Genetics

Genetics

Genetics is the study of

assort independently or be separated from their homologous pair during sexual reproduction wherein haploid gametes are formed. In this way new combinations of genes can occur in the offspring of a mating pair. Genes on the same chromosome would theoretically never recombine. However, they do via the cellular process of chromosomal crossover. During crossover, chromosomes exchange stretches of DNA, effectively shuffling the gene alleles between the chromosomes.[50] This process of chromosomal crossover generally occurs during meiosis, a series of cell divisions that creates haploid cells.

The probability of chromosomal crossover occurring between two given points on the chromosome is related to the distance between the points. For an arbitrarily long distance, the probability of crossover is high enough that the inheritance of the genes is effectively uncorrelated.[51] For genes that are closer together, however, the lower probability of crossover means that the genes demonstrate genetic linkage; alleles for the two genes tend to be inherited together. The amounts of linkage between a series of genes can be combined to form a linear linkage map that roughly describes the arrangement of the genes along the chromosome.[52]

Gene expression

Genetic code

The genetic code: Using a triplet code, DNA, through a messenger RNA intermediary, specifies a protein.

Genes generally express their functional effect through the production of proteins, which are complex molecules responsible for most functions in the cell. Proteins are made up of one or more polypeptide chains, each of which is composed of a sequence of amino acids, and the DNA sequence of a gene (through an RNA intermediate) is used to produce a specific amino acid sequence. This process begins with the production of an RNA molecule with a sequence matching the gene's DNA sequence, a process called transcription.

This messenger RNA molecule is then used to produce a corresponding amino acid sequence through a process called translation. Each group of three nucleotides in the sequence, called a codon, corresponds either to one of the twenty possible amino acids in a protein or an instruction to end the amino acid sequence; this correspondence is called the genetic code.[53] The flow of information is unidirectional: information is transferred from nucleotide sequences into the amino acid sequence of proteins, but it never transfers from protein back into the sequence of DNA—a phenomenon Francis Crick called the central dogma of molecular biology.[54]

The specific sequence of amino acids results in a unique three-dimensional structure for that protein, and the three-dimensional structures of proteins are related to their functions.[55][56] Some are simple structural molecules, like the fibers formed by the protein collagen. Proteins can bind to other proteins and simple molecules, sometimes acting as enzymes by facilitating chemical reactions within the bound molecules (without changing the structure of the protein itself). Protein structure is dynamic; the protein hemoglobin bends into slightly different forms as it facilitates the capture, transport, and release of oxygen molecules within mammalian blood.

A single nucleotide difference within DNA can cause a change in the amino acid sequence of a protein. Because protein structures are the result of their amino acid sequences, some changes can dramatically change the properties of a protein by destabilizing the structure or changing the surface of the protein in a way that changes its interaction with other proteins and molecules. For example, sickle-cell anemia is a human genetic disease that results from a single base difference within the coding region for the β-globin section of hemoglobin, causing a single amino acid change that changes hemoglobin's physical properties.[57] Sickle-cell versions of hemoglobin stick to themselves, stacking to form fibers that distort the shape of red blood cells carrying the protein. These sickle-shaped cells no longer flow smoothly through blood vessels, having a tendency to clog or degrade, causing the medical problems associated with this disease.

Some genes are transcribed into RNA but are not translated into protein products—such RNA molecules are called non-coding RNA. In some cases, these products fold into structures which are involved in critical cell functions (e.g. ribosomal RNA and transfer RNA). RNA can also have regulatory effect through hybridization interactions with other RNA molecules (e.g. microRNA).

Nature and nurture

Siamese cats have temperature-sensitive pigment-production mutation.

Although genes contain all the information an organism uses to function, the environment plays an important role in determining the ultimate phenotypes an organism displays. This is the complementary relationship often referred to as "Siamese cat. In this case, the body temperature of the cat plays the role of the environment. The cat's genes code for dark hair, thus the hair producing cells in the cat make cellular proteins resulting in dark hair. But these dark hair-producing proteins are sensitive to temperature (i.e. have a mutation causing temperature-sensitivity) and denature in higher-temperature environments, failing to produce dark-hair pigment in areas where the cat has a higher body temperature. In a low-temperature environment, however, the protein's structure is stable and produces dark-hair pigment normally. The protein remains functional in areas of skin that are colder – such as its legs, ears, tail and face – so the cat has dark-hair at its extremities.[58]

Environment plays a major role in effects of the human genetic disease phenylketonuria.[59] The mutation that causes phenylketonuria disrupts the ability of the body to break down the amino acid phenylalanine, causing a toxic build-up of an intermediate molecule that, in turn, causes severe symptoms of progressive mental retardation and seizures. However, if someone with the phenylketonuria mutation follows a strict diet that avoids this amino acid, they remain normal and healthy.

