Introduction to genetics: Difference between revisions

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'''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 organism's [[morphology (biology)|physical appearance]], such as [[eye]] [[Eye color|color]], height 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 genes, which is the reason why tall and thin people tend to have tall and thin children. Other traits come from interactions between genes and the environment, so a child who inherited the tendency of being tall 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 it, carrying genetic information. The language used by DNA is called [[genetic code]], which lets organisms read the information in the genes. This information is the instructions for the construction and operation of a living organism.

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=== Example of mixing ===

The effects of 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://www.ncbi.nlm.nihomim.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.]]

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[[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.