# Genetics

**Genetics** (frae the Auncient Greek γενετικός **genetikos** meanin "genitive"/"generative", in turn frae γένεσις **genesis** meanin "oreegin"), a field in biology, is the science o genes, heredity, an variation in livin organisms.

The faither o genetics is Gregor Mendel, a late 19t-century scientist an Augustinian freear. Mendel studied "trait inheritance", patterns in the wey traits are haundit doun frae paurents tae affspring. He observed that organisms (pease plants) inherit traits bi wey o discrete "units o inheritance". This term, still uised the day, is a somewhit ambiguous definition o whit is referred tae as a gene.

Trait inheritance an molecular inheritance mechanisms o genes are still primar principles o genetics in the 21st century, but modren genetics haes expaundit ayont inheritance tae studyin the function an behaviour o genes. Gene structur andfunction, variation, an distribution are studied within the context o the cell, the organism (e.g. dominance), an within the context o a population. Genetics haes gien rise tae a nummer o subfields, includin epigenetics an population genetics. Organisms studied within the broad field span the domeen o life, includin bacteria, plants, ainimals, an humans.

Genetic processes wark in combination wi an organism's environment an experiences tae influence development an behaviour, eften referred tae as naitur versus nurtur. The intracellular or extracellular environment o a cell or organism mey switch gene transcription on or off. A clessic ensaumple is twa seeds o genetically identical maize, ane placed in a temperate climate an ane in an arid climate. While the average hicht o the twa maize stauks mey be genetically determined tae be equal, the ane in the arid climate anerly growes tae hauf the hicht o the ane in the temperate climate due tae lack o watter an nutrients in its environment.

## Etymology

The wird *genetics* stems frae the auncient Greek γενετικός **genetikos** meanin "genitive"/"generative", that in turn derives frae γένεσις **genesis** meanin "oreegin".

## History

DNA, the molecular basis for biological heirship. Ilk straund o DNA is a cheen o nucleotides, matchin ilk ither in the centre tae form whit leuk lik rungs on a twisted ledder.
The observation that leevin things faw heir tae traits frae thair parents haes been uised syne prehistoric times tae impruive crap plants an ainimals throu selective breedin. The modren science o genetics, seekin tae unnerstaund this process, begoud wi the wark o the Augustinian freear Gregor Mendel in the mid-19t century.

Prior tae Mendel, Imre Festetics, a Hungarian noble, wha lived in Kőszeg afore Mendel, wis the first wha uised the wird "genetics." He descrived several rules o genetic heirship in his wirk *The genetic law o the Naitur* (Die genetische Gesätze der Natur, 1819). His seicont law is the same as whit Mendel published. In his third law, he developed the basic principles o mutation (he can be conseedert a forerunner o Hugo de Vries).

Modren genetics stairtit wi Mendel's studies o the naitur o heirship in plants. In his paper "*Versuche über Pflanzenhybriden*" ("Experiments on Plant Hybridisation"), presentit in 1865 tae the *Naturforschender Verein* (Society for Resairch in Naitur) in Brünn, Mendel traced the heirship patterns o certaint traits in pease plants an descrived them mathematically. Awtho this pattern o heirship coud anerly be observed for a few traits, Mendel's wark suggestit that heirship wis pairticulate, nae acquired, an that the heirship patterns o mony traits coud be explained throu semple rules an ratios.

The importance o Mendel's wirk did nae gain wide unnderstaundin till 1900, efter his daith, whan Hugo de Vries an ither scientists rediscovered his resairch. William Bateson, a proponent o Mendel's wark, mentioned the wird *genetics* in 1905 (the adjective *genetic*, derived frae the Greek wird *genesis*—γένεσις, "oreegin", predates the noun an wis first uised in a biological sense in 1860).

### Molecular genetics

Awtho genes war kent tae exist on chromosomes, chromosomes are componed o baith protein an DNA, an scientists did nae ken that o the twa is responsible for heirship. In 1928, Frederick Griffith discovered the phenomenon o transformation (see Griffith's experiment): deid bacteria coud transfer genetic material tae "transform" ither still-leevin bacteria. Saxteen years later, in 1944, the Avery–MacLeod–McCarty experiment identifee'd DNA as the molecule responsible for transformation.

James Watson an Francis Crick determined the structur o DNA in 1953, uisin the X-ray crystallografie wark o Rosalind Franklin an Maurice Wilkins that indicatit DNA haes a helical structur (i.e., shapit lik a corkscrew). Thair dooble-helix model haed twa strands o DNA wi the nucleotides pyntin inward, each matching a complementary nucleotide on the ither strand tae form what leuk lik rungs on a twisted ladder.