A popular method in determining how genes and environment ("nature and nurture") contribute to a phenotype is by studying identical and fraternal twins or siblings of multiple births.[60] Because identical siblings come from the same zygote, they are genetically the same. Fraternal siblings are as genetically different from one another as normal siblings. By analyzing statistics on how often a twin of a set has a certain disorder compared other sets of twins, scientists can determine whether that disorder is caused by genetic or environmental factors (i.e. whether it has 'nature' or 'nurture' causes). One famous example is the multiple birth study of the Genain quadruplets, who were identical quadruplets all diagnosed with schizophrenia.[61]

Gene regulation

The genome of a given organism contains thousands of genes, but not all these genes need to be active at any given moment. A gene is expressed when it is being transcribed into mRNA and there exist many cellular methods of controlling the expression of genes such that proteins are produced only when needed by the cell. Transcription factors are regulatory proteins that bind to DNA, either promoting or inhibiting the transcription of a gene.[62] Within the genome of Escherichia coli bacteria, for example, there exists a series of genes necessary for the synthesis of the amino acid tryptophan. However, when tryptophan is already available to the cell, these genes for tryptophan synthesis are no longer needed. The presence of tryptophan directly affects the activity of the genes—tryptophan molecules bind to the tryptophan repressor (a transcription factor), changing the repressor's structure such that the repressor binds to the genes. The tryptophan repressor blocks the transcription and expression of the genes, thereby creating negative feedback regulation of the tryptophan synthesis process.[63]

Transcription factors bind to DNA, influencing the transcription of associated genes.

Differences in gene expression are especially clear within intercellular signals and gradually establishing different patterns of gene expression to create different behaviors. As no single gene is responsible for the development of structures within multicellular organisms, these patterns arise from the complex interactions between many cells.

Within eukaryotes, there exist structural features of chromatin that influence the transcription of genes, often in the form of modifications to DNA and chromatin that are stably inherited by daughter cells.[64] These features are called "epigenetic" because they exist "on top" of the DNA sequence and retain inheritance from one cell generation to the next. Because of epigenetic features, different cell types grown within the same medium can retain very different properties. Although epigenetic features are generally dynamic over the course of development, some, like the phenomenon of paramutation, have multigenerational inheritance and exist as rare exceptions to the general rule of DNA as the basis for inheritance.[65]

Genetic change

Mutations

Gene duplication allows diversification by providing redundancy: one gene can mutate and lose its original function without harming the organism.

During the process of DNA polymerases.[66][67] Processes that increase the rate of changes in DNA are called mutagenic: mutagenic chemicals promote errors in DNA replication, often by interfering with the structure of base-pairing, while UV radiation induces mutations by causing damage to the DNA structure.[68] Chemical damage to DNA occurs naturally as well and cells use DNA repair mechanisms to repair mismatches and breaks. The repair does not, however, always restore the original sequence.

In organisms that use chromosomal crossover to exchange DNA and recombine genes, errors in alignment during meiosis can also cause mutations.[69] Errors in crossover are especially likely when similar sequences cause partner chromosomes to adopt a mistaken alignment; this makes some regions in genomes more prone to mutating in this way. These errors create large structural changes in DNA sequence – duplications, inversions, deletions of entire regions – or the accidental exchange of whole parts of sequences between different chromosomes (chromosomal translocation).

Natural selection and evolution

Mutations alter an organism's genotype and occasionally this causes different phenotypes to appear. Most mutations have little effect on an organism's phenotype, health, or reproductive fitness.[70] Mutations that do have an effect are usually deleterious, but occasionally some can be beneficial.[71] Studies in the fly Drosophila melanogaster suggest that if a mutation changes a protein produced by a gene, about 70 percent of these mutations will be harmful with the remainder being either neutral or weakly beneficial.[72]

An orthologous gene sequences.

