Wikia

Psychology Wiki

Changes: Introduction to genetics

Edit

Back to page

(Other)
(update WP)
Line 1: Line 1:
 
{{BioPsy}}
 
{{BioPsy}}
   
'''Genetics''' (from the [[Greek language|Greek]] genno '''γεννώ'''= give birth) is the [[science]] of [[gene]]s, [[heredity]], and the [[variation]] of [[organism]]s. The word genetics was first suggested to describe the study of inheritance and the science of variation by the 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.
+
[[Image:ADN animation.gif|frame|150px|right|A section of [[DNA]], the sequence of the plate-like units ([[nucleotide]]s) in the center carries information.]]
   
[[Human]]s applied knowledge of genetics in prehistory with the [[domestication]] and [[selective breeding|breeding]] of [[plant]]s and [[animal]]s. In modern research, genetics provides important tools for the investigation of the function of a particular gene, e.g., analysis of [[genetic interactions]]. Within [[organism]]s, genetic information generally is carried in [[chromosome]]s, where it is represented in the [[DNA sequence|chemical structure]] of particular [[DNA]] [[molecule]]s.
+
{{Introductory article|Genetics}}
   
[[Gene]]s encode the information necessary for synthesizing the [[amino-acid]] sequences in [[protein]]s, 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.
+
'''Genetics''' studies how living [[organism]]s inherit many of the features of their ancestors – for example, children usually look and act like other people in their family. Genetics tries to identify which features are inherited, and work out the details of how these features are passed from generation to generation.
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 (genetics)|transcription]] is regulated. Genes code for the [[nucleotide]] sequences in [[messenger RNA|mRNA]], [[transfer RNA|tRNA]] and [[ribosomal RNA|rRNA]], required for protein synthesis.
 
   
Genetics determines much (but not all) of the appearance of organisms, including humans, and possibly how they act. [[Environment]]al differences and [[randomness|random]] factors also play a part. [[Twin|Monozygotic ("identical") twins]], a [[cloning|clone]] resulting from the early splitting of an embryo, have the same DNA, but different [[personality|personalities]] and [[fingerprint]]s. Genetically-identical plants grown in colder [[climate]]s incorporate shorter and less-saturated [[fatty acid]]s to avoid stiffness.
+
In genetics, a feature of an organism is called a "[[trait (biology)|trait]]". Some traits are features of an organism's [[morphology (biology)|physical appearance]], for example, a person's eye color, height or weight. There are many other types of traits and these range from aspects of behavior to resistance to disease. Traits are often inherited, for example tall and thin people tend to have tall and thin children. Other traits come from the interaction between inherited features and the environment. For example a child might inherit the tendency to be tall, but if there is very little food where they live and they are poorly nourished, they will still be short. The way genetics and environment interact to produce a trait can be complicated: for example, the chances of somebody dying of [[cancer]] or [[heart disease]] seem to depend on both their family history and their lifestyle.
   
==History==
+
Genetic information is carried by a long molecule called [[DNA]] and this DNA is copied and inherited across generations. Traits are carried in DNA as instructions for constructing and operating an organism. These instructions are contained in segments of DNA called [[gene]]s. DNA is made of a sequence of simple units, with the order of these units spelling out instructions in the [[genetic code]]. This is similar to the orders of letters spelling out words. The organism "reads" the sequence of these units and decodes the instruction.
{{main|History of genetics}}
 
   
In his paper "Versuche über Pflanzenhybriden" ("Experiments in 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. 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.
+
Not all the genes for a particular instruction are exactly the same. Different forms of one type of gene are called different [[allele]]s of that gene. As an example, one allele of a gene for hair color could carry the instruction to produce a lot of the pigment in black hair, while a different allele could give a garbled version of this instruction, so that no pigment is produced and the hair is white. [[Mutation]]s are random events that change the sequence of a gene and therefore create a new allele. Mutations can produce a new trait, such as turning an allele for black hair into an allele for white hair. The appearance of new traits is important in [[Introduction to evolution|evolution]].
  +
{{clear}}
   
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.
+
{{Introduction to genetics glossary}}
   
Mendel did not understand the nature of inheritance. We now know that some heritable information is carried in [[DNA]]. ([[Retrovirus]]es, including [[influenza]], [[oncovirus]]es and [[HIV]], and many [[plant virus]]es, carry information as [[RNA]].) Manipulation of DNA can in turn alter the inheritance and features of various organisms.
+
==Inheritance in biology==
===Timeline of notable discoveries===
+
===Genes and inheritance===
:[[1859]] [[Charles Darwin]] publishes ''[[The Origin of Species]]''
+
Genes are inherited as units, with parents dividing out their genes to their offspring. You can think of this process like mixing two hands of cards, shuffling them, and then dealing them out again. Humans have two copies of each of their genes and when people reproduce they make copies of their genes in eggs or sperm, but only put in one copy of each type of gene. An egg then joins with a sperm to give a child with a new set of genes. This child will have the same number of genes as its parents but for any gene one of their two copies will come from the father, and one from the mother.<ref name=Utah/>
:[[1865]] [[Gregor Mendel]]'s paper, ''Experiments on Plant Hybridization''
 
:[[1903]] [[Chromosome]]s 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
 
:[[1913]] [[Gene map]]s 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 [[mutation]]s
 
:[[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 [[protein]]s; 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 [[transposon]]s in [[maize]]
 
:[[1952]] The [[Hershey-Chase experiment]] proves the genetic information of [[phage]]s (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 [[semiconservative replication|semiconservatively replicated]]
 
:[[1961]] The [[genetic code]] is arranged in triplets
 
:[[1964]] [[Howard Temin]] showed using [[RNA virus]]es 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 [[sequencing|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]] [[Phi-X174 phage|&Phi;-X174;]].
 
