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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|
Some traits are part of an organisms' [[morphology (biology)|physical appearance]]; such as a person's eye-color, height or weight. Other sorts of traits are not easily seen and include [[blood type]]s or resistance to diseases. Some traits are inherited through our genes, so 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 might inherit the tendency to be tall, but if they are poorly nourished, they will still be short. 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
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 a lot of 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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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 a single allele of a gene and are inherited in families. These include [[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 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
==Genetic engineering==
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