Genetic Terminology

Genetic Terminology

Where applicable, the definitions have been phrased in language relevant to blood banking.

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Allele: An alternative form of a gene that occurs at the same locus on homologous chromosomes, e.g., A, B, and O genes are alleles.
Amorph: A silent gene that does not produce a detectable product (antigen), e.g., O genes in the ABO BGS.
Aneuploidy: Having an abnormal number of chromosomes, i.e., not an exact multiple of the haploid number. For example, Downs syndrome (three #21 chromosomes) or Klinefelter syndrome (XXY males).
Anticodon: A sequence of three bases in tRNA that is complementary to a codon in mRNA. Enables tRNA to sequence amino acids in the order specified by mRNA.
Antithetical: Alternative forms of the same antigen produced by allelic genes, e.g., K and k antigens in the Kell BGS or C and c antigens in the Rh BGS.
Autosome: A non-sex chromosome. Synonymous with somatic chromosomes (chromosome pairs 1-22).
Balanced polymorphism: An equilibrium of two or more alleles that has remained constant over long periods of time.
Barr body: The sex chromatin, the visible inactive X chromosome on the somatic cell nuclear membrane.
Beneficial gene: A gene that confirms a trait that is advantageous to survival and that increases in frequency, e.g., the Fy gene that produces the Fy(a-b-) phenotype which makes West Africans resistant to certain types of malarial parasites.
Chimera: An extremely rare person composed of cells derived from different zygotes. Blood group chimerism is shown by mixed field agglutination when antigen typing red cells. Chimerism can be caused by dizygotic twins exchanging hematopoietic stem cells in utero and continuing to form blood cells that are genetically different, or by dispermic chimerism in which two separate zygotes develop into one person.
Chromosome: Rod-shaped structures within the cell nucleus that carry genes encoded by DNA.
Cis position: Genes in the cis position are on the same chromosome of a pair of homologous chromosomes. Mainly relates to the Rh BGS, e.g., in the genotype CDe/cde, D and C genes are in the cis position.
Cloned gene: A recombinant DNA molecule with the gene of interest. (Also see recombinant DNA.)
Co-dominant: Genes are co-dominant if both alleles are expressed in the heterozygous state, e.g., K and k genes in the Kell BGS.
Codon: A sequence of three bases in DNA or RNA that codes for a single amino acid. Enables specific proteins to be made by specific genes.
Consanguinity: Having a common ancestor, i.e., being blood relatives. Mating between two first cousins, for example, can be termed a consanguineous mating and is indicated in a pedigree by a double bar between the two parents. Such mating can result in an increased frequency of offspring who are homozygous for a recessive autosomal trait possessed by both parents, e.g. cystic fibrosis or the amorphic type of Rh null.
Crossing over: The exchange of genetic material between members of a pair of homologous chromosomes. For example, if a mating between a male (MS/Ns) and a female (MS/MS) results in an offspring who is MS/Ms, the recombinant child has occurred due to crossing over in the father.
Deletion: An abnormality in which part of a chromosome (carrying genetic material) is lost.
Deleted phenotype: The condition in which antigens that are normally present are missing, e.g., the Rh null phenotype in the Rh BGS. Deleted phenotypes can be caused by inheritance of regulatory genes that do not allow functional (antigen-producing) genes to make their products.
Diploid number of chromosomes: The number of chromosomes found in somatic cells, which in humans is 46.
Dizygotic twins: Twins produced from two separate ova that are separately fertilized, i.e. fraternal twins. Only dizygotic twins can exhibit blood group chimerism (shown by mixed field agglutination when antigen typing red cells).
DNA: Deoxyribonucleic acid. Composed of nucleic acids, these molecules encode the genes that allow genetic information to be passed to offspring.
DNA polymerases: Enzymes that can synthesize new DNA strands using previously synthesized DNA (or RNA) as a template.
DNA probe: A cloned DNA molecule labelled with a radioactive isotope (e.g., ³²P or ³⁵S) or a nonisotopic label (e.g., biotin). Used in molecular genetics to identify complementary DNA sequences by hybridizing to them.
