Terms Starting with A–D
- Allele: a form of a gene that occurs at the same locus on homologous chromosomes:
- Different alleles of ABO blood groups are an example.
- A null allele is a variant that has no function.
- Anticodon: a sequence of 3 bases in tRNA that is complementary to an mRNA codon. When bound with mRNA, the tRNA facilitates linking of amino acids.
- Autosome: a nonsex chromosome:
- Synonymous with somatic chromosomes (chromosome pairs 1–22)
- DNA in autosomes is known as atDNA or auDNA.
- Balanced polymorphism: an equilibrium of ≥ 2 alleles that has remained constant over long periods of time
- Barr body: sex chromatin, visible inactive X chromosome:
- Inactivated through lyonization, an inactivation process that occurs during embryogenesis
- Visualized as a darkly staining mass
- Chimera: extremely rare organism composed of cells derived from different genotypes:
- Organisms may have different phenotypes
- Possible for these organisms to have both male and female sex organs
- Chromosome: rod-shaped structures within the cell nucleus that carry genes encoded by DNA:
- Chromosome recombination during meiosis promotes genetic diversity.
- Chromosomes can be seen with a microscope during the metaphase of cell division.
- Cloned gene: a recombinant DNA molecule that shares the same DNA; occurs either naturally or artificially
- Codon: a sequence of 3 bases in DNA or RNA that codes for a single amino acid:
- Enables specific proteins to be made by specific genes
- 64 codons: 61 code for amino acids; 3 are used as stop signals
- Consanguinity: having a common ancestor:
- In other words, blood relatives
- Coefficient of relationship is defined by r, where r is the homozygosity fraction.
- Crossing over: the exchange of genetic material between pairs of homologous chromosomes
- Deletion: a change in which part of a chromosome carrying genetic material is lost
- Diploid: a cell that contains 2 complete copies of each chromosome within each cell nucleus:
- The number of chromosomes found in somatic cells, which in humans is 46
- All human cells except for gamete cells are diploid.
- Dizygotic twins: twins produced from 2 separate ova that are separately fertilized
- DNA: deoxyribonucleic acid:
- Composed of nucleic acids; these molecules encode the genes that allow genetic information to be passed to offspring.
- DNA is organized into chromosomes.
- DNA polymerases: enzymes that can synthesize new DNA strands using previously synthesized DNA (or RNA) as a template
- Dominant gene: A gene is dominant if it is expressed when heterozygous but its allele is not.
Terms Starting with E–L
- Gamete: a reproductive sex cell (ovum or sperm) with the haploid number (23) of chromosomes:
- Produced during meiosis
- Gametes hold half the genetic information from each individual.
- Gene: a DNA or RNA molecule that codes for the synthesis of a single polypeptide:
- DNA is copied into RNA, which is copied into a protein.
- Genes come in variants called alleles.
- Genome: entire DNA sequence 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:
- A genotype determines the phenotype, or observable characteristics.
- Genotype testing can be performed through DNA sequencing.
- Haploid: number of chromosomes found in sex cells, which in humans is 23:
- Half the number of usual chromosomes
- Gametes are haploid because of the process of meiosis.
- Hemizygous: 1 copy of a gene is present in a diploid cell or organism:
- Inheritance of an X-linked gene in males
- The gene for hemophilia A is said to be hemizygous in males because they have only 1 X chromosome.
- Heterozygous: 2 loci contain different alleles:
- This genotype is described by an uppercase and a lowercase letter (e.g., Yy).
- Some traits are advantageous for heterozygotes, and this is referred to as a heterozygote advantage.
- Homologous chromosomes: a matched pair of chromosomes, 1 from each parent
- Homozygous: Alleles of a gene are both present:
- Homozygous-dominant refers to 2 copies of the dominant allele.
- Homozygous-recessive refers to 2 copies of an allele that code for a recessive trait.
- Karyotype: the whole, or complete, set of chromosomes
- Linkage: Genes are linked if they are on the same chromosome within a measurable distance of each other and are normally inherited together.
- These genes often travel together during meiosis.
- The closer 2 genes are on a chromosome, the more likely they will be inherited together.
- Locus: the location of allelic genes on a chromosome:
- Plural = loci
- A, B, and O genes occur at the ABO locus.
Terms Starting with M–Q
- Meiosis: cell division that results in gametes:
- Occurs in germ cells
- Produces haploid cells, containing 1 set of each chromosome
- Genetic crossover occurs during the process of meiosis, which leads to genotype variability in offspring.
- Messenger RNA (mRNA): type of RNA that uses DNA as a template for protein synthesis:
- Read by ribosome
- Single-stranded
- Creates protein after binding with tRNA
- Mitosis: cell division that results in the formation of 2 cells, each with the same number of chromosomes as the parent cell:
- Creates genetically identical offspring cells, unlike meiosis
- Happens only in eukaryotic cells
- Monozygotic twins: identical twins:
- Twins derived from a single fertilized ovum
- Occurs in approximately 1 in every 300 live births
- Mutation: change in nucleotide sequence of the genome:
- Caused by:
- Errors in cell division
- DNA damage
- Insertion or deletion caused by a mobile genetic element
- Mutations do not always cause a phenotype change.