## Featurs o heirship

### Discrete heirship an Mendel's laws

At its maist fundamental level, heirship in organisms occurs bi passin discrete heritable units, cried genes, frae paurents tae affspring. This property wis first observed bi Gregor Mendel, wha studied the segregation o heritable traits in pease plants. In his experiments studyin the trait for flouer colour, Mendel observed that the flouers o ilk pease plant war aither purpie or white—but niver an intermediate atween the twa colours. Thir different, discrete versions o the same gene are cried alleles.

In the case o the pease, that is a diploid species, ilk individual plant haes twa copies o ilk gene, ane copy inheritit frae ilk parent. Mony species, includin humans, hae this pattern o heirship. Diploid organisms wi twa copies o the same allele o a gien gene are cried homozygous at that gene locus, while organisms wi twa different alleles o a gien gene are cried heterozygous.

The set o alleles for a gien organism is cried its genoteep, while the observable traits o the organism are cried its phenoteep. When organisms are heterozygous at a gene, eften ane allele is cried dominant as its qualities dominate the phenoteep o the organism, while the ither allele is cried recessive as its qualities recede an are nae observed. Some alleles dae nae hae complete dominance an insteid hae incomplete dominance bi expressin an intermediate phenoteep, or codominance bi expressin baith alleles at ance.

### Multiple gene interactions

Human height is a trait wi complex genetic causes. Francis Galton's data frae 1889 shaws the relationship atween affspring hicht as a function o mean paurent hicht.
Organisms hae thoosands o genes, an in sexually reproducin organisms thir genes generally assort independently o ilk ither. This means that the inheritance o an allele for yellae or green pease colour is unrelatit tae the inheritance o alleles for white or purpie flouers. This phenomenon, kent as "Mendel's seicont law" or the "law o independent assortment," means that the alleles ofdifferent genes get shuffled atween paurents tae form affspring wi mony different combinations. (Some genes dae nae assort independently, demonstratin genetic linkage, a tapic discussed later in this airticle.)

Eften different genes can interact in a wey that influences the same trait. In the Blue-eed Mary (*Omphalodes verna*), for ensaumple, thare exeests a gene wi alleles that determine the colour o flouers: blue or magenta. Anither gene, houever, controls whither the flouers hae colour at aw or are white. Whan a plant haes twa copies o this white allele, its flouers are white—regairdless o whither the first gene haes blue or magenta alleles. This interaction atween genes is cried epistasis, wi the seicont gene epistatic tae the first.

Mony traits are nae discrete featurs (e.g. purpie or white flouers) but are insteid conteenous featurs (e.g. human hicht an skin colour). Thir complex traits are products o mony genes. The influence o thir genes is mediatit, tae varyin degrees, bi the environment an organism haes experienced. The degree tae which an organism's genes contreibute tae a complex trait is cried heritability. Meisurment o the heritability o a trait is relative—in a mair variable environment, the environment haes a bigger influence on the tot variation o the trait. For ensaumple, human hicht is a trait wi complex causes. It haes a heritability o 89% in the Unitit States. In Nigerie, houever, whaur fowk experience a mair variable access tae guid nutreetion an heal care, hicht haes a heritability o anerly 62%.

## Molecular basis for inheritance

### DNA an chromosomes

The molecular structur o DNA. Bases pair throu the arrangement o hydrogen bondin atween the straunds.
The molecular basis for genes is deoxyribonucleic acid (DNA). DNA is componed o a cheen o nucleotides, o that thare are fower teeps: adenine (A), cytosine (C), guanine (G), an thymine (T). Genetic information exeests in the sequence o thir nucleotides, an genes exeest as stretches o sequence alang the DNA cheen. Viruses are the anerly exception tae this rule—whiles viruses uise the verra seemilar molecule RNA insteid o DNA as thair genetic material. Viruses canna reproduce withoot a host an are unaffectit bi mony genetic processes, sae tend nae tae be conseedert leevin organisms.

DNA normally exeests as a dooble-straundit molecule, cyled intae the shape o a dooble helix. Ilk nucleotide in DNA preferentially pairs wi its pairtner nucleotide on the opposite straund: A pairs wi T, an C pairs wi G. Sicweys, in its twa-straundit form, ilk straund effectively conteens aw necessar information, redundant wi its pairtner straund. This structur o DNA is the pheesical basis for heirship: DNA replication duplicates the genetic information bi splittin the straunds an uisin ilk straund as a template for synthesis o a new pairtner straund.