Dominant gene: A gene is dominant if it is expressed when heterozygous but its allele is not, e.g. in the Lewis system the Le gene is dominant (expressed in both Le Le and Le le genotypes) and the le gene is recessive.
Functional genes: Genes that produce proteins, e.g., blood group genes that produce antigens.
Gamete: A reproductive sex cell (ovum or sperm) with the haploid number (23) of chromosomes that results from meiosis.
Gene: A segment of a DNA molecule that codes for the synthesis of a single polypeptide.
Gene flow: Changes in gene frequencies that occur over long periods of time due to migration in which different populations interbreed. An example is the transfer of genes between racial groups, e.g., the "white" genes of the Duffy blood group system (Fy^a Fy^b) have an increased frequency in U.S. blacks compared to African blacks.
Gene interaction: The situation in which genes inherited at different loci interact to produce red cell phenotypes, e.g., Le le genes interact with Hh and Se se genes to produce the various Lewis red cell phenotypes.
Genome: Term used to denote the entire DNA sequence (gene content) of a gamete, person, population, or species.
Genotype: All of the alleles present at the locus (or closely linked loci) of a blood group system, indicating chromosomal alignment if appropriate, e.g., AO in the ABO BGS, CDe/cde in the Rh BGS, or MS/Ns in the MNSs BGS. Genotypes are indicated by superscripts, underlining, or italics.
Haploid number of chromosomes: The number of chromosomes found in sex cells, which in humans is 23.
Hardy-Weinberg law: A law developed in 1908 independently by George Hardy (an English mathematician) and Wilhelm Weinberg (a German physician) that is the basis for calculations used in population genetics. The law is described by the formula p² + 2pg + q² = 100%, where p is the frequency of one allele, q is the frequency of the other, p² and q² are the homozygous frequencies, and 2pg is the heterozygous frequency. The formula allows us to calculate the frequencies of genes, phenotypes, and genotypes when the frequency of a genetic trait is known.
Harmful gene: A gene that confirms a harmful trait such that it is reduced to a level at which it is maintained only by recurrent mutation, e.g., the gene for hemophilia A, which has a mutation rate of 1 in 10,000.
Hemizygous: Inheritance of an X-linked gene in males, e.g. the Xg^a gene or the gene for hemophilia A is said to be hemizygous in males since they have only one X chromosome.
Heterozygous: The situation in which allelic genes are different, e.g. the Kk genotype in the Kell BGS or the Fy^a Fy^b genotype in the Duffy BGS.
Homologous chromosomes: A matched pair of chromosomes, one from each parent, e.g., two #6 chromosomes.
Homozygous: The situation in which allelic genes are identical, e.g., the KK genotype or the Fy^a Fy^a genotype.
HUGO: Acronym for Human Genome Organization, an international organization conceived in 1988 to co-ordinate the Human Genome Project.
Human Genome Project: A worldwide project to map and sequence the human genome. The ultimate goal is to produce the complete nucleotide sequence of every human chromosome. (Also see HUGO.)
Immune response genes: Name given to genes that appear to be able to control whether a person is likely or unlikely to make red cell antibodies. Help explain why some transfusion recipients are hyper-responders (make multiple alloantibodies) and others, even when transfused with a very immunogenic antigen like D from the Rh system, never produce antibodies. (About 30% of D-negative people appear incapable of making anti-D.) The genes that regulate the immune response may be linked to the genes of the major histocompatibility complex (MHC) or may be the MHC genes themselves.
Index case: See proband.
Karyotype: A photomicrograph (photograph taken through a microscope) of all the chromosomes in a person, arranged in standard classification (from #1 chromosomes through to the sex chromosomes).
Linkage: Genes are linked if they are on the same chromosome within a measurable distance of each other and are normally inherited together, e.g., Lutheran and Secretor genes are linked as are the Dd, Cc, Ee subloci in the Rh BGS.
