Genetics (from the Greek γεννάω gennao, to beget or produce) is the science of genes, heredity, and the variation of organisms. The word "genetics" was first suggested to describe the study of inheritance and the science of variation by the prominent British scientist William Bateson  in a personal letter to Adam Sedgwick, dated April 18, 1905. Bateson first used the term "genetics" publicly at the Third International Conference on Genetics (London, England) in 1906.
Heredity and variations form the basis of genetics. Humans applied knowledge of genetics in prehistory with the domestication and breeding of plants and animals. In modern research, genetics provides important tools for the investigation of the function of a particular gene, e.g., analysis of genetic interactions. Within organisms, genetic information generally is carried in chromosomes, where it is represented in the chemical structure of particular DNA (deoxyribonucleic acid) molecules.
Genes encode the information necessary for synthesizing the amino-acid sequences in proteins, which in turn play a large role in determining the final phenotype, or physical appearance, of the organism. In diploid organisms, a dominant allele on one chromosome will mask the expression of a recessive gene on the other.
The phrase to code for is often used to mean a gene contains the instructions about how to build a particular protein, as in the gene codes for the protein. The "one gene, one protein" concept is now known to be simplistic. For example, a single gene may produce multiple products, depending on how its transcription is regulated. Genes code for the nucleotide sequences in mRNA, tRNA and rRNA, required for protein synthesis.
Genetics determines much (but not all) of the appearance of organisms, including humans, and possibly how they act. Environmental differences and random factors also play a part. Monozygotic ("identical") twins, a clone resulting from the early splitting of an embryo, have the same DNA, but different personalities and fingerprints. Genetically-identical plants grown in colder climates incorporate shorter and less-saturated fatty acids to avoid stiffness.
Gregor Mendel studied the basis of inheritance of simple traits such as flower color in pea plants. In his paper "Versuche über Pflanzenhybriden" ("Experiments in Plant Hybridization"), presented in 1865 to the Brunn Natural History Society, Mendel traced the inheritance patterns of certain traits in pea plants and showed that they could be described mathematically. Although not all features show these patterns of Mendelian inheritance, his work suggested the utility of the application of statistics to the study of inheritance. Since that time many more complex forms of inheritance have been demonstrated. The significance of Mendel's work was not understood until early in the twentieth century, after his death, when his research was re-discovered by other scientists working on similar problems.
Mendel's laws of inheritance hold even though they were formulated well before the actual molecules involved in heredity were known. Scientists now know that the alleles described by Mendel are physically real; each allele is a certain version of a gene, which in turn is a string of DNA molecules within our cell. (In contrast, retroviruses, including influenza, oncoviruses and HIV, and many plant viruses, carry information as RNA.) Manipulation of DNA can in turn alter the inheritance and features of various organisms. An allele becomes a trait when the gene expresses itself, i.e., when it is translated into a protein. The dominance of some alleles that Mendel observed is caused when certain alleles are more efficiently expressed than others. Mendel predicted that genes, or "heritable factors" as he called them, could order themselves randomly in the creation of gametes; this random assortment is now recognized as an important stage in meiotic cell divisions.
As an example of modern genetic's later verification of Mendel's discoveries, his law of independent assortment states that the two heritable factors responsible for a trait will separate into gametes independently of anything else. It is now understood that random assortment occurs in meiosis because there are two copies of each chromosome, each composed of two sister chromatids, and chance decides which particular gamete receives each chromatid.
Mendel's conclusions are applicable to any traits that are either present or absent in an individual, rather than varied along a continuum. But simple modifications can be made to Mendelian genetics to account for the existence of multiple alleles (for instance, in the coding for blood type), co-dominance and partial dominance, and sex-linked traits. Traits that are controlled by an interaction of different genes such as human eye color, however, are too complex to be explained using Mendelian logic.