External links

  • Bruce Alberts; Dennis Bray; Karen Hopkin; Alexander Johnson; Julian Lewis; Martin Raff; Keith Roberts; Peter Walter (2013). Essential Cell Biology, 4th Edition. Garland Science.  
  • Griffiths, Anthony J. F.; Miller, Jeffrey H.; Suzuki, David T.; Lewontin, Richard C.; Gelbart, eds. (2000). An Introduction to Genetic Analysis (7th ed.). New York: W. H. Freeman.  
  • Hartl D, Jones E (2005). Genetics: Analysis of Genes and Genomes (6th ed.). Jones & Bartlett.  
  • King, Robert C; Mulligan, Pamela K; Stansfield, William D (2013). A Dictionary of Genetics (8th ed.). New York: Oxford University Press.  
  • Lodish H, Berk A, Zipursky LS, Matsudaira P, Baltimore D, and Darnell J (2000). Molecular Cell Biology (4th ed.). New York: Scientific American Books.  

Further reading

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  2. ^ Hartl D, Jones E (2005)
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  4. ^ "Genesis (γένεσις)". Henry George Liddell, Robert Scott, A Greek-English Lexicon. Perseus Digital Library, Tufts University. Retrieved 20 February 2012. 
  5. ^ "Genetic". Online Etymology Dictionary. Retrieved 20 February 2012. 
  6. ^ DK Publishing (2009). Science: The Definitive Visual Guide. Penguin. p. 362.  
  7. ^ Weiling, F (1991). "Historical study: Johann Gregor Mendel 1822–1884.". American journal of medical genetics 40 (1): 1–25; discussion 26.  
  8. ^ Matthew Hamilton (2011). Population Genetics. Georgetown University. p. 26.  
  9. ^ Lamarck, J-B (2008). In Encyclopædia Britannica. Retrieved from Encyclopædia Britannica Online on 16 March 2008.
  10. ^ Singer, Emily (4 February 2009). "A Comeback for Lamarckian Evolution?".  
  11. ^ Peter J. Bowler, The Mendelian Revolution: The Emergency of Hereditarian Concepts in Modern Science and Society (Baltimore: Johns Hopkins University Press, 1989): chapters 2 & 3.
  12. ^ a b Blumberg, Roger B. "Mendel's Paper in English". 
  13. ^ genetics, n., Oxford English Dictionary, 3rd ed.
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    Initially titled the "International Conference on Hybridisation and Plant Breeding", the title was changed as a result of Bateson's speech. See: Cock AG, Forsdyke DR (2008). Treasure your exceptions: the science and life of William Bateson. Springer. p. 248.  
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  33. ^ Griffiths, Anthony J. F.; Miller, Jeffrey H.; Suzuki, David T.; Lewontin, Richard C.; Gelbart, eds. (2000). "Mendel's experiments". An Introduction to Genetic Analysis (7th ed.). New York: W. H. Freeman.  
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References

See also

bioinformatics, which uses computational approaches to analyze large sets of biological data. A common problem to these fields of research is how to manage and share data that deals with human subject and personal identifiable information. See also genomics data sharing.

As sequencing has become less expensive, researchers have genome assembly, computational tools to stitch together sequences from many different fragments.[89] These technologies were used to sequence the human genome in the Human Genome Project completed in 2003.[31] New high-throughput sequencing technologies are dramatically lowering the cost of DNA sequencing, with many researchers hoping to bring the cost of resequencing a human genome down to a thousand dollars.[90]

DNA sequencing, one of the most fundamental technologies developed to study genetics, allows researchers to determine the sequence of nucleotides in DNA fragments. The technique of chain-termination sequencing, developed in 1977 by a team led by Frederick Sanger, is still routinely used to sequence DNA fragments.[88] Using this technology, researchers have been able to study the molecular sequences associated with many human diseases.

DNA sequencing and genomics

DNA can also be amplified using a procedure called the polymerase chain reaction (PCR).[87] By using specific short sequences of DNA, PCR can isolate and exponentially amplify a targeted region of DNA. Because it can amplify from extremely small amounts of DNA, PCR is also often used to detect the presence of specific DNA sequences.

The use of clones of bacteria cells). ("Cloning" can also refer to the various means of creating cloned ("clonal") organisms.)