:[[1983]] [[Kary Banks Mullis]] discovers 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]] [http://www.genoscope.cns.fr/externe/English/Actualites/Presse/HGP/HGP_press_release-140403.pdf]
 
   
==Areas of genetics==
+
The effects of this mixing depends on the types (the alleles) of the gene you are interested in. If the father has two alleles specifying green eyes, and the mother has two alleles specifying brown eyes, all their children will get two alleles giving different instructions, one for green eyes and one for brown. The eye color of these children depends on how these alleles work together. If one allele overrides the instructions from another, it is called the ''[[Dominance_relationship#Dominant_allele|dominant]]'' allele, and the allele that is overridden is called the ''[[Dominance_relationship#Recessive_allele|recessive]]'' allele. In the case of a daughter with both green and brown alleles, brown is dominant and she ends up with brown eyes.<ref name=Athro>[http://www.athro.com/evo/gen/geframe.html How are human eye colors inherited?], Athro Limited, Accessed 20 May 2008</ref>
===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 [[Clone (genetics)|cloning]] as [[restriction enzyme]]s, 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 [[Mendelian inheritance|Mendel's laws]]. Patterns of inheritance still remain a useful tool for the study of [[genetic disease]]s.
+
[[Image:greeneyes.jpg|thumb|180px|left|Green eyes are a [[Dominance_relationship#Recessive_allele|recessive]] trait.]]
   
===Behavioral genetics===
+
However, the green eye color allele is still there in this brown-eyed girl, it just doesn't show. This is a difference between what you see on the surface (the set of observable traits of an organism, also called its [[phenotype]]) and which genes are in this organism (its [[genotype]]). In this example you can call the brown allele "B" and the green allele "g". (It is normal to write dominant alleles with capital letters and recessive ones with lower-case letters.) The brown-eyed daughter has the "brown eye phenotype" but her genotype is Bg, with one copy of the B allele, and one of the g allele.
:''Main article:'' [[Behavioural genetics]] (British spelling)
 
Behavioral genetics studies the influence of varying genetics on animal behavior.
 
   
===Clinical genetics===
+
Now imagine that this woman grows up and has children with a brown-eyed man who also has a Bg genotype. Her eggs will be a mixture of two types, one sort containing the B allele, and one sort the g allele. Similarly, her partner will produce a mix of two types of sperm containing one or the other of the two alleles. Now, when the alleles are mixed up in the offspring, these children have a chance of getting either brown or green eyes, since they could get a genotype of BB = brown eyes, Bg = brown eyes or gg = green eyes. In this generation, there is therefore a chance of the recessive allele showing itself in the phenotype of the children - some of them may have green eyes like their grandfather.<ref name=Athro/>
{{main|Clinical genetics}}
 
   
[[Physician]]s who are trained as Geneticists diagnose, treat, and counsel patients with [[genetic disorder]]s or [[syndrome]]s.
+
Many traits are inherited in a more complicated way than the example above. This can happen when there are several genes involved, each contributing a small part to the end result. Tall people tend to have tall children because their children get a package of many alleles that each contribute a bit to how much they grow. However, there are not clear groups of "short people" and "tall people", like there are groups of people with brown or green eyes. This is because of the large number of genes involved; this makes the trait very variable and people are many different heights.<ref>[http://www.childrensnyp.org/mschony/P02134.html Multifactorial Inheritance] Health Library, Morgan Stanley Children's Hospital, Accessed 20 May 2008</ref> Inheritance can also be complicated when the trait depends on the interaction between genetics and the environment. This is quite common, for example, if a child does not eat enough nutritious food this will not change traits like eye color, but it could stunt their growth.<ref>[http://www.med.umich.edu/opm/newspage/2003/kidheight.htm Low income kids' height doesn't measure up by age 1] University of Michigan Health System, Accessed 20th May 2008</ref>
These doctors are typically trained in a genetics [[Residency (medicine)|residency]] and/or [[fellowship]]. Although many are [[pediatrician]]s, some are not.
 
   
===Molecular genetics===
+
===Inherited diseases===
{{main|Molecular genetics}}
+
Some diseases are hereditary and run in families; others, such as [[infectious disease]]s, are caused by the environment. Other disorders are caused by a combination of hereditary and environmental factors.<ref>[http://www.genome.gov/19016930 Frequently Asked Questions About Genetic Disorders] NIH, Accessed 20 May 2008</ref>
   