Timeline of notable discoveries
- 1859 Charles Darwin publishes The Origin of Species
- 1865 Gregor Mendel's paper, Experiments on Plant Hybridization
- 1869 Friedrich Miescher discovers a weak acid in the nuclei of white blood cells that today we call DNA (Hartl and Jones).
- 1903 Chromosomes are discovered to be hereditary units
- 1906 The term "genetics" is first introduced publicly by the British biologist William Bateson at the Third International Conference on Genetics in London, England.
- 1910 Thomas Hunt Morgan shows that genes reside on chromosomes, and discovered linked genes on chromosomes that do not follow Mendel's law of independent allele segregation
- 1913 Alfred Sturtevant makes the first genetic map of a chromosome
- 1913 Gene maps show chromosomes containing linear arranged genes
- 1918 Ronald Fisher publishes On the correlation between relatives on the supposition of Mendelian inheritance - the modern synthesis starts.
- 1927 Physical changes in genes are called mutations
- 1928 Frederick Griffith discovers a hereditary molecule that is transmissible between bacteria (see Griffiths experiment)
- 1931 Crossing over is the cause of recombination (see Barbara McClintock and cytogenetics)
- 1941 Edward Lawrie Tatum and George Wells Beadle show that genes code for proteins; see the original central dogma of genetics
- 1944 Oswald Theodore Avery, Colin McLeod and Maclyn McCarty isolate DNA as the genetic material (at that time called transforming principle)
- 1950 Erwin Chargaff shows that the four nucleotides are not present in nucleic acids in stable proportions, but that some general rules appear to hold (e.g., the nucleotide bases Adenine-Thymine and Cytosine-Guanine always remain in equal proportions). Barbara McClintock discovers transposons in maize
- 1952 The Hershey-Chase experiment proves the genetic information of phages (and all other organisms) to be DNA
- 1953 DNA structure is resolved to be a double helix by James D. Watson and Francis Crick, with the help of Rosalind Franklin
- 1956 Jo Hin Tjio and Albert Levan established the correct chromosome number in humans to be 46
- 1958 The Meselson-Stahl experiment demonstrates that DNA is semiconservatively replicated
- 1961 The genetic code is arranged in triplets
- 1964 Howard Temin showed using RNA viruses that Crick's central dogma is not always true
- 1970 Restriction enzymes were discovered in studies of a bacterium, Haemophilus influenzae, enabling scientists to cut and paste DNA
- 1977 DNA is sequenced for the first time by Fred Sanger, Walter Gilbert, and Allan Maxam working independently. Sanger's lab complete the entire genome of sequence of Bacteriophage Φ-X174;.
- 1983 Kary Banks Mullis invents the polymerase chain reaction enabling the easy amplification of DNA
- 1985 Alec Jeffreys discovers genetic finger printing.
- 1989 The first human gene is sequenced by Francis Collins and Lap-Chee Tsui. It encodes the CFTR protein. Defects in this gene cause cystic fibrosis
- 1995 The genome of Haemophilus influenzae is the first genome of a free living organism to be sequenced
- 1996 Saccharomyces cerevisiae is the first eukaryote genome sequence to be released
- 1998 The first genome sequence for a multicellular eukaryote, C. elegans is released
- 2001 First draft sequences of the human genome are released simultaneously by the Human Genome Project and Celera Genomics.
- 2003 (14 April) Successful completion of Human Genome Project with 99% of the genome sequenced to a 99.99% accuracy 
- 2006 Marcus Pembrey and Olov Bygren publish Sex-specific, male-line transgenerational responses in humans, a proof of epigenetics. 
Areas of genetics
Classical genetics consists of the techniques and methodologies of genetics that predate the advent of molecular biology. After the discovery of the genetic code and such tools of cloning as restriction enzymes, the avenues of investigation open to geneticists were greatly broadened. Some classical genetic ideas have been supplanted with the mechanistic understanding brought by molecular discoveries, but many remain intact and in use, such as Mendel's laws. Patterns of inheritance still remain a useful tool for the study of genetic diseases.
