Introduction to genetics: Difference between revisions

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{{Introduction to genetics glossary}}

'''Genetics''' is the study of [[genes]] and tries to explain what they are and how they work. Genes are how living [[organism]]s inherit features or [[Phenotypic trait|traits]] from their ancestors; for example, children usually look like their parents because they have inherited their parents' genes. Genetics tries to identify which traits are inherited, and to explain how these traits are passed from generation to generation.

Some traits are part of an organismsorganism's [[morphology (biology)|physical appearance]];, such as a person's [[eye]] [[Eye color, height|color]] or weightheight. Other sorts of traits are not easily seen and include [[blood type]]s or resistance to [[Disease|diseases]]. Some traits are inherited through our genes, sowhich is the reason why tall and thin people tend to have tall and thin children. Other traits come from interactions between our genes and the environment, so a child mightwho inheritinherited the tendency toof bebeing tall, but if they are poorly nourished, they will still be short if [[Malnutrition|poorly]] [[Nutrition|nourished]]. The way our genes and environment interact to produce a trait can be complicated. For example, the chances of somebody dying of [[cancer]] or [[heart disease]] seems to depend on both their genes and their lifestyle.

Genes are made from a long [[molecule]] called [[DNA]], which is copied and inherited across generations. DNA is made of [[nucleotide|simple units]] that line up in a particular order within thisit, large molecule. The order of these units carriescarrying genetic information, similar to how the order of letters on a page carries information. The language used by DNA is called the [[genetic code]], which lets organisms read the information in the genes. This information is the instructions for constructingthe construction and operatingoperation of a living organism.

The information within a particular gene is not always exactly the same between one organism and another, so different copies of a gene do not always give exactly the same instructions. Each unique form of a single gene is called an [[allele]]. As an example, one allele for the gene for hair color could instruct the body to produce much pigment, producing black hair, while a different allele of the same gene might give garbled instructions that fail to produce any pigment, giving white hair. [[Mutation]]s are random changes in genes and can create new alleles. Mutations can also produce new traits, such as when mutations to an allele for black hair produce a new allele for white hair. This appearance of new traits is important in [[Introduction to evolution|evolution]].

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[[File:DNA animation.gif|frame|left|A section of [[DNA]]; the sequence of the plate-like units ([[nucleotide]]s) in the center carries information.]]

Genes are pieces of DNA that contain information for the synthesis of [[RNA|ribonucleic acids]] (RNAs) or [[Peptide|polypeptides]]. Genes are inherited as units, with two parents dividing out copies of their genes to their offspring. Humans have two copies of each of their genes, but each [[Egg cell|egg]] or [[sperm]] cell only gets ''one'' of those copies for each gene. An egg and sperm join to form a [[zygote]] with a complete set of genes. The resulting offspring has the same number of genes as their parents, but for any gene, one of their two copies comes from their father, and one from their mother.<ref name=Utah/>

=== Example of mixing ===

The effects of this mixing depend on the types (the [[allele]]s) of the gene. If the father has two copies of an allele for red hair, and the mother has two copies for brown hair, all their children get the two alleles that give different instructions, one for red hair and one for brown. The hair color of these children depends on how these alleles work together. If one allele [[dominance (genetics)|dominates]] 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 alleles for both red and brown hair, brown is dominant and she ends up with brown hair.<ref name="OMIM">[https://wwwomim.ncbi.nlm.nih.govorg/search?index=entry&start=1&limit=10&search=omim/155555&sort=score+desc&field=number Melanocortin 1 Receptor], Accessed 27 November 2010</ref>

[[File:Hair colors punnett.png|thumb|right|A [[Punnett square]] showing how two brown haired parents can have red or brown haired children. 'B' is for brown and 'b' is for red.]]

[[File:Redhead close up.jpg|thumb|180px|left|Red hair is a [[Dominance relationship#Recessive allele|recessive]] trait.]]

Although the red color allele is still there in this brown-haired girl, it doesn't show. This is a difference between what youis seeseen on the surface (the traits of an organism, called its [[phenotype]]) and the genes within the organism (its [[genotype]]). In this example you can call, the allele for brown can be called "B" and the allele for red "b". (It is normal to write dominant alleles with capital letters and recessive ones with lower-case letters.) The brown hair daughter has the "brown hair phenotype" but her genotype is Bb, with one copy of the B allele, and one of the b allele.

Now imagine that this woman grows up and has children with a brown-haired man who also has a Bb genotype. Her eggs will be a mixture of two types, one sort containing the B allele, and one sort the b allele. Similarly, her partner will produce a mix of two types of sperm containing one or the other of these two alleles. When the transmitted genes are joined up in their offspring, these children have a chance of getting either brown or red hair, since they could get a genotype of BB = brown hair, Bb = brown hair or bb = red hair. 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 red hair like their grandfather.<ref name=OMIM/>

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===Genes make proteins===

{{main|Genetic code}}

The function of genes is to provide the information needed to make molecules called [[protein]]s in cells.<ref name=Utah>{{Cite book| 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| access-date =2008-01-24| url-status =dead| archive-url =https://web.archive.org/web/20080210023634/http://learn.genetics.utah.edu/units/basics/tour/| archive-date =10 February 2008| df =dmy-all}}</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 the damage.<ref name=NIGMS>[http://publications.nigms.nih.gov/structlife/chapter1.html The Structures of Life] {{Webarchive|url=https://web.archive.org/web/20140607084902/http://publications.nigms.nih.gov/structlife/chapter1.html |date=7 June 2014 }} National Institute of General Medical Sciences, Accessed 20 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.

