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This article is about the general scientific term. Genetics_sentence_0

For the scientific journal, see Genetics (journal). Genetics_sentence_1

For a more accessible and less technical introduction to this topic, see Introduction to genetics. Genetics_sentence_2

Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms. Genetics_sentence_3

Though heredity had been observed for millennia, Gregor Mendel, a scientist and Augustinian friar working in the 19th century, was the first to study genetics scientifically. Genetics_sentence_4

Mendel studied "trait inheritance", patterns in the way traits are handed down from parents to offspring. Genetics_sentence_5

He observed that organisms (pea plants) inherit traits by way of discrete "units of inheritance". Genetics_sentence_6

This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene. Genetics_sentence_7

Trait inheritance and molecular inheritance mechanisms of genes are still primary principles of genetics in the 21st century, but modern genetics has expanded beyond inheritance to studying the function and behavior of genes. Genetics_sentence_8

Gene structure and function, variation, and distribution are studied within the context of the cell, the organism (e.g. dominance), and within the context of a population. Genetics_sentence_9

Genetics has given rise to a number of subfields, including molecular genetics, epigenetics and population genetics. Genetics_sentence_10

Organisms studied within the broad field span the domains of life (archaea, bacteria, and eukarya). Genetics_sentence_11

Genetic processes work in combination with an organism's environment and experiences to influence development and behavior, often referred to as nature versus nurture. Genetics_sentence_12

The intracellular or extracellular environment of a living cell or organism may switch gene transcription on or off. Genetics_sentence_13

A classic example is two seeds of genetically identical corn, one placed in a temperate climate and one in an arid climate (lacking sufficient waterfall or rain). Genetics_sentence_14

While the average height of the two corn stalks may be genetically determined to be equal, the one in the arid climate only grows to half the height of the one in the temperate climate due to lack of water and nutrients in its environment. Genetics_sentence_15

Etymology Genetics_section_0

The word genetics stems from the ancient Greek γενετικός genetikos meaning "genitive"/"generative", which in turn derives from γένεσις genesis meaning "origin". Genetics_sentence_16

History Genetics_section_1

Main article: History of genetics Genetics_sentence_17

The observation that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding. Genetics_sentence_18

The modern science of genetics, seeking to understand this process, began with the work of the Augustinian friar Gregor Mendel in the mid-19th century. Genetics_sentence_19

Prior to Mendel, Imre Festetics, a Hungarian noble, who lived in Kőszeg before Mendel, was the first who used the word "genetics." Genetics_sentence_20

He described several rules of genetic inheritance in his work The genetic law of the Nature (Die genetische Gesätze der Natur, 1819). Genetics_sentence_21

His second law is the same as what Mendel published. Genetics_sentence_22

In his third law, he developed the basic principles of mutation (he can be considered a forerunner of Hugo de Vries). Genetics_sentence_23

Other theories of inheritance preceded Mendel's work. Genetics_sentence_24

A popular theory during the 19th century, and implied by Charles Darwin's 1859 On the Origin of Species, was blending inheritance: the idea that individuals inherit a smooth blend of traits from their parents. Genetics_sentence_25

Mendel's work provided examples where traits were definitely not blended after hybridization, showing that traits are produced by combinations of distinct genes rather than a continuous blend. Genetics_sentence_26

Blending of traits in the progeny is now explained by the action of multiple genes with quantitative effects. Genetics_sentence_27

Another theory that had some support at that time was the inheritance of acquired characteristics: the belief that individuals inherit traits strengthened by their parents. Genetics_sentence_28

This theory (commonly associated with Jean-Baptiste Lamarck) is now known to be wrong—the experiences of individuals do not affect the genes they pass to their children, although evidence in the field of epigenetics has revived some aspects of Lamarck's theory. Genetics_sentence_29

Other theories included the pangenesis of Charles Darwin (which had both acquired and inherited aspects) and Francis Galton's reformulation of pangenesis as both particulate and inherited. Genetics_sentence_30

Mendelian and classical genetics Genetics_section_2

Further information: Mutationism and Modern synthesis (20th century) Genetics_sentence_31

Modern genetics started with Mendel's studies of the nature of inheritance in plants. Genetics_sentence_32

In his paper "Versuche über Pflanzenhybriden" ("Experiments on Plant Hybridization"), presented in 1865 to the Naturforschender Verein (Society for Research in Nature) in Brünn, Mendel traced the inheritance patterns of certain traits in pea plants and described them mathematically. Genetics_sentence_33

Although this pattern of inheritance could only be observed for a few traits, Mendel's work suggested that heredity was particulate, not acquired, and that the inheritance patterns of many traits could be explained through simple rules and ratios. Genetics_sentence_34

The importance of Mendel's work did not gain wide understanding until 1900, after his death, when Hugo de Vries and other scientists rediscovered his research. Genetics_sentence_35

William Bateson, a proponent of Mendel's work, coined the word genetics in 1905 (the adjective genetic, derived from the Greek word genesis—γένεσις, "origin", predates the noun and was first used in a biological sense in 1860). Genetics_sentence_36