Locus: The location of allelic genes on the chromosome, e.g., A, B, and O genes occur at the ABO locus. (Plural = loci)
Lyon hypothesis: The hypothesis proposed by Mary Lyon in 1961-2 that in the somatic cells of females one X chromosome is inactivated and becomes a Barr body. Because the process is random, which X is inactivated is due to chance; once inactivated, however, all of a cell's descendants will have the same inactive X. The hypothesis explains how males with only a single dose of an X-linked gene can have the same amount of genetic product as females with a double dose of the X-linked gene.
Mapping of genes: A variety of processes that include discovering that a gene is linked to another gene (which can serve as a marker for it), assigning genes to particular chromosomes, assigning genes to specific regions on chromosomes, and determining nucleotide sequences on chromosomes.
Meiosis: The type of cell division that occurs in sex cells by which gametes having the haploid number of chromosomes are produced from diploid cells.
Messenger RNA (mRNA): Type of RNA polymerase using DNA as a template. Contains the codons that encompass the genetic codes to be translated into protein.
Mitosis: Cell division that results in the formation of two cells, each with the same number of chromosomes as the parent cells, i.e., cell division that forms all new cells except sex cells.
Modifying gene: A regulatory gene (usually at a different locus than blood group genes) that in some way alters the expression of the blood group genes. Also called suppressor genes.
Monozygotic twins: Twins derived from a single fertilized ovum, i.e., identical twins.
Mutation: A permanent inheritable change in a single gene (point mutation) that results in the existence of two or more alleles occurring at the same locus. Blood group polymorphism has been caused by mutations occurring over long periods of time.
Nondisjunction: The failure of two members of a chromosome pair to disjoin during anaphase. For example, an offspring with the AB/O genotype can be produced if a group AB male mates with a group O female and nondisjunction happens in the father.
Northern blot: A blotting method used to analyze and detect RNA by using a DNA probe that will hybridize with its complementary RNA strand. Named for its similarity to the Southern blot used to analyze DNA.
Nucleic acids: Polymers of phosphorylated nucleosides, the building blocks of DNA and RNA.
Nucleoside: The building blocks of RNA and DNA. Compounds consisting of a purine (adenine or guanine) or pyrimidine (thymine or cytosine) attached to ribose (in RNA) or deoxyribose (in DNA) at the 1¹ carbon.
Nucleoside analogue: Synthetic nucleosides that are similar to nucleosides but differ at a key location. When incorporated into DNA, they terminate DNA chains and thus are useful as antiviral drugs. Examples are zidovudine (azidothymidine or AZT) and dideoxyinosine (ddI) used to treat AIDS.
Operator: A short sequence of nucleotides that controls the adjacent structural (functional) genes.
Operon: A postulated unit of gene action that consists of an operator and the closely linked functional genes it controls.
PCR: See polymerase chain reaction.
Pedigree: A diagram representing a family tree.
Phenotype: The antigens (traits) that result from those genes that are directly expressed (can be directly antigen typed), e.g., group A in the ABO BGS or D+C+E- c+e+ in the Rh BGS.
Plasmid: Extrachromosomal circular DNA in bacteria. Plasmids can independently replicate and encode a product for drug resistance or some other advantage. Used in molecular genetics as vectors for cloned segments of DNA.
Polymerase chain reaction: An in vitro method of amplifying DNA sequences hundreds of millions to billions of times in a few hours. Developed in 1984-1985 by Mullis, Saiki, et al.
Polymorphism: The existence of two or more different phenotypes resulting from two or more alleles, each with an appreciable frequency. Most blood group systems are polymorphic.
Polypeptides: Polymers of amino acids that form the building blocks of proteins.
Population genetics: The branch of genetics that deals with how genes are distributed in populations and how gene and genotype frequencies stay constant or change. Calculations are based on the Hardy-Weinberg law.
Proband: The family member whose phenotype leads to a family study. Also called an index case.
Proposita: A female proband.
Propositus: A male proband.
Recessive: Genes are recessive if the phenotype that they code for is only expressed when the genes are homozygous, e.g., le le genes, in the Lewis system or h h genes in the ABO BGS.
Recombinant: A person who has a new combination of genes not found together on the chromosome in either parent, e.g., an MS/Ns offspring whose parents are Ms/NS and MS/MS. A recombinant results from crossing over in one parent.