Genes are arranged linearly alang lang cheens o DNA base-pair sequences. In bacteria, ilk cell uisually conteens a single circular genophore, while eukaryotic organisms (sic as plants an ainimals) hae thair DNA arranged in multiple linear chromosomes. Thir DNA strands are eften extremely lang; the lairgest human chromosome, for ensaumple, is aboot 247 million base pairs in lenth. The DNA o a chromosome is associatit wi structural proteins that organize, compact, an control access tae the DNA, formin a material cried chromatin; in eukaryotes, chromatin is uisually componed o nucleosomes, segments o DNA woond aroond cores o histone proteins. The full set o hereditary material in an organism (uisually the combined DNA sequences o aw chromosomes) is cried the genome.

While haploid organisms hae anerly ane copy o ilk chromosome, maist ainimals an mony plants are diploid, conteenin twa o ilk chromosome an sicweys twa copies o ivery gene.

Mony species hae sae-cried sex chromosomes that determine the gender o ilk organism. In humans an mony ither ainimals, the Y chromosome conteens the gene that triggers the development o the speceefically male chairactereestics.

### Reproduction

When cells divide, thair full genome is copied an ilk dauchter cell inherits ane copy. This process, cried mitosis, is the semplest form o reproduction an is the basis for asexual reproduction. Asexual reproduction can an aa occur in multicellular organisms, producin affspring that inherit thair genome frae a single paurent. Affspring that are genetically identical tae thair paurents are cried clones.

Eukaryotic organisms eften uise sexual reproduction tae generate affspring that conteen a mixtur o genetic material inheritit frae twa different paurents. The process o sexual reproduction alternates atween forms that conteen single copies o the genome (haploid) an dooble copies (diploid).

## Gene expression

### Genetic code

The genetic code: Uisin a triplet code, DNA, throu a messenger RNA intermediary, specifees a protein.
Genes generally express thair functional effect throu the production o proteins, that are complex molecules responsible for maist functions in the cell. Proteins are made up o ane or mair polypeptide cheens, ilk o that is componed o a sequence o amino acids, an the DNA sequence o a gene (throu an RNA intermediate) is uised tae produce a speceefic amino acid sequence. This process begins wi the production o an RNA molecule wi a sequence matchin the gene's DNA sequence, a process cried transcription.

This messenger RNA molecule is then uised tae produce a correspondin amino acid sequence throu a process cried translation. Ilk group o three nucleotides in the sequence, cried a codon, corresponds aither tae ane o the twinty possible amino acids in a protein or an instruction tae end the amino acid sequence; this correspondence is cried the genetic code.

### Naitur an nurtur

Siamese cats hae a temperature-sensitive pigment-production mutation.
Awtho genes conteen aw the information an organism uises tae function, the environment plays an important role in determinin the ultimate phenoteeps an organism displays. The phrase "naitur an nurtur" refers tae this complementary relationship. The phenoteep o an organism depends on the interaction o genes an the environment. An interestin ensaumple is the coat colouration o the Siamese cat. In this case, the bouk temperatur o the cat plays the role o the environment. The cat's genes code for daurk hair, sicweys the hair-producin cells in the cat mak cellular proteins resultin in daurk hair. But thir daurk hair-producin proteins are sensitive tae temperatur (i.e. hae a mutation causin temperature-sensitivity) an denaitur in heicher-temperatur environments, failin tae produce daurk-hair pigment in auries whaur the cat haes a heicher bouk temperatur. In a law-temperatur environment, houever, the protein's structureis stable an produces daurk-hair pigment normally. The protein remeens functional in auries o skin that are caulder—sic as its legs, lugs, tail an face—sae the cat haes daurk-hair at its extremities.

## Genetic cheenge

### Mutations

Gene duplication allous diversification bi providin redundancy: ane gene can mutate an lose its oreeginal function withoot haurmin the organism.
In the process o DNA replication, errors occasionally occur in the polymerisation o seicont strand. Thir errors, cried mutations, can affect the phenoteep o an organism, especially if thay occur within the protein codin sequence o a gene. Error rates are uisually verra law—1 error in ivery 10–100 million bases—due tae the "pruifreadin" ability o DNA polymerases. Processes that increase the rate o cheenges in DNA are cried mutagenic: mutagenic chemicals promote errors in DNA replication, eften bi interferin wi the structur o base-pairin, while UV radiation induces mutations bi causin damage tae the DNA structur. Chemical damage tae DNA occurs naiturally as well an cells uise DNA repair mechanisms tae repair mismatches an breaks. The repair daes nae, houever, alweys restore the oreeginal sequence.

Ower mony generations, the genomes o organisms can cheenge signeeficantly, resultin in evolution. In the process cried adaptation, selection for beneficial mutations can cause a species tae evolve intae forms better able tae survive in thair environment.