DNA can be manipulated in the laboratory. Restriction enzymes are commonly used enzymes that cut DNA at specific sequences, producing predictable fragments of DNA.[86] DNA fragments can be visualized through use of gel electrophoresis, which separates fragments according to their length.

cellular cloning. A similar methodology is often used in molecular cloning.

Research methods

Normally, a cell divides only in response to signals called metastasis. Although there are some genetic predispositions in a small fraction of cancers, the major fraction is due to a set of new genetic mutations that originally appear and accumulate in one or a small number of cells that will divide to form the tumor and are not transmitted to the progeny (somatic mutations). The most frequent mutations are a loss of function of p53 protein, a tumor suppressor, or in the p53 pathway, and gain of function mutations in the ras proteins, or in other oncogenes.

Individuals differ in their inherited tendency to develop cancer,[84] and cancer is a genetic disease.[85] The process of cancer development in the body is a combination of events. Mutations occasionally occur within cells in the body as they divide. Although these mutations will not be inherited by any offspring, they can affect the behavior of cells, sometimes causing them to grow and divide more frequently. There are biological mechanisms that attempt to stop this process; signals are given to inappropriately dividing cells that should trigger cell death, but sometimes additional mutations occur that cause cells to ignore these messages. An internal process of natural selection occurs within the body and eventually mutations accumulate within cells to promote their own growth, creating a cancerous tumor that grows and invades various tissues of the body.

pharmacogenetics: the study of how genotype can affect drug responses.[83]

Schematic relationship between biochemistry, genetics and molecular biology.

Medicine

Organisms were chosen, in part, for convenience—short generation times and easy Escherichia coli, the plant Arabidopsis thaliana, baker's yeast (Saccharomyces cerevisiae), the nematode Caenorhabditis elegans, the common fruit fly (Drosophila melanogaster), and the common house mouse (Mus musculus).

Although geneticists originally studied inheritance in a wide range of organisms, researchers began to specialize in studying the genetics of a particular subset of organisms. The fact that significant research already existed for a given organism would encourage new researchers to choose it for further study, and so eventually a few gene regulation and the involvement of genes in development and cancer.

The model organism in genetics research.

Model organisms

Research and technology

By comparing the homology between different species' genomes, it is possible to calculate the evolutionary distance between them and when they may have diverged. Genetic comparisons are generally considered a more accurate method of characterizing the relatedness between species than the comparison of phenotypic characteristics. The evolutionary distances between species can be used to form evolutionary trees; these trees represent the common descent and divergence of species over time, although they do not show the transfer of genetic material between unrelated species (known as horizontal gene transfer and most common in bacteria).[79]

Over many generations, the genomes of organisms can change significantly, resulting in evolution. In the process called adaptation, selection for beneficial mutations can cause a species to evolve into forms better able to survive in their environment.[77] New species are formed through the process of speciation, often caused by geographical separations that prevent populations from exchanging genes with each other.[78] The application of genetic principles to the study of population biology and evolution is known as the "modern synthesis".

[76].migration and artificial selection [75],genetic draft, genetic drift, mutation as well as other factors such as [74]

Recombination and genetic linkage

Although they do not use the haploid/diploid method of sexual reproduction, bacteria have many methods of acquiring new genetic information. Some bacteria can undergo conjugation, transferring a small circular piece of DNA to another bacterium.[48] Bacteria can also take up raw DNA fragments found in the environment and integrate them into their genomes, a phenomenon known as transformation.[49] These processes result in horizontal gene transfer, transmitting fragments of genetic information between organisms that would be otherwise unrelated.

gametes such as sperm or eggs.

When cells divide, their full genome is copied and each clones.

Reproduction

is similar to the other chromosomes and contains many genes. The X and Y chromosomes form a strongly heterogeneous pair. X chromosome contains the gene that triggers the development of the specifically male characteristics. In evolution, this chromosome has lost most of its content and also most of its genes, while the Y chromosome In humans and many other animals, the [47] Many species have so-called

Walther Flemming's 1882 diagram of eukaryotic cell division. Chromosomes are copied, condensed, and organized. Then, as the cell divides, chromosome copies separate into the daughter cells.

While diploid, containing two of each chromosome and thus two copies of every gene.[34] The two alleles for a gene are located on identical loci of the two homologous chromosomes, each allele inherited from a different parent.