Molecular genetics builds upon the foundation of classical genetics but focuses on the structure and function of [[gene|genes]] at a [[Molecule | 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]].
+
Diseases that are caused by a single allele of a gene and are inherited in families are called [[genetic disorder]]s. These include diseases like [[Huntington's disease]], [[Cystic fibrosis]] or [[Duchenne muscular dystrophy]]. Cystic fibrosis, for example, is caused by mutations in a single gene called ''[[CFTR (gene)|CFTR]]'' and is inherited as a recessive trait.<ref>[http://ghr.nlm.nih.gov/condition=cysticfibrosis Cystic fibrosis] Genetics Home Reference, NIH, Accessed 16 May 2008</ref> Other diseases are influenced by genetics, but which alleles a person gets from their parents only changes their risk of getting a disease. Most of these diseases are inherited in a complex way, with either multiple genes involved, or both genes and the environment being important.
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 [[molecule|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]].
+
As an example, the risk of [[breast cancer]] is 50 times higher in the families most at risk, compared to the families least at risk. This variation is probably due to a large number of alleles. Each of them changes the risk a little bit.<ref>{{cite journal |author=Peto J |title=Breast cancer susceptibility-A new look at an old model |journal=Cancer Cell |volume=1 |issue=5 |pages=411–2 |year=2002 |month=June |pmid=12124169 |doi=10.1016/S1535-6108(02)00079-X}}</ref> Several of the genes involved have been identified, such as ''[[BRCA1]]'' and ''[[BRCA2]]'', but not all of them. However, although some of the risk is genetic, being overweight, drinking a lot of alcohol, or not exercising, all increase the risk of this cancer.<ref>[http://www.cancer.org/docroot/CRI/content/CRI_2_4_2X_What_are_the_risk_factors_for_breast_cancer_5.asp What Are the Risk Factors for Breast Cancer?] American Cancer Society, Accessed 16 May 2008</ref> A woman's risk of breast cancer is therefore the result of a large number of alleles and her environment, so it is very hard to predict.
   
===Population, quantitative and ecological genetics===
+
==How genes work==
:''Main articles:'' [[Population genetics]], [[Quantitative genetics]], [[Ecological genetics]]
+
===Genes make proteins===
  +
The function of genes is to provide the information needed to make molecules called [[protein]]s in cells.<ref name=Utah>{{citation| title =University of Utah Genetics Learning Center animated tour of the basics of genetics| publisher =Howstuffworks.com | url =http://learn.genetics.utah.edu/units/basics/tour|format = web resource|accessdate=2008-01-24 }}</ref> Cells are the smallest independent parts of organisms: the human body contains about 100 trillion cells, while very small organisms like [[bacteria]] are just a single cell. A cell is like a miniature and very complex factory that can make all the parts needed to produce a copy of itself, which happens when cells [[cell division|divide]]. There is a simple division of labor in cells - genes give instructions and proteins carry out these instructions, tasks like building a new copy of a cell, or repairing damage.<ref name=NIGMS>[http://publications.nigms.nih.gov/structlife/chapter1.html The Structures of Life] National Institute of General Medical Sciences, Accessed 20th May 2008</ref> Each type of protein is a specialist that only does one job, so if a cell needs to do something new, it must make a new protein to do this job. Similarly, if a cell needs to do something faster or slower than before, it makes more or less of the protein responsible. Genes tell cells what to do by telling them which proteins to make and in what amounts.
   
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 [[population]]s 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 frequency|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 (biology)|adaptation]] and [[speciation]].
+
[[Image:Genetic code.svg|thumb|image|right|280px|Genes are expressed by being [[transcription (genetics)|transcribed]] into RNA, and this RNA then [[protein biosynthesis|translated]] into protein.]]
   
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.
+
Proteins are made of a chain of 20 different types of [[amino acid]]s. This chain folds up into a compact shape, rather like an untidy ball of rope. The shape of the protein is determined by the sequence of amino acids along its chain and it is this shape that, in turn, determines what the protein will do.<ref name=NIGMS/> For example, some proteins have depressions in their surface that perfectly match another molecule, allowing the protein to bind to this molecule very tightly. Other proteins are [[enzyme]]s, which are like tiny machines that can alter other molecules.<ref>[http://www.howstuffworks.com/cell2.htm Enzymes] HowStuffWorks, Accessed 20th May 2008</ref>
   
Ecological genetics again builds upon the basic principles of population genetics but is more explicitly focused on [[ecology|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.
+
In DNA information is held in the sequence of the repeating units along the DNA chain.<ref name=nih>[http://ghr.nlm.nih.gov/handbook/basics/dna What is DNA?] Genetics Home Reference, Accessed 16 May 2008</ref> These units are four types of [[nucleotide]]s (A,T,G and C) and the sequence of nucleotides stores information in an alphabet called the [[genetic code]]. When a gene is read by a cell the DNA sequence is copied into a very similar molecule called [[RNA]] (this process is called [[Transcription (genetics)|transcription]]). Transcription is controlled by other DNA sequences (such as [[promoter]]s), which show a cell where genes are, and control how often they are copied. The RNA copy made from a gene is then fed through a structure called a [[ribosome]], which translates the sequence of nucleotides in the RNA into the correct sequence of amino acids and joins these amino acids together to make a complete protein chain. The new protein then folds up into its active form. The process of moving information from the language of DNA into the language of amino acids is called [[protein biosynthesis|translation]].<ref name=nobel>[http://nobelprize.org/educational_games/medicine/dna/index.html DNA-RNA-Protein] Nobelprize.org, Accessed 20th May 2008</ref>
   