- Caused by:
- Nondisjunction: the failure of 2 members of a chromosome pair to disjoin during cell division
- Nucleic acids: molecules composed of nucleotides:
- Compose DNA and RNA
- Crucial for protein synthesis
- Nucleoside: nucleobase and a 5-carbon sugar:
- Similar to nucleotides; however, they lack a phosphate group
- Operator: a short sequence of nucleotides that controls the adjacent structural (functional) genes
- Operon: a set of genes contained within a promoter region:
- These genes are often expressed together.
- Operon regulation may increase or decrease expression.
- Pedigree: a diagram representing a family tree
- Phenotype: traits that result from an organism’s genes:
- Phenotype is distinct from genotype, which describes the genetic code.
- The phenotype depends on expression and environmental factors that influence expression.
- Plasmid: extrachromosomal circular DNA in bacteria:
- Plasmids can independently replicate and encode a product for a physiologic advantage.
- Plasmids can be thought of as units of DNA that are able to independently replicate within a host.
- Polymorphism: the existence of ≥ 2 different phenotypes resulting from ≥ 2 alleles, each with an appreciable frequency
- Population genetics: the branch of genetics that deals with how genes are distributed in populations and how gene and genotype frequencies change
- Population genetics explains the concept of dominance, a phenotypic effect when 1 allele is present.
- The study of population genetics attempts to explain selection and genetic drift.
- Proband: the family member whose phenotype leads to a family study
- Also called an index case
- Within a pedigree, this individual is a shaded square (male) or circle (female).
Terms Starting with R–Z
- Recessive: a heritable characteristic controlled by genes that are expressed in offspring only when inherited from both parents:
- For a recessive trait to become the phenotype, an individual must carry all of the recessive alleles.
- Recessive alleles are often denoted with lower case letters and dominant alleles with uppercase letters.
- Recombinant DNA: DNA molecules that are made by binding of genetic material from multiple sources
- Restriction endonucleases: DNA enzymes that cleave DNA at internal positions on a strand because they recognize specific sequences:
- Length of strand cleaved is often 4–6 base pairs
- Bacterial origin
- Restriction fragment length polymorphisms: regions of DNA of varying lengths that can be cut out of DNA by restriction endonucleases:
- Because fragment lengths vary among individuals, they are polymorphic and can be used as genetic markers.
- Restriction fragment length polymorphism analysis is a test that fragments a DNA sample through restriction enzymes and sorts the fragments by size.
- Reverse transcriptase: an RNA-dependent DNA polymerase that synthesizes DNA from an RNA template:
- Used by HIV
- This enzyme allows for single-stranded RNA to be converted into double-stranded DNA.
- Ribosomal RNA (rRNA): type of RNA found in ribosomes, the site of protein synthesis:
- Noncoding RNA
- rRNA is the most common form of RNA in cells.
- Ribosomes: complexes of rRNA and protein in cytoplasm that serve as platforms for translation of mRNA into protein
- Ribonucleic acid (RNA): nucleic acids that are formed using DNA as a template:
- Similar to DNA except it has ribose in place of deoxyribose and uracil in place of thymine
- Important in coding, decoding, and expressing genes
- Sex chromosomes: the chromosomes that determine sex:
- XX: females
- XY: males
- Transcription: synthesis of single-stranded RNA by RNA polymerase using DNA as a template:
- DNA that encodes protein is made into mRNA.
- Sections of DNA that do not code for proteins are transcribed into noncoding RNA.
- 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
- tRNA is necessary for the process of translation.
- 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:
- 3 phases: initiation, elongation, termination
- Translation facilitated by ribosomes
- Transposition: horizontal gene transfer:
- Describes the exchange of genetic material
- Does not rely on reproduction
- A common mechanism of antibiotic resistance
- X chromosome: the sex chromosome present twice in females (XX) and once in males (XY)
- X-linked: describes a gene on the X chromosome
- Y chromosome: the sex chromosome that is present only in males (XY)
References
- Letendre, P. Genetic terminology. Retrieved July 26, 2020, from https://sites.ualberta.ca/~pletendr/tm-modules/genetics/70gen-term.html
- McGrath, C. (2020). Human genetics: a look in the mirror. Genome Biology and Evolution 12:1256–1257. https://doi.org/10.1093/gbe/evaa139
- Portin, P., Wilkins, A. (2017). The evolving definition of the term “gene.” Genetics 205:1353–1364. https://doi.org/10.1534/genetics.116.196956
- Jackson M, Marks L, May GHW, Wilson JB. (2018). The genetic basis of disease. Essays Biochem 62:643–723. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6279436/