Genes are arranged linearly along long chains of DNA base-pair sequences. In genome.

DNA normally exists as a double-stranded molecule, coiled into the shape of a double helix. Each nucleotide in DNA preferentially pairs with its partner nucleotide on the opposite strand: A pairs with T, and C pairs with G. Thus, in its two-stranded form, each strand effectively contains all necessary information, redundant with its partner strand. This structure of DNA is the physical basis for inheritance: DNA replication duplicates the genetic information by splitting the strands and using each strand as a template for synthesis of a new partner strand.[44]

The molecular basis for genes is deoxyribonucleic acid (DNA). DNA is composed of a chain of nucleotides, of which there are four types: adenine (A), cytosine (C), guanine (G), and thymine (T). Genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence along the DNA chain.[42] Viruses are the only exception to this rule—sometimes viruses use the very similar molecule RNA instead of DNA as their genetic material.[43] Viruses cannot reproduce without a host and are unaffected by many genetic processes, so tend not to be considered living organisms.

The molecular structure of DNA. Bases pair through the arrangement of hydrogen bonding between the strands.

DNA and chromosomes

Molecular basis for inheritance

Many traits are not discrete features (e.g. purple or white flowers) but are instead continuous features (e.g. human height and heritability.[40] Measurement of the heritability of a trait is relative—in a more variable environment, the environment has a bigger influence on the total variation of the trait. For example, human height is a trait with complex causes. It has a heritability of 89% in the United States. In Nigeria, however, where people experience a more variable access to good nutrition and health care, height has a heritability of only 62%.[41]

Often different genes can interact in a way that influences the same trait. In the Blue-eyed Mary (Omphalodes verna), for example, there exists a gene with alleles that determine the color of flowers: blue or magenta. Another gene, however, controls whether the flowers have color at all or are white. When a plant has two copies of this white allele, its flowers are white—regardless of whether the first gene has blue or magenta alleles. This interaction between genes is called epistasis, with the second gene epistatic to the first.[38]

Organisms have thousands of genes, and in sexually reproducing organisms these genes generally assort independently of each other. This means that the inheritance of an allele for yellow or green pea color is unrelated to the inheritance of alleles for white or purple flowers. This phenomenon, known as "Mendel's second law" or the "Law of independent assortment", means that the alleles of different genes get shuffled between parents to form offspring with many different combinations. (Some genes do not assort independently, demonstrating genetic linkage, a topic discussed later in this article.)

Human height is a trait with complex genetic causes. Francis Galton's data from 1889 shows the relationship between offspring height as a function of mean parent height. While correlated, remaining variation in offspring heights indicates environment is also an important factor in this trait.

Multiple gene interactions

When studying human genetic diseases, geneticists often use pedigree charts to represent the inheritance of traits.[37] These charts map the inheritance of a trait in a family tree.

In fertilization and breeding experiments (and especially when discussing Mendel's laws) the parents are referred to as the "P" generation and the offspring as the "F1" (first filial) generation. When the F1 offspring mate with each other, the offspring are called the "F2" (second filial) generation. One of the common diagrams used to predict the result of cross-breeding is the Punnett square.

Geneticists use diagrams and symbols to describe inheritance. A gene is represented by one or a few letters. Often a "+" symbol is used to mark the usual, non-mutant allele for a gene.[36]

Genetic pedigree charts help track the inheritance patterns of traits.

Notation and diagrams

When a pair of organisms reproduce sexually, their offspring randomly inherit one of the two alleles from each parent. These observations of discrete inheritance and the segregation of alleles are collectively known as Mendel's first law or the Law of Segregation.

The set of alleles for a given organism is called its recessive as its qualities recede and are not observed. Some alleles do not have complete dominance and instead have incomplete dominance by expressing an intermediate phenotype, or codominance by expressing both alleles at once.[35]

In the case of pea, which is a heterozygous.

At its most fundamental level, inheritance in organisms occurs by passing discrete heritable units, called genes, from parents to progeny.[32] This property was first observed by Gregor Mendel, who studied the segregation of heritable traits in pea plants.[12][33] In his experiments studying the trait for flower color, Mendel observed that the flowers of each pea plant were either purple or white—but never an intermediate between the two colors. These different, discrete versions of the same gene are called alleles.

