that code for proteins, starting from the start codon and ending with the stop codon.

Reduction — the halving of the chromosome number during meiosis.

Recombination — the redistribution of Genetic information in offspring resulting from the formation of new gene combinations during meiosis and Fertilization.

Renaturation — the process by which DNA strands rejoin based on complementary base pairing, restoring the double-stranded DNA Structure (used in molecular genetic diagnostics).

DNA Repair — the restoration of damage that occurs in the DNA structure As a result of mutagenic factors, replication errors, etc.

Replication (reduplication) — the synthesis of daughter DNA strands on an original template DNA molecule (self-duplication of DNA).

Restriction enzymes (Restriction Endonucleases) — bacterial enzymes that cleave double-stranded DNA at specific nucleotide sequences (restriction sites); used in genetic engineering.

DNA Restriction analysis — a method for analyzing DNA using restriction endonucleases.

Restriction — the Cleavage of DNA by a restriction endonuclease.

Retrovirus — an RNA-containing virus that during its life cycle passes through a double-stranded DNA stage that integrates into the genome of the infected cell. Used as a vector in gene therapy.

RNA polymerase — a DNA-dependent RNA polymerase enzyme that catalyzes the synthesis of RNA on a DNA template.

Mobile genetic elements — see Transposable elements.

Silencers — regulatory DNA nucleotide sequences that interact with specific transcription factors to repress or downregulate The activity of a specific gene.

Silent mutation — a type of gene mutation that does not result in a change in amino acids due to the redundancy of The Genetic Code.

Site — a specific location (position) within a DNA molecule.

Restriction site — a specific 4-12 nucleotide sequence recognized and cleaved by a restriction enzyme; the site of interaction between a restriction enzyme and DNA.

Splice site — a specific nucleotide sequence at the exon-intron boundary that serves an important signaling function for accurate splicing.

Centimorgan (cM) — a unit of measurement for genetic distance between loci (genes) on the same chromosome; 1 cM corresponds to a 1% recombination frequency between loci (genes).

Satellite DNA — a type of repetitive DNA in the genome; simple DNA sequence repeats arranged tandemly, forming blocks spanning several million base pairs or more. It is found predominantly in the centromeric regions of chromosomes.

Southern blotting — see Southern blot hybridization.

Chromosome segment — light or dark bands on a chromosome distinguishable under specific staining techniques.

Segregation — the separation among offspring of parental chromosomes, specific chromosomal segments, or alleles of particular genes (markers), as well as the segregation of certain phenotypic traits in progeny.

Sequencing — determination of The nucleotide sequence in a DNA molecule.

Sibs (siblings) — offspring of the same set of parents, brothers and sisters.

DNA sequence — the sequence of DNA nucleotides along a chromosome.

Syndrome — a complex of characteristic pathological signs that are causally related; in medical genetics, often used as a synonym for "disease".

Syndromological Analysis — the identification of a clinical symptom complex from the entire set of pathological signs that allows for the diagnosis of a specific genetic syndrome; frequently used in diagnosing hereditary multiple congenital malformation syndromes.

Chromosome instability syndrome — disorders characterized by a high frequency of chromosomal breaks or exchanges involving chromosome or chromatid segments.

Somatic cells — all Cells of the body, excluding germline cells. Human somatic cells are diploid.

Hereditary information — information regarding the structure of all organismal proteins and RNAs, along with the order of Implementation of this information during ontogeny.

Heredity — the property of organisms to ensure material and functional continuity between generations; the capacity of parents to transmit their traits and developmental features to offspring; the ability of living organisms to reproduce similar traits and properties across successive generations.

Splicing — the process of excising introns followed by the end-to-end ligation of RNA molecule fragments (exons).

Alternative Splicing — the generation of several mature mRNAs from a primary mRNA transcript through The Use of different exons (and occasionally introns); as a result, multiple proteins are produced under the control of a single gene.

Sporadic case — a single instance of a hereditary disease within the family being examined.

Stop codon — see Termination codon.

Suppressors (inhibitors) — genes that, in a homozygous or heterozygous state, suppress the action of non-allelic genes.

Satellite — a chromosomal segment located distal to the secondary constriction.

Tandem repeats — DNA sequences consisting of a large number of copies of identical segments arranged consecutively one after another.

Telomere — the terminal regions of a chromosome possessing specific DNA STRUCTURE AND FUNCTIONS.

Telomerase — the enzyme responsible for the synthesis of DNA at the telomeric regions of a chromosome.

Teratogenic factors — any factors whose action on the embryo induces Congenital Malformations.

Teratology — the science that studies developmental anomalies in animals and humans.

Terminator — a region marking the termination of transcription.

Tetraploid — an organism whose cells contain four haploid sets of chromosomes — 4n.

Barr bodies (sex chromatin body) — an inactivated, condensed X chromosome visible in somatic cell nuclei as a dark, densely stained mass; normally, females have one Barr body, whereas males have none.