Behavioral genetics studies the influence of varying genetics on animal behavior. Behavioral genetics studies the effects of human disorders as well as its causes. Behavioral genetics has yielded some very interesting questions about the evolution of various behaviors, and even some fundamental principles of evolution in general. For example, guppies and meerkats seem to be genetically driven to post a lookout to watch for predators. This lookout stands a significantly slimmer chance of survival than the others, so because of the mechanism of natural selection, it would seem that this trait would be lost after a few generations. However, the gene has remained, leading evolutionary philosopher/scientists such as Richard Dawkins and W. D. Hamilton to propose explanations, including the theories of kin selection and reciprocal altruism. The interactions and behaviors of gregarious creatures is partially genetic in cause and must therefore be approached by evolutionary theory.
Clinical genetics is also the study of genetic causes of clinical diseases.
Molecular genetics builds upon the foundation of classical genetics but focuses on the structure and function of genes at a molecular level. Molecular genetics employs the methods of both classical genetics (such as hybridization) and molecular biology. It is so-called to differentiate it from other sub fields of genetics such as ecological genetics and population genetics. An important area within molecular genetics is the use of molecular information to determine the patterns of descent, and therefore the correct scientific classification of organisms: this is called molecular systematics. The study of inherited features not strictly associated with changes in the DNA sequence is called epigenetics.
Some take the view that life can be defined, in molecular terms, as the set of strategies which RNA polynucleotides have used and continue to use to perpetuate themselves. This definition grows out of work on the origin of life, specifically the RNA world hypothesis.
Population, quantitative and ecological genetics
Population, quantitative and ecological genetics are all very closely related subfields and also build upon classical genetics (supplemented with modern molecular genetics). They are chiefly distinguished by a common theme of studying populations of organisms drawn from nature but differ somewhat in the choice of which aspect of the organism on which they focus. The foundational discipline is population genetics which studies the distribution of and change in allele frequencies of genes under the influence of the four evolutionary forces: natural selection, genetic drift, mutation and migration. It is the theory that attempts to explain such phenomena as adaptation and speciation.
The related subfield of quantitative genetics, which builds on population genetics, aims to predict the response to selection given data on the phenotype and relationships of individuals. A more recent development of quantitative genetics is the analysis of quantitative trait loci. Traits that are under the influence of a large number of genes are known as quantitative traits, and their mapping to a location on the chromosome requires accurate phenotypic, pedigree and marker data from a large number of related individuals.
Ecological genetics again builds upon the basic principles of population genetics but is more explicitly focused on ecological issues. While molecular genetics studies the structure and function of genes at a molecular level, ecological genetics focuses on wild populations of organisms, and attempts to collect data on the ecological aspects of individuals as well as molecular markers from those individuals.
Population genetics is closely linked with the methods of genetic epidemiology. One method to study gene-disease associations is using the principle of Mendelian randomization.
A more recent development is the rise of genomics, which attempts the study of large-scale genetic patterns across the genome for (and in principle, all the DNA in) a given species. Genomics depends on the availability of whole genome sequences, and computational tools developed in the field of bioinformatics for analysis of large set of data.
The science which grew out of the union of biochemistry and genetics is widely known as molecular biology. The term "genetics" is often conflated with the notion of genetic engineering, where the DNA of an organism is modified for some kind of practical end, but most research in genetics is aimed at understanding and explaining the effect of genes on phenotypes and in the role of genes in populations (see population genetics and ecological genetics). Genetic engineering is used as one experimental tool in many areas of genetics to assist ivestigations of how genes work inside organisms.
Based on the principles of genetic mutations, computational methods called genetic algorithms have been developed for optimizing search functions.
- Daniel Hartl and Elizabeth Jones. (2006). "Genetics: Analysis of Genes and Genomes": 2.
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