A Punnett square depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms.

Discrete inheritance and Mendel's laws

Features of inheritance

With the newfound molecular understanding of inheritance came an explosion of research.[28] One important development was chain-termination DNA sequencing in 1977 by Frederick Sanger. This technology allows scientists to read the nucleotide sequence of a DNA molecule.[29] In 1983, Kary Banks Mullis developed the polymerase chain reaction, providing a quick way to isolate and amplify a specific section of DNA from a mixture.[30] The efforts of the Human Genome Project, Department of Energy, NIH, and parallel private effort by Celera Genomics led to the sequencing of the human genome in 2003.[31]

Although the structure of DNA showed how inheritance works, it was still not known how DNA influences the behavior of cells. In the following years, scientists tried to understand how DNA controls the process of protein production.[26] It was discovered that the cell uses DNA as a template to create matching messenger RNA, molecules with nucleotides very similar to DNA. The nucleotide sequence of a messenger RNA is used to create an amino acid sequence in protein; this translation between nucleotide sequenced and amino acid sequences is known as the genetic code.[27]

James D. Watson and Francis Crick determined the structure of DNA in 1953, using the X-ray crystallography work of Rosalind Franklin and Maurice Wilkins that indicated DNA had a helical structure (i.e., shaped like a corkscrew).[22][23] Their double-helix model had two strands of DNA with the nucleotides pointing inward, each matching a complementary nucleotide on the other strand to form what looks like rungs on a twisted ladder.[24] This structure showed that genetic information exists in the sequence of nucleotides on each strand of DNA. The structure also suggested a simple method for replication: if the strands are separated, new partner strands can be reconstructed for each based on the sequence of the old strand. This property is what gives DNA its' semi-conservative nature where one strand of new DNA is from an original parent strand.[25]

Although genes were known to exist on chromosomes, chromosomes are composed of both protein and DNA, and scientists did not know which of these is responsible for inheritance. In 1928, Frederick Griffith discovered the phenomenon of transformation (see Griffith's experiment): dead bacteria could transfer genetic material to "transform" other still-living bacteria. Sixteen years later, in 1944, Oswald Theodore Avery, Colin McLeod and Maclyn McCarty identified DNA as the molecule responsible for transformation.[19] The role of the nucleus as the repository of genetic information in eukaryotes had been established by Hämmerling in 1943 in his work on the single celled alga Acetabularia.[20] The Hershey-Chase experiment in 1952 confirmed that DNA (rather than protein) is the genetic material of the viruses that infect bacteria, providing further evidence that DNA is the molecule responsible for inheritance.[21]

Molecular 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.

After the rediscovery of Mendel's work, scientists tried to determine which molecules in the cell were responsible for inheritance. In 1911, chromosomes, based on observations of a sex-linked white eye mutation in fruit flies.[17] In 1913, his student Alfred Sturtevant used the phenomenon of genetic linkage to show that genes are arranged linearly on the chromosome.[18]

The importance of Mendel's work did not gain wide understanding until the 1890s, after his death, when other scientists working on similar problems re-discovered his research. William Bateson, a proponent of Mendel's work, coined the word genetics in 1905.[13][14] (The adjective genetic, derived from the Greek word genesis—γένεσις, "origin", predates the noun and was first used in a biological sense in 1860.)[15] Bateson popularized the usage of the word genetics to describe the study of inheritance in his inaugural address to the Third International Conference on Plant Hybridization in London, England, in 1906.[16]

Modern genetics started with Gregor Johann Mendel, a scientist and Augustinian friar who studied the nature of inheritance in plants. In his paper "Versuche über Pflanzenhybriden" ("Experiments on Plant Hybridization"), presented in 1865 to the Naturforschender Verein (Society for Research in Nature) in Brünn, Mendel traced the inheritance patterns of certain traits in pea plants and described them mathematically.[12] Although this pattern of inheritance could only be observed for a few traits, Mendel's work suggested that heredity was particulate, not acquired, and that the inheritance patterns of many traits could be explained through simple rules and ratios.

