Introduction to Genetics definitions

Genetics definition can be stated as the biological field of study focusing on heredity, genes, and genetic variation. Heredity has been a matter of interest and contemplation for millennia. Still, the Augustinian Friar and Moravian scientist Gregor Mendel conducted the first-ever scientific study of genetics. Mendel noticed the patterns of inheritance in leguminous plants (pea plants) and formulated his theories of inheritance. His studies concerned the "trait inheritance" phenomenon in almost all organisms. He called the discrete inherited traits in legume plants "inheritance units".

According to popular genetic definitions, genetic mechanisms such as molecular and trait inheritance persist in being the fundamental principles of genetic study. However, the branch has emerged past inheritance to investigate the behaviour and function of genes. We study Genetics from multiple layers of contexts. In addition, within the contexts of cells, we study organisms, populations, the function of genes and their structure, distribution, and variation. Sub-disciplines of Genetics include: 

• Epigenetics
• Population genetics
• Molecular genetics.

Genetic processes in organisms are influenced directly by their experiences and environment, contributing to behaviour and development. A living cell's Internal or external environment may switch the transcription of genes, active or inactive. An example to portray this would be the experiment of placing genetically identical corn plants. One should place them in different climatic conditions to find that lack of suitable environment and nutrients affect those corn. In the end, one should compare it to the corn placed in suitable conditions even though they are genetically identical.

History

The term genetics arises from the Greek word genetikos. Genetics means "generative" or "genitive". It is from the word genesis, which means "origin".

Since ancient times, we have observed the natural phenomenon of heredity to improve their animals and plants through selective breeding. With Gregor Mendel's studies and findings in the nineteenth century, the exploration of the characteristics of this phenomenon of heredity and the scientific study of it began.

Before Mendel, the Hungarian noble Imre Festetics first used the term "genetics". In his book about nature's genetic laws published in 1819, Festetics describes various rules of inheritance and genetics. Festetics's second law has a lot of similarities with Mendel's. Still, the third law deals with mutation and the principles behind it. Hence, Festetics is considered a predecessor of Hugo Marie de Vries.

Before Mendel published his work, there were a few other theories regarding inheritance. The popular theory of blending inheritance was one among them. Charles Darwin even mentioned it in his famous Origin of Species (1859). However, Mendel's experiments pointed out with examples that, after hybridization, traits are never smoothly blended. Instead, the creation of traits is from gene combinations that are distinct and non-continuous. With this, the reason behind the trait blending in progenies was the quantitative effects formed from the presence of several genes.

Another concept that had a lot of support during the eighteenth century was the acquired characteristics theory advocated by Jean-Baptiste Lamarck. As per the latest research in the field, this theory is incorrect- the progenitor's experiences do not influence the genes passed down to their offspring. However, some cases in epigenetics revived some of the aspects discussed in the Lamarckian theory. 

Classical and Mendelian genetics definition

Gregor Mendel is the one who initiated the study on the characteristics of inheritance from a botanical perspective, and with it began modern genetics and formed the modern genetic definition. His work "Plant Hybridization Experiments" (1865), which he presented to the Nature Research Society in Brno, was an important step forward. In it, Mendel outlined the inheritance pattern of traits found in legume plants and gave mathematical illustrations. Mendel's experiments and findings led to the understanding that heredity is not acquired but is particulate. As a result, we can find the occurrence of inherent traits with simple ratios and rules.

The value of Mendel's work was recognized posthumously when it was discovered again by scientists such as William Bateson and Hugo Marie de Vries. Bates coined the term genetics from genesis, which meant "origin" in classical Greek. The recovery of Gregor Mendel's work led to numerous scientific explorations, including the attempts to discover the cellular molecules that carry out the inheritance process. Nettie Stevens initiated her studies on mealworms in 1900. Her investigations led to the finding that male chromosomes influence sex in organisms. Through his observations of white-eye mutations found in certain fruit flies conducted in 1911, Thomas Hunt inferred that genes exist in chromosomes.

Molecular genetics definition

Early definitions

Early molecular genetics definition formed in the 18th century when scientists discovered the chromosomes to be the carrier of genes. However, chromosomes contain DNA and protein, and the molecules behind inheritance are not available yet. Scientist Frederick Griffith found out about the occurrence of transformation in 1928. Griffith's experiment found that dead bacteria could influence and " transform" other living bacteria by transferring genetic material. Later on, the McCarty-MacLeod-Avery experiment of 1944 ensued in the finding that DNA is the molecule inflicting transformation in bacteria. This led to the formation of the transformation genetics definition. With Chase's and Hershey's experiments conducted in 1952, it became certain that DNA, not protein, was responsible for genetic influence in organisms.

In 1953, Francis Crick and James Watson discovered DNA's structure to be helical (similar to the shape of a corkscrew) through the crystallography work done by Maurice Wilkins and Rosalind Franklin. In the double-helix DNA model, there were two DNA strands with pairs of nucleotides pointing towards the inside, complementing each other, arranged like ladder rungs. This arrangement of nucleotides suggested how reconstruction occurs seamlessly in DNAs, replicating the former sequences and replacing them with matching strands.

Modern definitions

The discovery of DNA's structure explained the process of inheritance. But, the aspect yet of being known was how DNAs influence cell behaviour in organisms. In the years that followed, scientists pursued learning the protein production process controlled by DNAs. According to recent research, single-stranded RNAs, which were much similar to DNAs, formed amino acid sequences in protein from nucleotide sequences. This process of translating nucleotide sequences to amino acids is called genetic coding.

