Introduction to genetics: Difference between revisions

Revision as of 06:22, 28 June 2025

Revision as of 17:01, 17 July 2025

Line 19:

=== 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.]]

Line 39:

A protein consists of a chain 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 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.]]

Line 64:

==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 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>[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.