Mendelian and classical genetics

Although the science of genetics began with the applied and theoretical work of Gregor Mendel in the mid-19th century, other theories of inheritance preceded Mendel. A popular theory during Mendel's time was the concept of blending inheritance: the idea that individuals inherit a smooth blend of traits from their parents.[8] Mendel's work provided examples where traits were definitely not blended after hybridization, showing that traits are produced by combinations of distinct genes rather than a continuous blend. Blending of traits in the progeny is now explained by the action of multiple genes with quantitative effects. Another theory that had some support at that time was the inheritance of acquired characteristics: the belief that individuals inherit traits strengthened by their parents. This theory (commonly associated with Jean-Baptiste Lamarck) is now known to be wrong—the experiences of individuals do not affect the genes they pass to their children,[9] although evidence in the field of epigenetics has revived some aspects of Lamarck's theory.[10] Other theories included the pangenesis of Charles Darwin (which had both acquired and inherited aspects) and Francis Galton's reformulation of pangenesis as both particulate and inherited.[11]

The observation that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding.[6] The modern science of genetics, seeking to understand this process, began with the work of Gregor Mendel in the mid-19th century.[7]

DNA, the molecular basis for biological inheritance. Each strand of DNA is a chain of nucleotides, matching each other in the center to form what look like rungs on a twisted ladder.

History

The sequence of nucleotides in a gene is read and translated by a cell to produce a chain of amino acids which in turn folds into a protein. The order of amino acids in a protein corresponds to the order of nucleotides in the gene. This relationship between nucleotide sequence and amino acid sequence is known as the genetic code. The amino acids in a protein determine how it folds into its unique three-dimensional shape, a structure that is ultimately responsible for the protein's function. Proteins carry out many of the functions needed for cells to live. A change to the DNA in a gene can change a protein's amino acid sequence, thereby changing its shape and function and rendering the protein ineffective or even malignant (e.g. sickle cell anemia). Changes to genes are called mutations.

A quick heuristic that is often used (but not always true) is "one gene, one protein" meaning a singular gene codes for a singular protein type in a cell (enzyme, transcription factor, etc.)

The modern working definition of a gene is a portion (or sequence) of DNA that codes for a known cellular function or process (i.e. the function "make melanin molecules"). A single 'gene' is most similar to a single 'word' in the English language. The nucleotides (molecules) that make up genes can be seen as 'letters' in the English language. A single gene may have a small number of nucleotides or a large number of nucleotides, in the same way that a word may be small or large (e.g. 'cell' vs. 'electrophysiology'). A single gene often interacts with neighboring genes to produce a cellular function and can even be ineffectual without those neighboring genes. This can be seen in the same way that a 'word' may have meaning only in the context of a 'sentence.' A series of nucleotides can be put together without forming a gene (non coding regions of DNA), like a string of letters can be put together without forming a word (e.g. udkslk). Nonetheless, all words have letters, like all genes must have nucleotides.

The Gene

The word genetics stems from the Ancient Greek γενετικός genetikos meaning "genitive"/"generative", which in turn derives from γένεσις genesis meaning "origin".[3][4][5]

Etymology

Contents

  • Etymology 1
  • The Gene 2
  • History 3
    • Mendelian and classical genetics 3.1
    • Molecular genetics 3.2
  • Features of inheritance 4
    • Discrete inheritance and Mendel's laws 4.1
    • Notation and diagrams 4.2
    • Multiple gene interactions 4.3
  • Molecular basis for inheritance 5
    • DNA and chromosomes 5.1
    • Reproduction 5.2
    • Recombination and genetic linkage 5.3
  • Gene expression 6
    • Genetic code 6.1
    • Nature and nurture 6.2
    • Gene regulation 6.3
  • Genetic change 7
    • Mutations 7.1
    • Natural selection and evolution 7.2
  • Research and technology 8
    • Model organisms 8.1
    • Medicine 8.2
    • Research methods 8.3
    • DNA sequencing and genomics 8.4
  • See also 9
  • References 10
  • Further reading 11
  • External links 12

Genetic processes work in combination with an organism's environment and experiences to influence development and behavior, often referred to as arid climate only grows to half the height of the one in the temperate climate, due to lack of water and nutrients in its environment.

Trait inheritance and bacteria, plants, animals, and humans.

The father of genetics is discrete "units of inheritance". This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene.

, but it intersects frequently with many of the life sciences and is strongly linked with the study of information systems. biology It is generally considered a field of [2][1]