Genetics as a subject matter has a lot of terms which we often come across as we view genetically related articles. These terms are however confusing and even complicate one’s ability to comprehend a given passage if one is not conversant with them. This post tends to solve this issue by outlining the meaning of these commonly used terms in genetics as follows:

Allele One of the different forms of a gene or DNA sequence that can exist at a single locus.

Aneuploid Not having the “correct” chromosome composition. An individual with an abnormal complement of chromosomes resulting from the absence of a chromosome(s) or the presence of an additional chromosome(s).

Annealing Formation of double-stranded nucleic acid from single stranded forms.

Apoptosis Programmed cell death (PCD); a process in which cellular DNA is degraded and the nucleus condensed; then cell is then devoured by neighboring cells or phagocytes.

Autosome Any chromosome other than the sex chromosomes or the mitochondrial chromosome.

Blastocysts In mammals, the embryo at the 16-cell stage of development through the 64-cell satge when the embryo implants.

Cancer genes Mutant alleles of naormal genes that lead to cancer

Carcinogen Physical or chemical agent which induces cancer.

Carrier In human genetics, an individual heterozygous for a mutant allele that generally causes disease only in the homozygous state. More generally, an individual who possesses a mutant allele but does not express it in the phenotype because of a dominant allelic partner; thus, an individual of genotype Aa is a carrier of a if there is complete dominance of A over a.

cDNA A duplex DNA where one strand is identical in sequence (except for T in place of U) and one is complementary to a particular RNA.

cDNA libraries Libraries which store sequences copied into DNA from RNA transcripts; typically these sequences carry only the exon information for making proteins.

Centimorgan (cM) A unit of measure of ecombination frequency. One cM is equal to 1% chance that a marker at one genetic locus will be separated from a marker at a second locus due to crossing-over in a single generation.

Chiasmata Observable regions in which nonsister chromatids of homologous chromosomes cross-over each other.

Chi square (c2) test A statistical test to determine the probability that an observed deviation from the expected event or outcome occurs solely by chance.

cis-acting locus Locus that affects the activity only of DNA sequences on the same molecule of DNA; usually implies that the locus does not code for protein.

cis configuration Two sites on the same molecule of DNA.

Clone A group of cells or molecules that are identical by having arisen from a single ancestral cell or molecule.

Chromosomes Self-replicating structures of cells that carry in their nucleotide sequences the linear array of genes.

Complementarity The chemical affinity between specific nitrogenous bases as a result of their hydrogen bonding properties. The property of two nucleic acid chains having base sequences such that an antiparallel duplex can form where the adenines and thymines (or uracils) are apposed to each other, and the guanines and cytosines are apposed to each other.

Complementation The production of a wild-type phenotype when two different mutations are combined in a diploid or a heterokaryon.

Chromatid One of the two side-by-side replicas produced by chromosome duplication.
Codon A triplet of nucleotides that represents an amino acid or a termination (STOP) signal.

Cross The deliberate mating of selected parents based on particular genetic traits desired in the offspring.

Cross-over During meiosis, the breaking of one maternal chromosome, resulting in the exchange of corresponding sections of DNA, and the rejoining of the chromosome. The process can result in the exchange of alleles between chromosomes. Compare recombination.

Cytoplasmic inheritance Inheritance via genes found in cytoplasmic organelles.

Degenerate code A genetic code in which some amino acids may be encoded by more than one codon each.

The separation of the two strands of a DNA double helix, or the severe disruption of the structure of any complex molecule without breaking the major bonds of its chains.

Domain of a protein A discrete continuous part of the amino acid sequence that can be equated with a particular function.

Dominance The expression of a trait in the heterozygous condition.
Downstream Sequences proceeding farther in the direction of transcription, for example, the coding region is downstream of the promoter.

Endonuclease An enzyme that cleaves the phosphodiester bond within a nucleotide chain.
Enzyme A protein that functions as a catalyst.

Eukaryotes Organisms (ranging from yeast to humans) which have nucleated cells.

Euploid Having the “correct” chromosome composition. Cells containing only complete sets of chromosomes.

Exon Any segment of an interrupted gene that is represented in the mature RNA product. The protein-coding sequences of a gene.

Exonuclease An enzyme that cleaves nucleotides one at a time from an end of a polynucleotide chain.

Familial trait Any trait that is more common in relatives of an affected individual than in the general population; could be due to genetic and/or environmental causes.

Frameshift mutations Mutations that arise by deletions or insertions that are not a multiple of 3 bp; they change the frame in which triplets are translated into protein.

