Some important terms used in genetics

Important terminologies


  • Genes are small sections or segments of DNA, called cistrons within the genome that code for proteins.
  • It determines particular character of an organism.
  • It is a hereditary unit which is transmitted from one generation to another.


  • It is a particular position or place where gene is located on a chromosome.
  • Different genes have different loci.

Allele (allelomorph):

  • It is one of the alternative forms of a gene.
  • For example: T and t are two alleles of the gene for plant height. In a diploid cell, there are usually two alleles of any gene which occupy the same relative position (locus) on homologous chromosomes. One allele is often dominant over the other. Within a population there may be several allele of a gene. Each has a unique nucleotide sequence.


  • The genetic composition of an organism, i.e. the combination of alleles it possesses.
  • For example:  the genotype of pure tall plant is TT and a hybrid tall plant is Tt.


  • The external appearance or observable characteristics of an organism is called phenotype.
  • These are determined by the genes, the dominance relationships between the alleles and by the interaction of the genes with the environment.
  • For example, the genotype of a hybrid tall is Tt and its phenotype is tall.


  • The individuals with two identical alleles of the same gene are called homozygous.
  • For example, pure tall (TT) or pure dwarf (tt) is homozygous.
  • The homozygous organisms breed true when crossed with genetically identical organisms.


  • The individuals with two different alleles of the same gene are known as heterozygous.
  • For example, hybrid tall (Tt) is heterozygous.
  • The heterozygous organisms do not breed true.


  • A group of organisms or cells that have arisen from a single individual by asexual reproduction are called clone.
  • They are genetically identical.

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  • Sudden heritable change in the genetic material of organisms is called mutation.


  • A gene or an organism that has undergone a heritable change, especially one with visible effects is called mutant.

Back cross:

  • A cross between F1 hybrids and one of the two parents from which they are derived is called back cross.
  • It is used to identify hidden recessive alleles.

Test cross:

  • The cross of F1 hybrids with the recessive parent is called test cross.
  • It is used to test whether an individual is homozygous or heterozygous or to determine the genotype of an unknown individual.
  • It is possible to know the unknown genotype within one generation.

Behavioural genetics:

  • Study of interaction of genes with the environment to reduce a particular pattern of behavior is called behavioural genetics.

Molecular genetics:

  • Study of the structure and function of gene and its regulation at molecular level is called molecular genetics.

Dominant allele:

  • An allele which affects the phenotype of a heterozygous organism just as much as when the organism is homozygous for this allele is called dominant alleles.

Recessive allele:

  • An allele affects only the phenotype of an organism when the dominant allele is not present.


  • It is a sequence of three bases in tRNA which is complementary to a codon in mRNA.
  • It enables tRNA in sequencing of amino acids in the order specified by mRNA.

Balanced polymorphism:

  • It is an equilibrium of two or more alleles that has remained constant over long periods of time.


  • It is the existence of two or more different phenotypes that results from two or more alleles, each with an appreciable frequency.
  • Most blood group systems are polymorphic.


  • The family member whose phenotype leads to a family study and also known as index case is proband.

Regulatory genes:

  • In the operon model, there are some genes that inhibit an operator gene so that it prevents its functional genes from producing proteins.


  • It is the process of translating the codon sequence in mRNA into polypeptides with the help of tRNA and ribosomes.


  • Synthesis of single stranded RNA by RNA polymerase using DNA as a template. The process in the nucleus whereby DNA is transcribed into mRNA.


  • The chromosomes that determine sex are called sex chromosomes. XX in females and XY in males.

Somatic chromosomes:

  • A non-sex chromosome (soma=body).
  • Its synonymous word is autosome.

Restriction fragment length polymorphism (RFLP):

  • Regions of DNA of varying lengths that can be cut out of DNA by restriction endonucleases.
  • They are polymorphic and can be used as genetic markers as the fragment length vary among individuals.




Some important terms used in genetics