Genetics is a field of science includes the study of inheritance and genetic variations by investigating the DNA, genes, genome, chromosome and other components of it. 

In a broader sense, we can say, that the study of genes, genome and chromosome and related inheritance traits is called genetics. 

The mechanism of inheritance was first explained by Gregor Johann Mendel during the late 18’s.

As per Mendel’s finding, “the traits inherited from parents to their offsprings.” Some traits are physical while some are biological. 

What he called a trait are now called genes which inherited from one generation to another generation.  

The genes are the sequence of DNA, encoding a specific protein. 

The physical traits are visible phenotypes such as the colour of the eye, skin colour, height and hair colour. 

The biochemical traits are involved in the regulation of biological reactions of ours which are not visible. 

For example, the reaction of lipid metabolism, the reaction of purine synthesis. 

Mutation in any of the traits results in the alteration of related phenotype, the resulting alteration might be helpful or harmful. 

Any alteration in the sequence of DNA which influence the related phenotype is called mutation. 

Different diseases are originated due to the stable mutations in the genome. 

Genes are made up of the sequence of DNA and located on the chromosome. 

The chromosomes are the complex network of DNA and proteins, the process called DNA packaging helps to arrange DNA on a chromosome. 

46 numbers chromosome are present in the somatic cells of us in which 22 pairs are autosomal chromosome and a pair of sex chromosome are found. 

In a male, one X and one Y chromosome are present while in female two XX chromosomes are present. 

The overview of how DNA arranged in the cell.

Interestingly, alteration in the number of chromosomes can also cause severe mental, structural or physical abnormalities. 

For instance, the down syndrome is caused by the presence of one extra 21 number of a chromosome, while the Klinefelter syndrome is caused because of the presence of one extra X chromosome in male (XXY).

Several structural chromosomal abnormalities also cause severe phenotypic defects. 

Broadly, the study of chromosome and related alteration is called cytogenetics while the study and analysis of DNA and genes at a molecular level are called molecular genetics. 

In the present article, we will discuss on genetics, its definition and different branches of it.

The content of the article is,

  • Definition of genetics
  • Application of genetics
  • History of genetics 
  • Branches of genetics
    • Molecular genetics 
    • Cytogenetics 
    • human genetics
    • Preimplantation genetics 
    • Clinical genetics
    • Plant genetics
    • Microbial genetics
    • Metagenomics
    • Population genetics 
    • Epigenetics
    • Biochemical genetics
    • Physiological genetics
    • Quantitative genetics 
    • Conservative genetics 
    • Behavioural genetics 
  • Tools used in genetics
  • Conclusion

Definition of genetics:

“The branch of the science deals with the study of the heredity and variation in genes and genotype is called genetics. “

Also,

“The study of structure and function of DNA, genes, chromosomes and related alterations are called genetics.”

The term “Genetics” was coined by William Bateson is 1905.

The term “genetics” was derived from the Greek word “genetikos” and “genesis”. Genetikos: generative and genesis: origin.

Common terminologies used in the present article: 

Terminology  Definition 
DNA  Inheritance unit of an organism. 
Gene  A sequence of DNA which encodes a protein. 
Genotype  A genotype is a heritable portion of a genome which produces a specific phenotype. 
Phenotype  A set of the observable characteristics of an organism. 

Several applications of genetics are given below,

Applications of genetic studies: 

  • Disease diagnosis and characterization
  • Identification of pathogenic mutations
  • Preserving biodiversity 
  • Identification and characterization of microbes
  • Studying inheritance pattern 
  •  Creating advanced plant species 
  • Creating genetically modified organisms 
  • DNA fingerprinting 
  • Antibiotic resistance study and drug discovery 
  • Genetic/DNA medicines 
  • Genetic engineering 
  • Crop improvement 
  • Animal and Plant Breeding program 
  • Infectious disease diagnosis
  • Screening, prognosis and diagnosis of cancer

History of Genetics:

Mendel was the pioneer in experimenting and establishing the base of genetics and hence Gregor Johann Mendel is called the father of genetics.