===Genomics===
+
[[Image:DNA replication split.svg|thumb|left|[[DNA replication]]. DNA is unwound and [[nucleotide]]s are matched to make two new strands.]]
{{main|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 availability of whole genome sequences, and computational tools developed in the field of [[bioinformatics]] for analysis of large set of data.
+
If the sequence of the nucleotides in a gene changes, the sequence of the amino acids in the protein it produces may also change - if part of a gene is deleted, the protein produced will be shorter and may not work any more.<ref name=NIGMS/> This is the reason why different alleles of a gene can have different effects in an organism. As an example, hair color depends on how much of a dark substance called [[melanin]] is put into the hair as it grows. If a person has a normal set of the genes involved in making melanin, they make all the proteins needed and they grow dark hair. However, if the alleles for a particular protein have different sequences and produce proteins that do not do the job correctly, no melanin will be produced and the hair will be white. This condition is called [[albinism]] and the person suffering from it is called an albino.<ref>[http://www.albinism.org/publications/what_is_albinism.html What is Albinism?] The National Organization for Albinism and Hypopigmentation, Accessed 20 May 2008</ref>
   
===Closely-related fields===
+
===Copies of genes are inherited===
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.
 
   
==See also==
+
When genes are passed from a parent to a child they are copied - the parent keeps the same number of genes as they had before and just passes on the new copies to their offspring. Genes are also copied each time a cell divides into two new cells. The process that copies DNA is called [[DNA replication]].<ref name=nih/>
* [[List of genetics-related topics]]
 
* [[List of genetic engineering topics]]
 
   
  +
DNA can be copied very easily and accurately because each piece of DNA can direct the creation of a new copy of its information. This is because DNA is made of two strands that pair together like the two sides of a zipper. The nucleotides are in the center, like the teeth in the zipper, and pair up to hold the two strands together. Importantly, the four different sorts nucleotides are different shapes, so in order for the strands to close up properly, an '''A''' nucleotide must go opposite a '''T''' nucleotide, and a '''G''' opposite a '''C'''. This exact pairing is called [[base pairing]].<ref name=nih/>
   
===See also===
+
When DNA is copied, the two strands of the old DNA are pulled apart by enzymes which move along each of the two single strands pairing up new nucleotide units and then zipping the strands closed. This produces two new pieces of DNA, each containing one strand from the old DNA and one newly-made strand. This process isn't perfect and sometimes the proteins will make mistakes and put the wrong nucleotide into the strand they are building. This causes a change in the sequence of that gene. These changes in DNA sequence are called [[mutation]]s.<ref>[http://learn.genetics.utah.edu/units/disorders/mutations/ Mutations] The University of Utah, Genetic Science Learning Center, Accessed 20th May 2008</ref> Mutations produce new alleles of genes. Sometimes these changes stop the gene from working properly, like the melanin genes discussed above. In other cases these mutations can change what the gene does or even let it do its job a little better than before. These mutations and their effects on the traits of organisms are one of the causes of [[evolution]].<ref name=Marshall/>
*[[Central dogma of molecular biology]]
 
*[[Chimera (genetics)|Chimerism]]
 
*[[Gene regulatory network]]
 
*[[Genetic counseling]]
 
*[[Genetic screen]]
 
*[[Genetic testing]]
 
*[[List of publications in biology#Genetics| Important publications in genetics]]
 
*[[List of genetics research organizations]]
 
*[[List of geneticists & biochemists]]
 
*[[Mitochondrial genetics]]
 
*[[Reprogenetics]]
 
*[[Punnett square]]
 