Gene The fundamental physical and functional unit of heredity, which carries information from one generation to the next; a segment of DNA, composed of a transcribed region and a regulatory sequence that makes transcription possible.

Genetic code The set of correspondences between nucleotide pair triplets in DNA and amino acids in protein.

Genetic heterogeneity A similar phenotype being caused by different mutations. Most commonly used for a similar phenotype being caused by mutations in different genes. Allelic heterogeneity refers to different mutations in the same gene.

Genetic markers Alleles of genes, or DNA polymorphisms, used as experimental probes to keep track of an individual, a tissue, a cell, a nucleus, a chromosome, or a gene.

Genome The total genetic material of an organism, i.e. an organism’s complete set of DNA sequences.

Genotype The actual alleles present in an individual..

Germ cells Specialized cells that form the reproductive organ where they ultimately undergo meiosis, thereby producing gametes that contain half the number of chromosomes as there body cells. Germ cells are responsible for transmitting genes to the next generation of an organism.

Haploid A single set of chromosomes present in the egg and sperm cells of animals, in the egg and pollen cells of plants, and in stable or transient life cycle forms of some other organisms such as yeast.

Haplotype A set of closely linked alleles (genes or DNA polymorphisms) inherited as a unit. A contraction of the phrase “haploid genotype.”

Heterozygous Having two different alleles at a given locus on a pair of homologous chromosomes.

Heterogeneous trait see Genetic Heterogeneity

Homologous chromosomes Chromosomes that pair with each other at meiosis.

Homozygote An individual possessing a pair of identical alleles at a given locus on a pair of homologous chromosomes.

Housekeeping gene Gene that is expressed in virtually all cells since it is fundamental to the any cell’s functions.

Hydrogen bond A weak bond involving the sharing of an electron with a hydrogen atom; hydrogen bonds are important in the specificity of base pairing in nucleic acids and in the determination of protein shape.

Hybridization The process of joining two complementary strands of DNA or one each of DNA and RNA to from a double-stranded molecule.

Introns The DNA base sequences interrupting the protein-coding sequences of a gene. These sequences are transcribed into RNA but are cut out of the message before it is translated into protein.

Inbred line A group of identical pure-breeding diploid or polyploid organisms, distinguished from other individuals of the same species by some unique phenotype or genotype, that are maintained by interbreeding.

Karyotype The entire chromosome complement of an individual or cell, as seen during mitotic metaphase.

Kilobase pair or kilobase (kb) 1000 base pairs of DNA or 1000 bases of RNA.

Lawn Bacteria immobilized in a nutrient agar, used as a field to test for the presence of viral particles.

Leader sequence The sequence at the 5′ end of an mRNA that is not translated into protein.

Library A set of cloned fragments together representing the entire genome, created then placed into storage.

Ligase DNA ligase; an enzyme that can rejoin a broken phosphodiester bond in a nucleic acid; requires a 5′ phosphate and a 3′ OH.

Linkage The proximity of two or more markers on a chromosome; the closer together the markers are, the lower the probability that they will be separated by recombination, thereby increasing the probability that specific alleles will be inherited together

Linkage group A group of genes chained together by linkage relationsships.

Locus A specific location on a chromosome.

Lod score The logarithm of the ratio of the odds that two loci are linked with a recombination fraction equal to or greater than 0 and less than 0.5 to the likelihood of independent assortment. Also called “Z.”

Marker same as Genetic marker

Melting Denaturation of DNA.

Missense mutation A single DNA base change which leads to a codon specifying a different amino acid.

Model organisms Creatures used in genomic analysis because they have many genetic mechanisms in common with each other and with humans. These organisms lend themselves well to classical breeding experiments and direct manipulation of the genome.

Morphogens Substances that define different cell fates in a concentration-dependent manner

mRNA (messenger RNA) An RNA molecule, transcribed from a gene, from which a protein is translated by the action of ribosomes.

Mutagen Any agent that is capable of increasing the mutation rate.

Mutant allele An allele differing from the allele found in the standard, or wild type.

Nonsense codon (also called STOP codon) Any one of three triplets (UAG, UAA, UGA) that cause termination of protein sysnthesis.

Nonsense mutation (also called STOP mutation) Any change in DNA that causes a (termination) codon to replace a codon representing an amino acid.
Nonsense suppressor (also called STOP suppressor) A gene coding for a mutant tRNA able to respond to one or more of the termination codons.

Northern blotting Procedure to transfer RNA from an agarose gel to a nylon membrane.