With the discovery, understanding molecules and the inheritance process took a great step. They resulted in an eruption of studies in the field. Among the prominent theories that arose during this period include Tomoko Ohta's theory of neutrality in the evolution of molecules (1973). In her work, Ohta talks about the necessity of the environment and natural selection to match the pace of genetic evolution. Other major developments in the 1970s included Frederick Sanger's DNA sequencing work in 1977, through the chain-termination method. With this method, scientists could look through the sequences of nucleotides in DNA.

In 2003, the field of genetics took a large step ahead with human genome sequencing through the endeavours of the Energy Department, Project for Human Genome (HGP), and NIH with the support of private firms such as Celera Genomics.

Chromosomes and DNAs

DNA or deoxyribonucleic acid is the molecular agent or the reason behind genes and genetic activity. DNA comprises sequences of nucleotides, comprising four kinds: adenine, cytosine, guanine, and thymine, usually denoted by their respective first letters (A, C, G, and T). In these nucleotide sequences lies genetic information, and along the chains of DNA, genes comprise the expanses of such sequences. But in viruses, the arrangement differs rather from DNA; RNA exists as genetic material in them. Viruses are not living beings since they can only reproduce inside a host and cannot carry out most genetic processes.

DNAs normally exist as double-stranded molecules in loops of double helix structures. Each nucleotide in DNAs favourably pairs with their partner nucleotides in the second strand- while 'A's pair with 'T's, 'C's pair with 'G's. Hence, both strands carry all essential genetic information in a two-stranded structure. This arrangement of DNA acts as the base for inheritance. DNA replication copies the information in the strands by splitting them and recording the information in each strand to synthesize a new partner strand.

In many species, some sex-chromosomes function as the determinant of the organism's gender. In several animals, including human beings, the chromosome 'Y' acts as the triggering factor for the formation of male characteristics. With evolution, the chromosome 'Y' gradually lost many genes and content. In contrast, chromosome 'X' remains akin to the rest of the chromosomes and consists of many genes. The aspects above are the base for the evolutionary genetics definition.

Reproduction

According to a recent cellular genetics definition, during a cell division, the entire genome gets replicated. And each daughter cell inherits a copy of it. Scientists call this process mitosis, and the most basic form of cellular reproduction is asexual reproduction. Certain multicellular organisms also reproduce asexually, giving birth to offspring that acquire their genomes from only one parent. When an offspring is identical to its progenitor genetically, we call it a clone.

Most eukaryotic organisms take part in sexual reproduction and generate progenies that consist of a combination of the genomes of both parents. The sexual reproduction in them occurs alternatively between the genome's single replica (haploid) and double replica (diploid). Haploid cells come together from combined genetic material and create a double copy or diploid with paired chromosomes. Diploid organisms divide to form haploids. They do not replicate their DNA but create offspring cells that arbitrarily inherit one from each chromosome pair. Most animals and plants exist in the diploid form, while single-celled gametes like sperms and eggs in the form of haploid.

Genetic change

Mutations

Sometimes errors can occur during the DNA replication process in the second strand's polymerization. These errors or anomalies are mutations. They can alter an organism, phenotype-especially if errors exist in the protein-coding sequences of the gene. Usually, error rates are very low- for every ten to a hundred million bases, only one error occurs- owing to the DNA polymerases' skill to "proofread".

Mutagenic processes are the activities that increase the chance of mutation in DNA. Mutagenic chemicals boost the chance of errors occurring during DNA replication, usually by causing interference in the arrangement of base-pairing. At the same time, ultra-violet radiations induce mutations by damaging the DNA's structure. Damages are not only inflicted on the DNAs artificially, as sometimes chemical damages occur naturally in DNAs. Through some mechanisms, the cells repair the mismatched and broken pairs. But this mechanism does not allow the restoration of pairs into their original sequence. Among the major sources of DNA damage include ROS (chemical molecules highly reactive to oxygen), formed by the aerobic respiration of cells, which can cause mutations.

Some organisms employ chromosomal crossover methods for exchanging DNAs and recombining the genes. In such organisms, mutations can form from the misalignment during meiosis. The chances of errors occurring while crossing over are higher if the partner chromosomes with similar sequences adopt a wrong alignment; this causes certain areas of genomes to mutate at a higher rate. Such errors can inflict large amounts of changes in the DNA sequence's structure-deletions, duplications, inversions of an entire region-or the mistaken exchange of entire sequence parts between various chromosomes. Such an occurrence is called a chromosomal translocation.

Evolution and natural selection

Mutations cause alterations in the genotype of organisms, which can sometimes lead to the formation of different phenotypes. Most mutations have negligible influence on a creature's health, reproductive fitness, and phenotype. Those that do have the ability to influence are usually harmful, but on some occasions, some mutations can act positively. Studies conducted on Drosophila melanogaster fly show that mutations that alter the proteins generated by genes are prone to be harmful at 70 per cent.

Population genetics concerns the genetic difference distributions within a collection of organisms or populations and the changes in these distributions over time. Alterations in the allele frequency in populations are mainly due to natural selection when a particular allele forms an organism's reproductive or genetic advantage. Other factors for altering allele frequency include genetic drift, mutation, artificial selection, genetic hitch-hiking, and migration.

Genomes of a species can have significant alterations after years of allele alteration, which gradually results in evolution. To adapt to a new climate, beneficial mutations are a must, and the species gradually adapt to its environment. The formation of new species occurs through the speciation process. This is when the members of a species get geographically part ways and do not exchange their genes.