Transgenic organisms (models) — experimental organisms whose genomes include foreign genetic material artificially introduced using Genetic engineering METHODS.

Transcription factors — see Transcription factors.

Transcription — the synthesis of RNA on a DNA template.

Reverse transcription — the synthesis of DNA on an RNA template using a special enzyme, RNA-dependent DNA polymerase (Reverse Transcriptase, retranscriptase).

Robertsonian translocation (or centric fusion) — occurs between any two acrocentric chromosomes from group D (pairs 13, 14, and 15) and group G (pairs 21 and 22); during centric fusion, two homologous or non-homologous chromosomes lose their short arms and one centromere, while their long arms fuse. Instead of two chromosomes, a single chromosome is formed containing the genetic material of the long arms of both chromosomes.

Translocation — The transfer of a chromosomal segment to a non-homologous chromosome or another linkage group.

Non-reciprocal translocation — the movement of a chromosomal segment either within the same chromosome or to another chromosome without reciprocal exchange.

Reciprocal translocation — the mutual exchange of segments between two non-homologous chromosomes.

Transposition — the movement of small DNA fragments from one site of a chromosome to another.

Transposons — mobile genetic elements; DNA segments capable of intra- and interchromosomal movement via a cut-and-paste mechanism.

Triplet — see Codon.

Triploid — an organism that has three haploid sets of chromosomes in its cells — 3n.

Trisomic — an individual whose diploid set contains an extra chromosome of any pair (2n+1).

Trisomy — a type of genomic mutation (aneuploidy) in which the diploid chromosome set contains an extra chromosome of any pair (2n+1).

Uniparental diploidy — the inheritance of a diploid chromosome set from only one parent.

Uniparental disomy — the inheritance of two homologous chromosomes from only one parent.

Uniparental isodisomy — the inheritance of two chromatids of the same chromosome from only one parent.

Inheritance — the transmission of parental traits to offspring.

Growth factors — proteins secreted by certain cells that affect other cells by stimulating or inhibiting their division.

Transcription factors — proteins that facilitate the binding of RNA polymerase to a promoter.

Pharmacogenetics — a branch of genetics that studies the GENETIC BASIS OF an organism's response to drugs (including adverse reactions).

Phenocopy — a clinical syndrome that manifests under the guise of an inherited disease but is caused by non-genetic factors.

Phenotype — the totality of all external and internal traits of an organism determined by the individual's genotype and realized under specific environmental conditions.

Fertile — an organism capable of producing offspring.

Fetoscopy — a method of prenatal diagnostics involving visual inspection of the fetus using endoscopic equipment.

Multifactorial diseases form a group of disorders caused by the interaction of numerous genetic and environmental factors acting with an additive effect.

Inherited diseases are disorders associated with alterations in the hereditary material, namely mutations (gene, chromosomal, and genomic).

X-linked genes are genes localized on the X chromosome.

Chimerism is the presence within an organism of two or more cell clones with different genotypes derived from separate zygotes, meaning they have distinct genetic origins.

Chromatin is the material that makes up chromosomes, consisting of a complex combination of proteins (histones and non-histone proteins) and DNA.

Ring chromosome is a chromosome shaped like a closed ring. It occurs when two telomeric fragments are lost, and the 'sticky' ends of the chromosome fuse to form a circle.

Marker chromosome is an abnormal, additional chromosome (specifically, a fragment of a chromosome or joined fragments of multiple chromosomes containing a centromere) arising from a chromosomal aberration; it typically appears as a very short acrocentric chromosome, and less commonly in other forms, such as metacentric or ring-shaped.

Acrocentric chromosomes are chromosomes whose centromeres are positioned very close to one of their ends.

Dicentric chromosomes are chromosomes containing two centromeres; they arise as a result of structural rearrangements.

Chromosomes with a satellite, or SAT-chromosomes, are chromosomes featuring a secondary constriction that separates a small spherical thickening (the satellite) from the main body of the chromosome; these secondary constrictions are associated with nucleolus formation and contain the nucleolar organizer.

Metacentric chromosomes are chromosomes with the centromere located approximately in the middle.

Submetacentric chromosomes are chromosomes with unequal arms.

Telocentric chromosomes are rod-shaped chromosomes with the centromere located at one end; they do not normally occur in the Human Karyotype.

Chromosomal map — see Genetic map.

Centric fusion — see Robertsonian translocation.

Centric fission is a phenomenon opposite to centric fusion, where a single chromosome divides into two, requiring the formation of a new centromere.

Centromere is the specialized region of A eukaryotic chromosome to which the spindle fibers attach during cell division.

Cyclin-dependent kinases (Cdks) are enzymes that form complexes with cyclins at specific Stages of the cell cycle; cyclin-Cdk complexes regulate the mitotic cell cycle.

Cyclins are specific regulatory proteins involved in controlling the cell cycle.

Cytogenetics is a branch of genetics that studies cellular structures carrying hereditary information, primarily chromosomes.