Genetic Terminology and Definitions List

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Gene: A segment of a DNA molecule that codes for the synthesis of a single polypeptide.

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.

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.

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.

Carcinogen: Physical or chemical agent which induces cancer.

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

Chromatid: One of the two side-by-side replicas produced by chromosome duplication.

Chromosome: Rod-shaped structures within the cell nucleus that carry genes encoded by DNA.

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

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.

Crossing over: The exchange of genetic material between members of a pair of homologous chromosomes.

Denaturation: 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.

Dominance: The expression of a trait in the heterozygous condition.

Dominant: Gene/trait that appears or expresses itself; shown with a capital letter (e.g. Tall = T, Brown = B, etc.).

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.

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

Gamete: male and female sex cells, sperm and eggs.

Genome: Term used to denote the entire DNA sequence (gene content) of a gamete, person, population, or species.

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

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.

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.

Hemizygous: Inheritance of an X-linked gene in males, e.g. the Xga 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 Fya Fyb 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 Fya Fya genotype.

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

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.

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

Locus: A specific location on a chromosome.

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.

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

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

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.

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: Stretches of codons in the same reading frame uninterrupted by STOP codons.

Operon: A postulated unit of gene action that consists of an operator and the closely linked functional genes it controls.

Phenotype: Observable characteristics of an organism.

Plasmid: Cytoplasmic, autonomously replicating extrachromosomal DNA molecule.

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.

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.

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

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.)

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

Trait: Any detectable phenotypic variation of a particular inherited character.

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

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