   
===Publications===
+
==Genes and evolution==
*''[[Genetics (journal)|Genetics]]''
+
{{Further|[[Introduction to evolution]]}}
*''[[Journal of Genetics]]''
+
[[Image:PCWmice1.jpg|thumb|right|[[Mouse|Mice]] with different coat colors.]]
*''[[Annals of Human Genetics]]''
+
A population of organisms [[Evolution|evolves]] when an inherited trait becomes more common or less common over time.<ref name=Marshall>{{citation|first = Marshall| last= Brain | contribution =How Evolution Works | contribution-url =http://science.howstuffworks.com/evolution.htm/printable | title =How Stuff Works: Evolution Library| publisher =Howstuffworks.com | url =http://science.howstuffworks.com/evolution-channel.htm|format = web resource|accessdate=2008-01-24 }}</ref> For instance, all the mice living on an island would be a single population of mice. If over a few generations, white mice went from being rare, to being a large part of this population, then the coat color of these mice would be evolving. In terms of genetics, this is called a change in [[allele frequency]]&mdash;such as an increase in the frequency of the allele for white fur.
*''[[Heredity (journal)|Heredity]]''
+
  +
Alleles become more or less common either just by chance (in a process called [[genetic drift]]), or through [[natural selection]].<ref>[http://evolution.berkeley.edu/evosite/evo101/IIIMechanisms.shtml Mechanisms: The Processes of Evolution] Understanding Evolution, Accessed 20th May 2008</ref> In natural selection, if an allele makes it more likely that an organism will survive and reproduce, then over time this allele will become more common. But if an allele is harmful, natural selection will make it less common. For example, if the island was getting colder each year and was covered with snow for much of the time, then the allele for white fur would become useful for the mice, since it would make them harder to see against the snow. Fewer of the white mice would be eaten by predators, so over time white mice would out-compete mice with dark fur. White fur alleles would become more common, and dark fur alleles would become more rare.
  +
  +
Mutations create new alleles. These alleles have new DNA sequences and can produce proteins with new properties.<ref>[http://evolution.berkeley.edu/evosite/evo101/IIICGeneticvariation.shtml Genetic Variation] Understanding Evolution, Accessed 20th May 2008</ref> So if an island was populated entirely by black mice, mutations could happen creating alleles for white fur. The combination of mutations creating new alleles at random, and natural selection picking out those which are useful, causes [[adaptation]]. This is when organisms change in ways that help them to survive and reproduce.
  +
  +
==Genetic engineering==
  +
  +
Since traits come from the genes in a cell, if you put a new piece of DNA into a cell, this can produce a new trait. This is how [[genetic engineering]] works. For example, crop plants can be given a gene from an Arctic fish, so they produce an [[antifreeze protein]] in their leaves.<ref>[http://www.nd.edu/~ndmag/ilum1f98.html Long underwear for water] Notre Dame magazine, 1998</ref> This can help prevent frost damage. Other genes that can be put into crops include a natural [[insecticide]] from the bacteria ''[[Bacillus thuringiensis]]''. The insecticide kills insects that eat the plants, but is harmless to people.<ref>[http://ars.usda.gov/is/ar/archive/nov99/pest1199.htm Tifton, Georgia: A Peanut Pest Showdown] USDA, accessed 16 May 2008</ref> In these plants the new genes are put into the plant before it is grown, so the genes will be in every part of the plant, including its seeds. The plant's offspring will then inherit the new genes, something which has lead to concern about the spread of new traits into wild plants.<ref>[http://www.geo-pie.cornell.edu/gmo.html Genetically engineered organisms public issues education] Cornell University, Accessed 16 May 2008</ref>
  +
  +
The kind of technology used in genetic engineering is also being developed to treat people with [[genetic disorder]]s in an experimental medical technique called [[gene therapy]].<ref>{{cite web| last = Staff|date= November 18, 2005| url = http://www.ornl.gov/sci/techresources/Human_Genome/medicine/genetherapy.shtml| title = Gene Therapy| format = FAQ| work = Human Genome Project Information| publisher = [[Oak Ridge National Laboratory]]| accessdate = 2006-05-28}}</ref> However, here the new gene is put in after the person has grown up and become ill, so any new gene will not be inherited by their children. Gene therapy works by trying to replace the allele that causes the disease with an allele that will work properly.
  +
  +
==References==
  +
{{reflist|2}}
  +
  +
==Further reading==
  +
*{{cite book | author=Jones, Steve | title=[[The Language of the Genes]]| publisher=Flamingo | year=2000 | id=ISBN 0-00-655243-9}} ([[The Aventis Prizes for Science Books|Aventis Prize winner]])
  +
*{{cite book | author=Schwartz, James | title=In Pursuit of the Gene: From Darwin to DNA| publisher=Harvard University Press | year=2008 | id=ISBN 0-67-402670-5}}
  +
*{{cite book | author=Hamer, Dean and Copeland, Peter | title=Living with Our Genes: Why They Matter More Than You Think | publisher=Anchor | year=1999 | id=ISBN 0-38-548584-0}}
  +
*{{cite book | author=Goodsell, David | title=Our Molecular Nature: The Body's Motors, Machines and Messages | publisher=Springer | year=1996 | id=ISBN 0-38-794498-2}}
   
 
==External links==
 
==External links==
{{book|Genetics}}
+
'''Genetics'''
===Related publications===
+
*[http://learn.genetics.utah.edu/ Introduction to Genetics], University of Utah
*''Advanced Genetics''
+
*[http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=gnd Introduction to Genes and Disease], NCBI open book
*[http://www.journals.uchicago.edu/AJHG/home.html ''American Journal of Human Genetics'']
+
*[http://www.genome.gov/10002096 Genetics glossary], A talking glossary of genetic terms.
*''Annual Reviews of Genetics''
 
*[http://www.nature.com/ejhg/ ''European Journal of Human Genetics'']
 
*''[http://www.genesdev.org/ Genes and Development]''
 
*[http://hmg.oupjournals.org/ ''Human Molecular Genetics'']
 
*''[http://jhered.oupjournals.org/ Journal of Heredity]''
 
*[http://www.bionews.in/index.php/archives/category/genetics// ''Latest Genetics News'']
 
*[http://www.nature.com/ng/ ''Nature Genetics'']
 
*''[[Nature Reviews Genetics]]'' ([http://www.nature.com/nrg/index.html journal home])
 
*[http://www.nature.com/genomics/ ''Nature Genome Gateway'']
 
*[http://www.jpharmacogenetics.com/ ''Pharmacogenetics'']
 
*''Journal of Medical Genetics''
 
   
=== Other ===
+
'''DNA and genes'''
*[http://www.jbpub.com/connections Exploring the Way Life Works]
+
*[http://www.blackwellpublishing.com/trun/artwork/Animations/cloningexp/cloningexp.html Animated guide to cloning]
*[http://gslc.genetics.utah.edu Genetic Science Learning Center]
+
*[http://www.ncbi.nlm.nih.gov/About/primer/genetics.html Genetics] NCBI, A Science Primer
*[http://www.jic.bbsrc.ac.uk/corporate/Library/letter.html Letter to Adam Sedgwick in 1905 from William Bateson]
 