[[File:Genetic code.svg|thumb|right|280px|Genes are expressed by being [[transcription (genetics)|transcribed]] into RNA, and this RNA then [[protein biosynthesis|translated]] into protein.]]

ProteinsA areprotein madeconsists of a chain of 20 different types of [[amino acid]] molecules, of which there are 20 types. This chain folds up into a compact shape, rather like an untidy ball of string. 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 does.<ref name=NIGMS/> For example, some proteins have parts of their surface that perfectly match the shape of another molecule, allowing the protein to bind to this molecule very tightly. Other proteins are [[enzyme]]s, which are like tiny machines that alter other molecules.<ref>[http://www.howstuffworks.com/cell2.htm Enzymes] HowStuffWorks, Accessed 20 May 2008</ref>

The information in DNA is held in the sequence of the repeating units along the DNA chain.<ref name=nih>[httphttps://ghr.nlm.nihmedlineplus.gov/handbookgenetics/understanding/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 (biology)|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 RNA 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 20 May 2008</ref>

[[File:DNA replication split.svg|thumb|left|[[DNA replication]]. DNA is unwound and [[nucleotide]]s are matched to make two new strands.]]

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==Inherited diseases==

Some diseases are hereditary and run in families; others, such as [[infectious disease]]s, are caused by the environment. Other diseases come from a combination of genes and the environment.<ref>[http://www.genome.gov/19016930 requently Asked Questions About Genetic Disorders] NIH, Accessed 20 May 2008</ref> [[Genetic disorder]]s are diseases that are caused by aone singleor allelemore ofabnormalities ain genethe genome and are inherited in families. These may be monogenic (caused by alterations in a single gene), polygenic (caused by alterations in multiple genes). Examples of monogenic gene disorders include [[Huntington's disease]], [[cystic fibrosis]] or [[Duchenne muscular dystrophy]]. Cystic fibrosis, for example, is caused by mutations in a single gene calledthe ''[[CFTR (gene)|CFTR]]'' gene and is inherited as a recessive trait.<ref>[https://web.archive.org/web/20030801073139/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 the genes a person gets from their parents only change their risk of getting a disease. Most of these diseases are inherited in a complex way, with either multiple genes involved, or coming from both genes and the environment. 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 changing the risk a little bit.<ref>{{cite journal |author=Peto J |title=Breast cancer susceptibility&nbsp;– A new look at an old model |journal=Cancer Cell |volume=1 |issue=5 |pages=411–2 |date=June 2002 |pmid=12124169 |doi=10.1016/S1535-6108(02)00079-X |issn=1535-6108|doi-access=free }}</ref> Several of the genes have been identified, such as ''[[BRCA1]]'' and ''[[BRCA2]]'', but not all of them. However, although some of the risks are genetic, the risk of this cancer is also increased by being overweight, heavy alcohol consumption and not exercising.<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?] {{webarchive|url=https://web.archive.org/web/20090429042057/http://www.cancer.org/docroot/CRI/content/CRI_2_4_2X_What_are_the_risk_factors_for_breast_cancer_5.asp |date=29 April 2009 }} American Cancer Society, Accessed 16 May 2008</ref> A woman's risk of breast cancer, therefore, comes from a large number of alleles interacting with her environment, so it is very hard to predict.

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==Genetic engineering==

{{main|Genetic engineering}}

Since traits come from the genes in a cell, putting a new piece of DNA into a cell can produce a new trait. This is how [[genetic engineering]] works. For example, rice can be given genes from a maize and a soil bacteria so the rice produces [[beta-carotene]], which the body converts to vitamin A.<ref>Staff [http://www.goldenrice.org/ Golden Rice Project] Retrieved 5 November 2012</ref> This can help children with Vitamin A deficiency. Another gene being put into some crops comes from the bacterium ''[[Bacillus thuringiensis]]''; the gene makes a protein that is an [[insecticide]]. The insecticide kills insects that eat the plants but is harmless to people.<ref>[httphttps://agresearchmag.ars.usda.gov/is1999/arnov/archive/nov99/pest1199.htmpest 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 are in every part of the plant, including its seeds.<ref>[http://www.gmo-safety.eu/basic-info/129.bacterial-arsenal-combat-chewing-insects.html Genetic engineering: Bacterial arsenal to combat chewing insects] {{Webarchive|url=https://web.archive.org/web/20110515134013/http://www.gmo-safety.eu/basic-info/129.bacterial-arsenal-combat-chewing-insects.html |date=15 May 2011 }} GMO Safety, Jul 2010</ref> The plant's offspring inherit the new genes, which has led to concern about the spread of new traits into wild plants.<ref>[httphttps://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]]| access-date = 2006-05-28}}</ref> However, here the new, properly working gene is put in targeted cells, not altering the chance of future children inheriting the disease causing alleles.