During the period of 1856 to 1865, he experimented on pea plant and discovered the phenomenon of “inheritance of traits”.

What is called a trait are now called a gene, that can be inherited from one generation to another generation.

In 1866, he published his research paper describing the law of inheritance and independent assortment.

Some of the milestone discoveries in the genetics are enlisted below, 

1842: Wilhelm von Nageli, a Swiss botanist, observed the plant cell.

1866: Mendel’s research work published under the title of “experiments on plant hybridization.”

 1869: Friedrich Miescher discovered the nucleic acid.

1888: Waldeyer identified the chromosome present in the cell.

1889: Richard Altmann purified DNA from the protein.

1905: William Bateson coined the term “genetics”.

1908: discovery of Hardy-Weinberg’s law.

1910: Morgan T, explained that the genes are located on the chromosomes. Also, experimented on Drosophila Melanogaster and determined the nature of sex-linked traits.

1923: Griffith F, experimented on bacteria and postulated that the DNA is genetic material.

1953: Watson and Crick identified the structure of DNA.

Read more on DNA: DNA story: The structure and function of DNA

Branches of genetics: 

The branches of genetics

Molecular genetics: 

Molecular genetics is an interdisciplinary sub-branch of genetics deals with the study of the structure and function of DNA as well as genes (at a molecular level) using techniques such as Polymerase chain reaction and DNA sequencing. 

Using the molecular genetics techniques one can screen pathogenic mutation, detect SNPs (single nucleotide polymorphism), minor deletion or duplication at DNA level, changes in the gene expression and identify mutant DNA sequence.

Using the state of the art techniques such as DNA sequencing we can also identify some of the novel mutations. 

Polymerase chain reaction, gene cloning, DNA sequencing and DNA quantification are some of the techniques used in it. 

Any of the molecular genetic experiment divided into the 4 sub-steps: 

  1. Separation of molecule 
  2. Purification of molecules 
  3. Processing of molecule
  4. Detection of molecule

Separation is a process of extracting molecules like DNA or mRNA from other cell debris. 

We have covered so many articles on DNA extraction. Some of the DNA extraction methods are enlisted here: 

  1. Phenol chloroform DNA extraction method
  2. Proteinase K DNA extraction method

After that DNA is purified using the ready to use-kit or using alcohol.

Once the purified – good quantity DNA is obtained, it is further processing for the downstream applications. 

PCR: 

The polymerase chain reaction is a process in which we can amplify millions of copies of a DNA segment of our interest in vitro.

The process is temperature-dependent, divided into three steps,

Denaturation: the double-stranded DNA denatured into single-stranded one.

Annealing: the Sequence-specific DNA primer binds/anneal to its complementary sequence on single-stranded DNA.

Extention: the taq DNA polymerase amplifies the DNA using the 3′ end of the primer.

A comprehensive handbook of long-range PCR

Read more on PCR: A Complete Guide of the Polymerase Chain Reaction 

DNA cloning: 

DNA cloning is a traditional method for the synthesis of DNA. Using a cloning vector our sequence of interest can be synthesised by the bacterial transformation. 

The method is time-consuming and not so accurate. 

DNA sequencing: 

The method of reading the sequence of DNA using a computational tool is called a DNA sequencing. 

In this method, we can actually analyse any variation or new mutation in our sequence of interest using the fluoro-labelled dNTPs. 

Read more on DNA sequencing: DNA sequencing

Cytogenetics: 

Cytogenetics is a sub-branch of genetics including the study of inheritance through chromosomal analysis using techniques such as karyotyping, chromosomal staining and chromosomal banding and FISH. 

Structural and numerical chromosomal abnormalities can be screened using cytogenetic techniques.

Major deletions and duplications (more 10kb) can only be detected using FISH or DNA microarray techniques.