*[http://www.ornl.gov/sci/techresources/Human_Genome/genetics.shtml The Virtual Library on Genetics]
 
*[http://www.gene-watch.org Council for Responsible Genetics]
 
*[http://geneticsmadeeasy.com Genetics made easy]
 
   
{{Genetics-footer}}
+
'''Evolution'''
{{Biology-footer}}
+
*{{citation | title =Understanding Evolution: your one-stop source for information on evolution| publisher = The University of California Museum of Paleontology, Berkeley | url =http://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_01|format = web resource|accessdate=2008-01-24 }}
   
[[Category:Genetics|*]]
+
'''Interactive'''
  +
*[http://museum.thetech.org/ugenetics/eyeCalc/eyecalculator.html What Color Eyes Would Your Children Have?] Genetics of human eye color: An interactive introduction.
  +
*[http://nobelprize.org/educational_games/medicine/dna_double_helix/index.html Double Helix Game] from the Nobel Prize website. Match CATG bases with each other, and other games.
  +
*[http://learn.genetics.utah.edu/units/basics/transcribe/ Transcribe and translate a gene]. University of Utah
   
  +
[[Category:Genetics]]
 
{{enWP| Genetics}}
 
{{enWP| Genetics}}

Revision as of 23:20, July 30, 2008

Assessment | Biopsychology | Comparative | Cognitive | Developmental | Language | Individual differences | Personality | Philosophy | Social |
Methods | Statistics | Clinical | Educational | Industrial | Professional items | World psychology |

Biological: Behavioural genetics · Evolutionary psychology · Neuroanatomy · Neurochemistry · Neuroendocrinology · Neuroscience · Psychoneuroimmunology · Physiological Psychology · Psychopharmacology (Index, Outline)


File:ADN animation.gif


Genetics studies how living organisms inherit many of the features of their ancestors – for example, children usually look and act like other people in their family. Genetics tries to identify which features are inherited, and work out the details of how these features are passed from generation to generation.

In genetics, a feature of an organism is called a "trait". Some traits are features of an organism's physical appearance, for example, a person's eye color, height or weight. There are many other types of traits and these range from aspects of behavior to resistance to disease. Traits are often inherited, for example tall and thin people tend to have tall and thin children. Other traits come from the interaction between inherited features and the environment. For example a child might inherit the tendency to be tall, but if there is very little food where they live and they are poorly nourished, they will still be short. The way genetics and environment interact to produce a trait can be complicated: for example, the chances of somebody dying of cancer or heart disease seem to depend on both their family history and their lifestyle.

Genetic information is carried by a long molecule called DNA and this DNA is copied and inherited across generations. Traits are carried in DNA as instructions for constructing and operating an organism. These instructions are contained in segments of DNA called genes. DNA is made of a sequence of simple units, with the order of these units spelling out instructions in the genetic code. This is similar to the orders of letters spelling out words. The organism "reads" the sequence of these units and decodes the instruction.

Not all the genes for a particular instruction are exactly the same. Different forms of one type of gene are called different alleles of that gene. As an example, one allele of a gene for hair color could carry the instruction to produce a lot of the pigment in black hair, while a different allele could give a garbled version of this instruction, so that no pigment is produced and the hair is white. Mutations are random events that change the sequence of a gene and therefore create a new allele. Mutations can produce a new trait, such as turning an allele for black hair into an allele for white hair. The appearance of new traits is important in evolution.

Template:Introduction to genetics glossary

Inheritance in biology

Genes and inheritance

Genes are inherited as units, with parents dividing out their genes to their offspring. You can think of this process like mixing two hands of cards, shuffling them, and then dealing them out again. Humans have two copies of each of their genes and when people reproduce they make copies of their genes in eggs or sperm, but only put in one copy of each type of gene. An egg then joins with a sperm to give a child with a new set of genes. This child will have the same number of genes as its parents but for any gene one of their two copies will come from the father, and one from the mother.[1]

The effects of this mixing depends on the types (the alleles) of the gene you are interested in. If the father has two alleles specifying green eyes, and the mother has two alleles specifying brown eyes, all their children will get two alleles giving different instructions, one for green eyes and one for brown. The eye color of these children depends on how these alleles work together. If one allele overrides the instructions from another, it is called the dominant allele, and the allele that is overridden is called the recessive allele. In the case of a daughter with both green and brown alleles, brown is dominant and she ends up with brown eyes.[2]

Greeneyes

Green eyes are a recessive trait.

However, the green eye color allele is still there in this brown-eyed girl, it just doesn't show. This is a difference between what you see on the surface (the set of observable traits of an organism, also called its phenotype) and which genes are in this organism (its genotype). In this example you can call the brown allele "B" and the green allele "g". (It is normal to write dominant alleles with capital letters and recessive ones with lower-case letters.) The brown-eyed daughter has the "brown eye phenotype" but her genotype is Bg, with one copy of the B allele, and one of the g allele.

