Genetics

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Genetics (from the Greek genno γεννώ = give birth) is the science of genes, heredity, evolution, and the variation of organisms. The phenomenon of inheritance has been implicitly utilized in breeding of organisms and selection for desired traits, and the scientific field of genetics seeks to understand the mechanisms of inheritance.

The genetic information of organisms is contained within the chemical structure of DNA (deoxyribonucleic acid) molecules. Individually inherited traits, corresponding to regions in the DNA sequence, are called genes. Genes encode the information necessary for synthesizing RNA and proteins -- complex molecules generally responsible for enzymatic reactions, synthesis, communication and structure within a cell. DNA sequence is transcribed into an intermediate molecule called "messenger RNA", and ribosomes translate this sequence to form a chain of amino acids to form a protein. This process is known as the central dogma of molecular biology.

Although genetics plays a large role in determining the appearance and behavior of organisms, it is the interaction of genetics with the environment that determines the ultimate outcome. Thus, while identical twins have the same DNA and genes, differences in their experiences during development and childhood results in different personalities and fingerprints.

 

History

Main article: History of genetics
Morgan's observation of sex-linked inheritance of a mutation causing white eyes in Drosophila led him to the hypothesis that genes are located upon chromosomes.
Morgan's observation of sex-linked inheritance of a mutation causing white eyes in Drosophila led him to the hypothesis that genes are located upon chromosomes.

Gregor Johann Mendel, a German-Czech Augustinian monk and scientist, is often called the "father of modern genetics", a title given to him due to his early work on the heredity of plants. In his paper "Versuche über Pflanzenhybriden" ("Experiments on Plant Hybridization"), presented in 1865 to the Brunn Natural History Society, Gregor Mendel traced the inheritance patterns of certain traits in pea plants and showed that they could be described mathematically.[3] 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.

The significance of Mendel's observations 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. The word "genetics" itself was coined by William Bateson, a significant proponent of Mendel's work, in a letter to Adam Sedgwick, dated April 18, 1905.[4] Bateson promoted the term "genetics" publicly in his inaugural address to the Third International Conference on Plant Hybridization (London, England) in 1906.[5]

In the decades following rediscovery and popularization of Mendel's work, numerous experiments sought to elucidate the molecular basis of DNA. In 1910 Thomas Hunt Morgan argued that genes reside on chromosomes, based observations of a sex-linked white eye mutation in fruit flies. In 1913 his student Alfred Sturtevant used the phenomenon of genetic linkage and the associated recombination rates to demonstrate and map the linear arrangement of genes upon the chromosome.

The chemical structure of DNA.
The chemical structure of DNA.

Although chromosomes were known to contain genes, chromosomes were composed of both protein and DNA -- it was unknown which was critical for heredity or how the process occurred. In 1928, Frederick Griffith published his discovery of the phenomenon of transformation (see Griffith's experiment); sixteen years later, in 1944, Oswald Theodore Avery, Colin McLeod and Maclyn McCarty used this phenomenon to isolate and identify the molecule responsible for transformation as DNA[6]. The Hershey-Chase experiment in 1952 identified DNA (rather than protein) as the genetic material of viruses, further evidence that DNA was the molecule responsible for inheritance.

James D. Watson and Francis Crick resolved the structure of DNA in 1953, using X-ray crystallography information that indicated the molecule had a helical structure. Their double-helix model paired a sequence of nucleotides with a "complement" on the other strand. This structure not only provided a physical explanation for information, contained within the order of the nucleotides, but also a physical mechanism for duplication through separation of strands and the reconstruction of a partner strand based on the nucleotide pairings. They famously observed this in their paper, stating: "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."

In the following decades, an explosion of research based on this understanding of the molecular nature of DNA became possible. The development of DNA sequencing in 1977 enabled the determination of nucleotide sequences on DNA,[7] and the PCR method developed by Kary Banks Mullis in 1983 allowed the isolation and amplification of arbitrary segments of DNA. These and other techniques, through the pooled efforts of the Human Genome Project and parallel private effort by Celera Genomics, culminated in the sequencing of the human genome in 2001.

Timeline of notable discoveries

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 and Muller's morphs. Patterns of inheritance still remain a useful tool for the study of genetic diseases.

Behavioral genetics

Main article: Behavioral genetics

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

Main article: Clinical genetics

Physicians who are trained as Geneticists diagnose, treat, and counsel patients with genetic disorders or syndromes. These doctors are typically trained in a genetics residency and/or fellowship.

Clinical genetics is also the study of genetic causes of clinical 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.

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.

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. The field typically depends on the availability of whole genome sequences, computational tools and Sequence profiling tool using bioinformatics approaches for analysis of large sets 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.

References

  1. ^ a b Daniel Hartl and Elizabeth Jones (2005). Genetics: Analysis of Genes and Genomes, 6th edition. Jones & Bartlett.  854 pages. ISBN 0-7637-1511-5.
  2. ^ Robert C. King, Willliam D. Stansfield, Pamela K. Mulligan (2006). A Dictionary of Genetics, 7th edition. New York: Oxford University Press.  596 pages. ISBN 0-19-530761-5 (paper).
  3. ^ a b Mendel, G.. "Versuche über Pflanzen-Hybriden". Verh. Naturforsch. Ver. Brünn 4: 3-47.  (in English in 1901, J. R. Hortic. Soc. 26: 1–32)
  4. ^ Online copy of William Bateson's letter to Adam Sedgwick
  5. ^ a b Bateson, William (1907). "The Progress of Genetic Research". Wilks, W. (editor) Report of the Third 1906 International Conference on Genetics: Hybridization (the cross-breeding of genera or species), the cross-breeding of varieties, and general plant breeding, London: Royal Horticultural Society. 
    Although the conference was titled "International Conference on Hybridisation and Plant Breeding", Wilks changed the title for publication as a result of Bateson's speech.
  6. ^ a b Avery, MacLeod, and McCarty (1944). "Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types: Induction of Transformation by a Desoxyribonucleic Acid Fraction Isolated from Pneumococcus Type III". Journal of Experimental Medicine 79 (1): 137-58. 35th anniversary reprint available
  7. ^ a b Sanger F, Air GM, Barrell BG, Brown NL, Coulson AR, Fiddes CA, Hutchison CA, Slocombe PM, Smith M., Nucleotide sequence of bacteriophage phi X174 DNA, Nature. 1977 Feb 24;265(5596):687-94
  8. ^ Ernest W. Crow and James F. Crow (2002). "100 Years Ago: Walter Sutton and the Chromosome Theory of Heredity". Genetics 160. 
  9. ^ Beadle GW, Tatum EL (1941). "Genetic control of biochemical reactions in neurospora". PNAS 27: 499-506. 
  10. ^ Watson JD and Crick FH (1953). "Molecular structure of nucleic acids; a structure for deoxyribose nucleic acid". Nature 171 (4356): 737-8. 
  11. ^ Meselson, M. and Stahl, F.W. (1958). "The Replication of DNA in Escherichia coli". PNAS 44: 671-82. 
  12. ^ Min Jou W, Haegeman G, Ysebaert M, Fiers W. (1972). "Nucleotide sequence of the gene coding for the bacteriophage MS2 coat protein". Nature 237 (5350): 82-8. 
  13. ^ Fiers W et al. (1976). "Complete nucleotide-sequence of Bacteriophage MS2-RNA - primary and secondary structure of replicase gene". Nature 260: 500-507. 
  14. ^ http://www.genoscope.cns.fr/externe/English/Actualites/Presse/HGP/HGP_press_release-140403.pdf

See also

Journals

External link

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