Introduction to genetics: Difference between revisions

Revision as of 22:27, 22 October 2023

Revision as of 01:56, 2 November 2023

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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, withbut onefor copyany gene, one of eachtheir genetwo copies comingcomes from their father and one from their mother.<ref name=Utah/>

=== Example of mixing ===

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Although the red color allele is still there in this brown-haired girl, it doesn't show. This is a difference between what is seen on the surface (the traits of an organism, called its [[phenotype]]) and the genes within the organism (its [[genotype]]). In this example, 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/> In the case of two parents with one recessive allele and one dominant allele, their offspring will have a 25% chance of inheriting BB, a 25% chance of inheriting bb, and a 50% chance of inheriting Bb.

Many traits are inherited in a more complicated way than the example above. This can happen when there are several genes involved, each contributing a small part to the result. Tall people tend to have tall children because their children get a package of many alleles that each contribute a bit to how much they grow. However, there are not clear groups of "short people" and "tall people", like there are groups of people with brown or red hair. This is because of the large number of genes involved; this makes the trait very variable and people are of many different heights.<ref>[http://www.childrensnyp.org/mschony/P02134.html Multifactorial Inheritance] Health Library, Morgan Stanley Children's Hospital, Accessed 20 May 2008</ref> Despite a common misconception, the green/blue eye traits are also inherited in this complex inheritance model.<ref name=Athro>[http://www.athro.com/evo/gen/inherit1.html#uncertainty Eye color is more complex than two genes], Athro Limited, Accessed 27 November 2010</ref> Inheritance can also be complicated when the trait depends on the interaction between genetics and environment. For example, malnutrition does not change traits like eye color, but can stunt growth.<ref>{{cite web |url=http://www.med.umich.edu/opm/newspage/2003/kidheight.htm |title=Low income kids' height doesn't measure up by age 1 |publisher=University of Michigan Health System |access-date=May 20, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20080526034018/http://www.med.umich.edu/opm/newspage/2003/kidheight.htm |archive-date=26 May 2008 |df=dmy-all }}</ref>

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===Genes make proteins===

{{main|Genetic code}}

The function of genes is to provide the information needed to generatemake 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.]]