Some of the structural and numerical anomalies are enlisted in the table below,

Disease  Abnormality  Cytogenetic indication 
Down syndrome  Numerical  Trisomy 21
Klinefelter syndrome  Numerical  One extra X chromosome in male (XXY).
Philadelphia syndrome  Structural  Translocation between chromosome 9 and 22 
Turner syndrome  Numerical  monosomy in female, single X chromosome.
Neuroblastoma  Structural  Chromosome 1p deletion 

We have already covered a beautiful article on cytogenetics, you can read it here: 

Read our article on Cytogenetics: A Brief Introduction To Cytogenetics.

Human genetics: 

The branch of genetics comprises the study of genetic alteration and its role in the development of the disease especially in humans is called human genetics.

Using the cytogenetic, molecular genetics, phylogenetic, population genetics and clinical genetic methods, any mutation can be characterised which involved in the development of the disease. 

We can study,

  • The inheritance pattern of disease.
  • The severity of the disease. 
  • Possibility of inheritance in the consecutive generation

Furthermore,

more recently, the genetic techniques are also used in the screening, prognosis and diagnosis of cancer. So many oncogenes are now known due to the advancement in genetic.

Preimplantation genetics: 

Characterizing or profiling the genetic composition of the embryo before implantation, the branch of genetics is known as preimplantation genetics. 

Even, the genetic profile of an oocyte or sperm is checked before fertilization is also covered in the preimplantation genetic study.

The major application of preimplantation genetic is to screen the high-risk pregnancy. 

A couple having the previous history of any genetic disorder can be suggested for preimplantation genetics. 

Selective abortions can be prevented by doing the preimplantation genetic analysis prior to embryo formation. 

Some of the cells from the pre-embryonic stage are taken and grown in a lab. 

That cells can be used for molecular genetic analysis or cytogenetic analysis, see the figure below,

The outline of preimplantation genetic diagnosis process

Broadly we can say that any of the genetic abnormality or disease can be identified prior to implantation. 

Although the field has some most promising applications, the PIGD still under pre-clinical trial phase. 

Clinical genetics: 

This genetic field is involved with the study of disease, finding the root of the disease, its adverse and related effects and its inheritance pattern. 

Plant genetics: 

The branch of the genetics deals with the study of genetic variation and chromosomal abnormalities in plants covered under the plant genetics. 

The mechanism of inheritance was developed from experimenting with plants. The foundation of genetics laid by experimenting on pea plant by father of genetics, Mendel. 

The state of the art genetic tools like karyotyping, PCR and DNA sequencing are used in the plant research and GMO studies too.

Genetically modified plant species have tremendous economic value. Plant genome is modified for creating new variation in native plant species to increase yields, creating disease resistance,  to increase the nutrition level and to create stress resistance. 

BT-cotton and BT- brinjal is the best example of genetically modified plant species. 

Ploidy level in the plant (polyploidy or aneuploidy) can be studied using standard karyotyping method whereas any alteration in the DNA or any gene can be studied using the PCR and sequencing method. 

Furthermore, species and speciation studies can also be possible using genetic tools. 

Nicotiana benthamiana, Arabidopsis thaliana and Brachypodium distachyon are some of the model organism used in the plant genetic studies. 

New variation in the plant species can be induced using the gene-editing method

One of the traditional gene-editing methods is Agarobacterium mediated gene transfer. 

Mainly the genome of the dicot plants is edited by introducing the gene in the T-plasmid of Agrobacterium. 

Modern methods like gene gun, particle bombardment and viral vector-mediated gene transfer are more accurate and advanced methods used for plant genomic research. 

Microbial genetics: 

The microbial genetics is an applied branch of genetics includes the study of the genes, genotypes ad gene expression of microorganism for various genetic engineering applications. 

The study includes bacteria, viruses, archaea, protozoa and some fungi. 

Excluding the RNA viruses, DNA is genetic material in both prokaryotes as well as in eukaryotes, in addition to this, the hereditary and genotypic processes in microbes are similar to eukaryotes hence the microbes can be used as a model organism for studying the genetic traits. 

For instance, the operon model has provided information on gene expression and regulation of different enzymes involved in the metabolism of biomolecules such as lactose. 

Some of the microbes are the proven causative agent for the disease. By studying those microorganisms infectious diseases can be prevented. 