Recombinant DNA: In molecular genetics, artificially made DNA composed of fragments of DNA from different chromosomes (often from different species) that have been joined together (spliced) by genetic engineering. For example, healthcare workers are routinely vaccinated with a recombinant hepatitis B vaccine made by inserting a piece of the hepatitis B virus genome (the part that codes for the HB_sAg) into yeast cells via a plasmid. The yeast cells then produce a large amount of HB_sAg, which is purified into the vaccine and stimulates the production of protective anti-HB_s antibodies.
Regulatory genes: In the operon model, genes that inhibit an operator gene so that it prevents its functional genes from producing proteins.
Restriction endonucleases: DNA enzymes of bacterial origin that can cleave DNA at internal positions on a strand because they recognize specific sequences (usually 4-6 base pairs). The enzymes evolved in bacteria as defenses against the invasion of foreign DNA in the form of viruses or plasmids and are used in molecular genetics to chop up DNA at particular locations.
Restriction fragment length polymorphisms (RFLP): Regions of DNA of varying lengths that can be cut out of DNA by restriction endonucleases. Because the fragment lengths vary among individuals, they are polymorphic and can be used as genetic markers.
Reverse transcriptase: An RNA-dependent DNA polymerase that synthesizes DNA from an RNA template. Used by retroviruses like the human immunodeficiency virus (HIV) to make proviral DNA from its RNA genome.
RFLP: See restriction fragment length polymorphism.
Ribosomal RNA (rRNA): Type of RNA found in ribosomes, the site of protein synthesis in the cytoplasm.
Ribosomes: Complexes of rRNA and protein in cytoplasm that serve as platforms for translation for mRNA into protein.
RNA: Ribonucleic acid. Nucleic acids that are formed using DNA as a template. Similar to DNA except has ribose in place of deoxyribose and uracil in place of thymine. (Also see messenger RNA, ribosomal RNA, and transfer RNA.)
Sex chromosomes: The chromosomes that determine sex. XX in females and XY in males.
Sex-linked: An outdated term for genes on the X chromosome. Historically synonymous for X-linked since, apart from genes essential for male sex determination, the Y chromosome appears to have few recognized gene loci.
Somatic chromosome: A non-sex chromosome (soma=body). Synonym is autosome.
Southern blot: A blotting method developed in 1975 by E.M. Southern that detects restriction enzyme-cleaved DNA by use of a labelled DNA probe that will hybridize with its complementary DNA strand.
Structural genes: See functional genes.
Suppressor genes: See regulatory genes.
Syntenic: Genes are on the same chromosome but are not close enough for linkage to be demonstrated.
Transcription: Synthesis of single-stranded RNA by RNA polymerase using DNA as a template. The process in the nucleus whereby DNA is transcribed into mRNA.
Transfer RNA (tRNA): Type of RNA that facilitates translation of mRNA into protein. Contains anticodons that provide the molecular link between the codons of mRNA and the amino acid sequences of proteins.
Transient polymorphism: A temporary polymorphism in which an allele (harmful gene) is disappearing or an allele (beneficial gene) is increasing in frequency.
Translation: The process of translating the codon sequence in mRNA into polypeptides with the help of tRNA and ribosomes.
Trans position: Genes in the trans position are on opposite chromosomes of a pair of homologous chromosomes. In the genotype CDe/cde, for example, D and c genes are in the trans position.
Western blot: An assay used to separate viral (and other) antigens and to identify corresponding antibodies to the viral antigens. For example, the western blot is a relatively specific and sensitive test for antibodies to HIV that is used as a confirmatory test for sera that are repeatedly reactive by EIA (enzyme immunoassay) screening tests. Named western blot as a joke due to its similarity to the Southern blot and since its discoverers worked in the western USA.
X-chromosome: The sex chromosome present in double dose in females (XX) and in single dose in males (XY).
X-linked: Genes on the X chromosome, e.g., genes for hemophilia A, hemophilia B, and Xg^a blood group genes.
Y-chromosome: The sex chromosome present only in males (XY).