Now imagine that this woman grows up and has children with a brown-eyed man who also has a Bg genotype. Her eggs will be a mixture of two types, one sort containing the B allele, and one sort the g allele. Similarly, her partner will produce a mix of two types of sperm containing one or the other of the two alleles. Now, when the alleles are mixed up in the offspring, these children have a chance of getting either brown or green eyes, since they could get a genotype of BB = brown eyes, Bg = brown eyes or gg = green eyes. In this generation, there is therefore a chance of the recessive allele showing itself in the phenotype of the children - some of them may have green eyes like their grandfather.[2]

Many traits are inherited in a more complicated way than the example above. This can happen when there are several genes involved, each contributing a small part to the end result. Tall people tend to have tall children because their children get a package of many alleles that each contribute a bit to how much they grow. However, there are not clear groups of "short people" and "tall people", like there are groups of people with brown or green eyes. This is because of the large number of genes involved; this makes the trait very variable and people are many different heights.[3] Inheritance can also be complicated when the trait depends on the interaction between genetics and the environment. This is quite common, for example, if a child does not eat enough nutritious food this will not change traits like eye color, but it could stunt their growth.[4]

Inherited diseases

Some diseases are hereditary and run in families; others, such as infectious diseases, are caused by the environment. Other disorders are caused by a combination of hereditary and environmental factors.[5]

Diseases that are caused by a single allele of a gene and are inherited in families are called genetic disorders. These include diseases like Huntington's disease, Cystic fibrosis or Duchenne muscular dystrophy. Cystic fibrosis, for example, is caused by mutations in a single gene called CFTR and is inherited as a recessive trait.[6] Other diseases are influenced by genetics, but which alleles a person gets from their parents only changes their risk of getting a disease. Most of these diseases are inherited in a complex way, with either multiple genes involved, or both genes and the environment being important.

As an example, the risk of breast cancer is 50 times higher in the families most at risk, compared to the families least at risk. This variation is probably due to a large number of alleles. Each of them changes the risk a little bit.[7] Several of the genes involved have been identified, such as BRCA1 and BRCA2, but not all of them. However, although some of the risk is genetic, being overweight, drinking a lot of alcohol, or not exercising, all increase the risk of this cancer.[8] A woman's risk of breast cancer is therefore the result of a large number of alleles and her environment, so it is very hard to predict.

How genes work

Genes make proteins

The function of genes is to provide the information needed to make molecules called proteins in cells.[1] Cells are the smallest independent parts of organisms: the human body contains about 100 trillion cells, while very small organisms like bacteria are just a single cell. A cell is like a miniature and very complex factory that can make all the parts needed to produce a copy of itself, which happens when cells divide. There is a simple division of labor in cells - genes give instructions and proteins carry out these instructions, tasks like building a new copy of a cell, or repairing damage.[9] Each type of protein is a specialist that only does one job, so if a cell needs to do something new, it must make a new protein to do this job. Similarly, if a cell needs to do something faster or slower than before, it makes more or less of the protein responsible. Genes tell cells what to do by telling them which proteins to make and in what amounts.

File:Genetic code.svg

Proteins are made of a chain of 20 different types of amino acids. This chain folds up into a compact shape, rather like an untidy ball of rope. The shape of the protein is determined by the sequence of amino acids along its chain and it is this shape that, in turn, determines what the protein will do.[9] For example, some proteins have depressions in their surface that perfectly match another molecule, allowing the protein to bind to this molecule very tightly. Other proteins are enzymes, which are like tiny machines that can alter other molecules.[10]

In DNA information is held in the sequence of the repeating units along the DNA chain.[11] These units are four types of nucleotides (A,T,G and C) and the sequence of nucleotides stores information in an alphabet called the genetic code. When a gene is read by a cell the DNA sequence is copied into a very similar molecule called RNA (this process is called transcription). Transcription is controlled by other DNA sequences (such as promoters), which show a cell where genes are, and control how often they are copied. The RNA copy made from a gene is then fed through a structure called a ribosome, which translates the sequence of nucleotides in the RNA into the correct sequence of amino acids and joins these amino acids together to make a complete protein chain. The new protein then folds up into its active form. The process of moving information from the language of DNA into the language of amino acids is called translation.[12]

File:DNA replication split.svg

If the sequence of the nucleotides in a gene changes, the sequence of the amino acids in the protein it produces may also change - if part of a gene is deleted, the protein produced will be shorter and may not work any more.[9] This is the reason why different alleles of a gene can have different effects in an organism. As an example, hair color depends on how much of a dark substance called melanin is put into the hair as it grows. If a person has a normal set of the genes involved in making melanin, they make all the proteins needed and they grow dark hair. However, if the alleles for a particular protein have different sequences and produce proteins that do not do the job correctly, no melanin will be produced and the hair will be white. This condition is called albinism and the person suffering from it is called an albino.[13]

Copies of genes are inherited

When genes are passed from a parent to a child they are copied - the parent keeps the same number of genes as they had before and just passes on the new copies to their offspring. Genes are also copied each time a cell divides into two new cells. The process that copies DNA is called DNA replication.[11]

DNA can be copied very easily and accurately because each piece of DNA can direct the creation of a new copy of its information. This is because DNA is made of two strands that pair together like the two sides of a zipper. The nucleotides are in the center, like the teeth in the zipper, and pair up to hold the two strands together. Importantly, the four different sorts nucleotides are different shapes, so in order for the strands to close up properly, an A nucleotide must go opposite a T nucleotide, and a G opposite a C. This exact pairing is called base pairing.[11]

