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Genetics


Genetics (from the Greek genno γεννώ= give birth) is the science of genes, heredity, and the variation of organisms. The word genetics was first applied to describe the study of inheritance and the science of variation by English scientist William Bateson in a letter to Adam Sedgewick, dated April 18, 1905.

Humans began applying knowledge of genetics in prehistory with the domestication and breeding of plants and animals. In modern research, genetics provides important tools in 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 molecules.

Genes encode the information necessary for synthesizing proteins, which, in turn play a large role in influencing, although, in many instances, do not completely determine, the final phenotype of the organism. The phrase to code for is often used to mean a gene contains the instructions on how to build a particular protein, as in the gene codes for the protein. Note that 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.

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History

Main article: history of genetics

It was not until 1865 that Gregor Mendel first traced inheritance patterns of certain traits in pea plants and showed that they obeyed simple statistical rules. Although not all features show these patterns of Mendelian inheritance, his work acted as a proof that application of statistics to inheritance could be highly useful. Since that time many more complex forms of inheritance have been demonstrated.

From his statistical analysis Mendel defined a concept that he described as an allele, which was the fundamental unit of heredity. The term allele as Mendel used it is nearly synonymous with the term gene, whilst the term allele now means a specific variant of a particular gene.

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 was unaware of the physical nature of the gene. We now know that genetic information is normally carried on DNA. (Certain viruses store their genetic information in RNA). Manipulation of DNA can in turn alter the inheritance and features of various organisms.

Timeline of notable discoveries

1859 Charles Darwin publishes The Origin of Species
1865 Gregor Mendel's paper, Experiments on Plant Hybridization
1903 Chromosomes are discovered to be hereditary units
1905 British biologist William Bateson coins the term "genetics" in a letter to Adam Sedgwick
1910 Thomas Hunt Morgan shows that genes reside on chromosomes
1913 Alfred Sturtevant makes the first genetic map of a chromosome
1918 Ronald Fisher publishes On the correlation between relatives on the supposition of Mendelian inheritance - the modern synthesis starts.
1913 Gene maps show chromosomes containing linear arranged genes
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
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., that the amount of adenine, A, tends to be equal to that of thymine, T). 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
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 Watson's central dogma is not always true
1970 Restriction enzymes were discovered in studies of a bacterium, Haemophilius 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 discovers the polymerase chain reaction enabling the easy amplification of DNA
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 sequnce 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 [1]

Areas of genetics

Classical genetics

Main articles: Classical genetics, Mendelian inheritance

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 inheritence still remain a useful tool for the study of genetic diseases.

Molecular genetics

Main article: Molecular genetics

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

Main articles: Population genetics, Quantitative genetics, 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.

Genomics

Main article: Genomics

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 availabilty of whole genome sequences, and compuational tools developed in the field of bioinformatics for analysis of large set of data.

Closely-related fields

The science which grew out of the union of biochemistry and genetics is widely known as molecular biology. The term "genetics" is often widely 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), rather than genetic engineering.


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