Basic Terms of Genetics

Genetics is the study of genes and their functions and behaviors. Genetics is an important topic for board examinations, and a thorough understanding of different words used in the study of genetics is an important piece of knowledge. Crucial terms in the study of genetics are listed and defined below, in alphabetical order.

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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.
  • 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)


  1. Letendre, P. Genetic terminology. Retrieved July 26, 2020, from
  2. McGrath, C. (2020). Human genetics: a look in the mirror. Genome Biology and Evolution 12:1256–1257.
  3. Portin, P., Wilkins, A. (2017). The evolving definition of the term “gene.” Genetics 205:1353–1364.
  4. Jackson M, Marks L, May GHW, Wilson JB. (2018). The genetic basis of disease. Essays Biochem 62:643–723.

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