When DNA is copied, the two strands of the old DNA are pulled apart by enzymes which move along each of the two single strands pairing up new nucleotide units and then zipping the strands closed. This produces two new pieces of DNA, each containing one strand from the old DNA and one newly-made strand. This process isn't perfect and sometimes the proteins will make mistakes and put the wrong nucleotide into the strand they are building. This causes a change in the sequence of that gene. These changes in DNA sequence are called mutations.[14] Mutations produce new alleles of genes. Sometimes these changes stop the gene from working properly, like the melanin genes discussed above. In other cases these mutations can change what the gene does or even let it do its job a little better than before. These mutations and their effects on the traits of organisms are one of the causes of evolution.[15]

Genes and evolution

Further information: Introduction to evolution
File:PCWmice1.jpg

A population of organisms evolves when an inherited trait becomes more common or less common over time.[15] For instance, all the mice living on an island would be a single population of mice. If over a few generations, white mice went from being rare, to being a large part of this population, then the coat color of these mice would be evolving. In terms of genetics, this is called a change in allele frequency—such as an increase in the frequency of the allele for white fur.

Alleles become more or less common either just by chance (in a process called genetic drift), or through natural selection.[16] In natural selection, if an allele makes it more likely that an organism will survive and reproduce, then over time this allele will become more common. But if an allele is harmful, natural selection will make it less common. For example, if the island was getting colder each year and was covered with snow for much of the time, then the allele for white fur would become useful for the mice, since it would make them harder to see against the snow. Fewer of the white mice would be eaten by predators, so over time white mice would out-compete mice with dark fur. White fur alleles would become more common, and dark fur alleles would become more rare.

Mutations create new alleles. These alleles have new DNA sequences and can produce proteins with new properties.[17] So if an island was populated entirely by black mice, mutations could happen creating alleles for white fur. The combination of mutations creating new alleles at random, and natural selection picking out those which are useful, causes adaptation. This is when organisms change in ways that help them to survive and reproduce.

Genetic engineering

Since traits come from the genes in a cell, if you put a new piece of DNA into a cell, this can produce a new trait. This is how genetic engineering works. For example, crop plants can be given a gene from an Arctic fish, so they produce an antifreeze protein in their leaves.[18] This can help prevent frost damage. Other genes that can be put into crops include a natural insecticide from the bacteria Bacillus thuringiensis. The insecticide kills insects that eat the plants, but is harmless to people.[19] In these plants the new genes are put into the plant before it is grown, so the genes will be in every part of the plant, including its seeds. The plant's offspring will then inherit the new genes, something which has lead to concern about the spread of new traits into wild plants.[20]

The kind of technology used in genetic engineering is also being developed to treat people with genetic disorders in an experimental medical technique called gene therapy.[21] However, here the new gene is put in after the person has grown up and become ill, so any new gene will not be inherited by their children. Gene therapy works by trying to replace the allele that causes the disease with an allele that will work properly.

References

  1. 1.0 1.1 (web resource)University of Utah Genetics Learning Center animated tour of the basics of genetics, Howstuffworks.com, http://learn.genetics.utah.edu/units/basics/tour, retrieved on 2008-01-24 
  2. 2.0 2.1 How are human eye colors inherited?, Athro Limited, Accessed 20 May 2008
  3. Multifactorial Inheritance Health Library, Morgan Stanley Children's Hospital, Accessed 20 May 2008
  4. Low income kids' height doesn't measure up by age 1 University of Michigan Health System, Accessed 20th May 2008
  5. Frequently Asked Questions About Genetic Disorders NIH, Accessed 20 May 2008
  6. Cystic fibrosis Genetics Home Reference, NIH, Accessed 16 May 2008
  7. Peto J (June 2002). Breast cancer susceptibility-A new look at an old model. Cancer Cell 1 (5): 411–2.
  8. What Are the Risk Factors for Breast Cancer? American Cancer Society, Accessed 16 May 2008
  9. 9.0 9.1 9.2 The Structures of Life National Institute of General Medical Sciences, Accessed 20th May 2008
  10. Enzymes HowStuffWorks, Accessed 20th May 2008
  11. 11.0 11.1 11.2 What is DNA? Genetics Home Reference, Accessed 16 May 2008
  12. DNA-RNA-Protein Nobelprize.org, Accessed 20th May 2008
  13. What is Albinism? The National Organization for Albinism and Hypopigmentation, Accessed 20 May 2008
  14. Mutations The University of Utah, Genetic Science Learning Center, Accessed 20th May 2008
  15. 15.0 15.1 Brain, Marshall, "How Evolution Works" (web resource), How Stuff Works: Evolution Library, Howstuffworks.com, http://science.howstuffworks.com/evolution.htm/printable, retrieved on 2008-01-24 
  16. Mechanisms: The Processes of Evolution Understanding Evolution, Accessed 20th May 2008
  17. Genetic Variation Understanding Evolution, Accessed 20th May 2008
  18. Long underwear for water Notre Dame magazine, 1998
  19. Tifton, Georgia: A Peanut Pest Showdown USDA, accessed 16 May 2008
  20. Genetically engineered organisms public issues education Cornell University, Accessed 16 May 2008
  21. Staff Gene Therapy. (FAQ) Human Genome Project Information. Oak Ridge National Laboratory. URL accessed on 2006-05-28.

Further reading

External links

Genetics

DNA and genes

Evolution

Interactive

This page uses Creative Commons Licensed content from Wikipedia (view authors).

Around Wikia's network

Random Wiki