Editing Genetics (section)

=== Nature and nurture ===
{{Main|Nature and nurture}}

[[File:Niobe050905-Siamese Cat.jpeg|thumb|upright|[[Siamese (cat)|Siamese cats]] have a 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. The phrase "[[nature and nurture]]" refers to this complementary relationship. The phenotype of an organism depends on the interaction of genes and the environment. An interesting example is the coat coloration of the [[Siamese (cat)|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 [[Denaturation (biochemistry)|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{{emdash}}so the cat has dark hair at its extremities.<ref>{{cite journal | vauthors = Imes DL, Geary LA, Grahn RA, Lyons LA | title = Albinism in the domestic cat (Felis catus) is associated with a tyrosinase (TYR) mutation | journal = Animal Genetics | volume = 37 | issue = 2 | pages = 175–178 | date = April 2006 | pmid = 16573534 | pmc = 1464423 | doi = 10.1111/j.1365-2052.2005.01409.x }}</ref>

Environment plays a major role in effects of the human genetic disease [[phenylketonuria]]. 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 intellectual disability and seizures. However, if someone with the phenylketonuria mutation follows a strict diet that avoids this amino acid, they remain normal and healthy.<ref>{{cite web |url=https://www.nlm.nih.gov/medlineplus/phenylketonuria.html |title=MedlinePlus: Phenylketonuria |access-date=15 March 2008 |publisher=NIH: National Library of Medicine |url-status=live |archive-url=https://web.archive.org/web/20080725183720/http://www.nlm.nih.gov/medlineplus/phenylketonuria.html |archive-date=25 July 2008}}</ref>

A common method for determining how genes and environment ("nature and nurture") contribute to a phenotype involves [[twin study|studying identical and fraternal twins]], or other siblings of [[multiple birth]]s.<ref>For example, {{cite book |title=Nature via Nurture: Genes, Experience and What Makes Us Human |vauthors=Ridley M |publisher=Fourth Estate |year=2003|isbn= 978-1-84115-745-0 |page=73}}</ref> Identical siblings are genetically the same since they come from the same zygote. Meanwhile, fraternal twins are as genetically different from one another as normal siblings. By comparing how often a certain disorder occurs in a pair of identical twins to how often it occurs in a pair of fraternal twins, scientists can determine whether that disorder is caused by genetic or postnatal environmental factors. One famous example involved the study of the [[Genain quadruplets]], who were [[Multiple birth|identical quadruplets]] all diagnosed with [[schizophrenia]].<ref name="Genain">{{Cite journal |title=The Genain Quadruplets: A Case Study and Theoretical Analysis of Heredity and Environment in Schizophrenia |journal=Behavioral Science |volume=9 |issue=4 | vauthors = Rosenthal D |year=1964 |page=371 |doi=10.1002/bs.3830090407 }}</ref>