Null allele An allele that makes no gene product or whose product has no activity of any kind; a deletion of a gene is necessarily a null allele.

Null hypothesis The prediction that an observed difference is due to chance alone and not due to a systematic cause; this hypothesis is tested by statisical analysis, and accepted or rejected.

Oligonucleotides Small single-stranded segments of DNA typically 20-30 nucleotide bases in size which are synthesized in vitro.

Oncogene An allele of a normal gene, called a proto-oncogene, that causes a cell to become cancerous.

Open reading frame Streteches of codons in the same reading frame uninterrupted by STOP codons.

Pedigree An orderly diagram of a family’s relevant genetic features, extending back to at least both sets of grandparents and preferably through as many generations as possible.

Penetrance The proportion of individuals with a specific genotype who manifest that genotype at the phenotype level.

Phage Short for bacteriophage; a virus for which the natural host is a bacterial cell. Literally “bacteria eaters.”

Phenotype Observable characterisics of an organism.

Plaque In bacterial virus analysis, a clear area of a petri dish, devoid of bacterial cells, indicating the presence of viral particles.

Plasmid Cytoplasmic, autonomously replicating extrachromosomal DNA molecule.

Point mutation A change in a single base pair.

Polarity An overall direction.

Polymorphic site A chromosome site with two or more identifiable alleleic DNA sequences. Also called a polymorphic locus.

Positional cloning The process where researchers obtain the clone of a gene without prior knowledge of its protein product or function; it uses large-scale physical and formal genetic linkage maps to find specific genes.

Primer Short, pre-existing oligonucleotide or polynucleotide cahin to which new DNA can be added by DNA polymerase.

Prokaryote An organism without a nucleus; eubacteria, archaebacteria, and blue-green algae.

Promoter A region of DNA involved in binding of RNA polymerase to initiate transcription.

Proto-oncogene A gene that can mutate to an allele, an oncogene, that causes a cell to become cancerous.

Reading frame A sequence of sense codons such that each suceeding triplet generates the correct order of amino acids resulting in a polypeptide chain.

Recessive allele An allele whose phenotypic effect is not expressed in a heterozygote.

Recombinant DNA molecules A combinatin of DNA molecules of different origin that are joined experimentally.

Recombination The formation of a new combination of alleles through independent assortment or crossing-over.

Renaturation The reassociation of denatured complementary single strands of a DNA double helix.

Restriction enzymes Proteins that recognize specific, short nucleotide sequences in DNA and catalyze cutting at those sites.

Silent mutation Mutation in which the function of the protein product of the gene is unaltered.

Somatic cells All the cells of an organism except those of the germ line.

STOP see Nonsense.

Suppression Changes that eliminate the effects of a mutation without reversing the original change in DNA.

Suppressor mutation A mutation that counteracts the effects of another mutation. A suppressor maps at a different site than the mutation it counteracts, either within the same gene or at a more distant locus. Different suppressors act in different ways.

Temperature-sensitive mutation A class of conditional mutations; the mutant phenotype is observed in one temperature range and the wild-type phenotype is observed in another temperature range.

Template strand The strand of the DNA double helix that is copied by base pair complementarity to make an RNA. The other, non-template strand of the DNA duplex has a sequence that is identical to the synthesized RNA (except in RNA, U replaces T).

Trait Any detectable phenotypic variation of a particular inherited character.

Transcription unit The distance between sites of initiation and termination by RNA polymerase; may include more than one gene.

Trans configuration The configuration of two sites refers to their presence on two different molecules of DNA (chromosomes).

Transfection of eukaryotic cells The acquisition of new genetic markers by incorporation of added DNA.

Transformation of bacteria or yeast The acquisition of new genetic markers by incorporation of added DNA.

Transformation of eukaryotic cells Their conversion to a state of unrestrained growth in culture, resembling or identical with the tumorigenic condition; usually applied to mammalian cells.

Transgenic organism One whose genome has been modified by externally applied new DNA; a term applied to metazoans.

Vector In cloning, the plasmid, phage, or yeast chromosomal sequences used to propagate a cloned DNA segment.

Western blotting A technique in which proteins are separated by gel electrophoresis and transferred to a nylon sheet. A specific protein is then identified through its reaction with a labeled antibody.

Wild type The genotype or phenotype that is found most commonly in nature or in the standard laboratory stock for a given organism.

X-ray crystallography A technique for deducing molecular structure by aiming a beam of X rays at a crystal of the test compound and measuring the scatter of rays.


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