Some important terms used in genetics

Important terminologies

Genes:

• 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.

Locus:

• 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.

Genotype:

• 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.

Phenotype:

• 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.

Homozygous:

• 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.

Heterozygous:

• 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.

Clone:

• 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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Mutation:

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

Mutant:

• 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.

Anticodon:

• 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.

Polymorphism:

• 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.

Proband:

• 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.

Translation:

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

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.

Sex-chromosomes:

• 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.

References: 

i) https://www.lecturio.com/concepts/basic-terms-of-genetics/

ii) https://sites.ualberta.ca/~pletendr/tm-modules/genetics/70gen-term.html

Some important terms used in genetics