Furthermore, vaccines and antibiotics against any microbes can be developed by studying their genetic profile. 

New antibiotic resistance species of microorganism can also found using tools like PCR or DNA sequencing. 

Apart from all these, the microbial genetic study helps in genetic engineering practices such as developments of therapeutic drugs and therapeutic proteins. 

All these studies can be possible because of the genetic tools used for the microbial genetic analysis.

Metagenomics: 

Meta: Vast or huge, Genomics: a study of genetics or inheritance

The branch of the genetics that deals with the study and identification of different microorganisms from the environmental sample using modern genetics techniques is called metagenomic studies. 

Different types of organisms can be studied and identified by studying the recovered environmental sample without using the cultivation or culturing method. 

The traditional microbiology techniques such as culturing or cultivation may not help in the identification of all organism present in any biological/ environmental samples. 

A majority of microbial diversity has been missed, contaminated or not cultured properly that creates a loophole in the study. 

Using genetic tools such as DNA sequencing or polymerase chain reaction, one can identify microbial diversity in any biological sample. 

Shotgun sequencing and PCR direct sequencing is commonly used for the metagenomic analysis. 

By extracting DNA from any biological sample, one can identify and characterise any microorganism present in that sample using the species-specific and sequence-specific primers and bioinformatic tools. 

A powerful tool sequencing can even identify any unknown organism or new microbe or strains of microorganism. 

The outline of metagenomics analysis

Population genetics: 

An interdisciplinary branch of the genetics includes the study of genetic difference within and between the population or individual is known as population genetics. 

By doing mathematical calculations, statistical analysis, fieldwork and genetic analysis one can calculate the genetic frequency, allelic frequency and other factors with respect to the population. 

We can also determine how natural selection, mutation, genetic drift and gene flow can influence the phenotype and genotype in the population. 

The Hardy-Weinberg’s equation is the basis of the population genetics 

H-D equation: P2 + 2pq + q2 = 1 

Epigenetics:

The branch of the genetics deals with the study of alterations in an organism caused by gene expression rather than alteration caused by a gene mutation. 

 In simple words, epigenetics is a study of a switch on and off of gene expression. 

Different genes expressed in different amount in different cells and create various tissue types. 

What we eat, how we sleep, how we exercise and stress has a major impact on our epigenetic profile.

Some of the epigenetic factors like methylation, ubiquitination, acetylation and histone modification and activate oncogene and causes cancer. 

Biochemical genetics: 

The branch of the genetics related to studying the chemistry of DNA, gene, chromosome, RNA and related biomolecules is called as biochemical genetics. 

Physiological genetics: 

Physiological genetics deals with the study of physiological characteristics such as sex differentiation and sex determination, blood group factor and sickle cell anaemia like physiological conditions. 

Quantitative genetics: 

It is actually a branch of the population genetics which studies the continuously varying phenotypes. The correlation between phenotype and related genotype is the base for quantitative genetics.  

Conservation genetics: 

Again, conservative genetics is a subfield of population genetics in which, using the genetic tools and by understanding the dynamics of genes and its expression profile, endangered species of plants, animals and other organisms can be conserved. 

Behavioural genetics:

The field of genetics study the behavioural phenotypes of an organism governed by the genetic factors are called behaviour genetics.

The behaviour of an organism is influenced by the interaction between the environment and genetic composition.

Some of the behaviours of ours are governed by inherited genetic factors. Read our article on it: Behavioural Genetics: Science behind the behaviour

Tools used genetics: 

Using any other tool listed below, any genetic abnormality like SNP, copy number variation, structural and numerical chromosomal abnormality, gene expression and any new variation can be studied. 

Conclusion:

In a broader sense, genetics is a study of genotype, its related phenotype and alterations in the genome.

Using genetics tools, nowadays, the diagnosis of inherited diseases is a common medical practice.

The Human Genome Project was completed in the year 2013. Now we have the entire genomic sequence of human. We can use this data for the identification of new mutation and alterations.

Further, genomic data of so many other organisms are now available which is used for identification and characterization of different organism and species.