Bateson both acted as a mentor and was aided significantly by the work of other scientists from Newnham College at Cambridge, specifically the work of Becky Saunders, Nora Darwin Barlow, and Muriel Wheldale Onslow. Genetics_sentence_37

Bateson popularized the usage of the word genetics to describe the study of inheritance in his inaugural address to the Third International Conference on Plant Hybridization in London in 1906. Genetics_sentence_38

After the rediscovery of Mendel's work, scientists tried to determine which molecules in the cell were responsible for inheritance. Genetics_sentence_39

In 1900, Nettie Stevens began studying the mealworm. Genetics_sentence_40

Over the next 11 years, she discovered that females only had the X chromosome and males had both X and Y chromosomes. Genetics_sentence_41

She was able to conclude that sex is a chromosomal factor and is determined by the male. Genetics_sentence_42

In 1911, Thomas Hunt Morgan argued that genes are on chromosomes, based on observations of a sex-linked white eye mutation in fruit flies. Genetics_sentence_43

In 1913, his student Alfred Sturtevant used the phenomenon of genetic linkage to show that genes are arranged linearly on the chromosome. Genetics_sentence_44

Molecular genetics Genetics_section_3

Although genes were known to exist on chromosomes, chromosomes are composed of both protein and DNA, and scientists did not know which of the two is responsible for inheritance. Genetics_sentence_45

In 1928, Frederick Griffith discovered the phenomenon of transformation (see Griffith's experiment): dead bacteria could transfer genetic material to "transform" other still-living bacteria. Genetics_sentence_46

Sixteen years later, in 1944, the Avery–MacLeod–McCarty experiment identified DNA as the molecule responsible for transformation. Genetics_sentence_47

The role of the nucleus as the repository of genetic information in eukaryotes had been established by Hämmerling in 1943 in his work on the single celled alga Acetabularia. Genetics_sentence_48

The Hershey–Chase experiment in 1952 confirmed that DNA (rather than protein) is the genetic material of the viruses that infect bacteria, providing further evidence that DNA is the molecule responsible for inheritance. Genetics_sentence_49

James Watson and Francis Crick determined the structure of DNA in 1953, using the X-ray crystallography work of Rosalind Franklin and Maurice Wilkins that indicated DNA has a helical structure (i.e., shaped like a corkscrew). Genetics_sentence_50

Their double-helix model had two strands of DNA with the nucleotides pointing inward, each matching a complementary nucleotide on the other strand to form what look like rungs on a twisted ladder. Genetics_sentence_51

This structure showed that genetic information exists in the sequence of nucleotides on each strand of DNA. Genetics_sentence_52

The structure also suggested a simple method for replication: if the strands are separated, new partner strands can be reconstructed for each based on the sequence of the old strand. Genetics_sentence_53

This property is what gives DNA its semi-conservative nature where one strand of new DNA is from an original parent strand. Genetics_sentence_54

Although the structure of DNA showed how inheritance works, it was still not known how DNA influences the behavior of cells. Genetics_sentence_55

In the following years, scientists tried to understand how DNA controls the process of protein production. Genetics_sentence_56

It was discovered that the cell uses DNA as a template to create matching messenger RNA, molecules with nucleotides very similar to DNA. Genetics_sentence_57

The nucleotide sequence of a messenger RNA is used to create an amino acid sequence in protein; this translation between nucleotide sequences and amino acid sequences is known as the genetic code. Genetics_sentence_58

With the newfound molecular understanding of inheritance came an explosion of research. Genetics_sentence_59

A notable theory arose from Tomoko Ohta in 1973 with her amendment to the neutral theory of molecular evolution through publishing the nearly neutral theory of molecular evolution. Genetics_sentence_60

In this theory, Ohta stressed the importance of natural selection and the environment to the rate at which genetic evolution occurs. Genetics_sentence_61

One important development was chain-termination DNA sequencing in 1977 by Frederick Sanger. Genetics_sentence_62

This technology allows scientists to read the nucleotide sequence of a DNA molecule. Genetics_sentence_63

In 1983, Kary Banks Mullis developed the polymerase chain reaction, providing a quick way to isolate and amplify a specific section of DNA from a mixture. Genetics_sentence_64

The efforts of the Human Genome Project, Department of Energy, NIH, and parallel private efforts by Celera Genomics led to the sequencing of the human genome in 2003. Genetics_sentence_65

Features of inheritance Genetics_section_4

Discrete inheritance and Mendel's laws Genetics_section_5

Main article: Mendelian inheritance Genetics_sentence_66

At its most fundamental level, inheritance in organisms occurs by passing discrete heritable units, called genes, from parents to offspring. Genetics_sentence_67

This property was first observed by Gregor Mendel, who studied the segregation of heritable traits in pea plants. Genetics_sentence_68

In his experiments studying the trait for flower color, Mendel observed that the flowers of each pea plant were either purple or white—but never an intermediate between the two colors. Genetics_sentence_69

These different, discrete versions of the same gene are called alleles. Genetics_sentence_70

In the case of the pea, which is a diploid species, each individual plant has two copies of each gene, one copy inherited from each parent. Genetics_sentence_71

Many species, including humans, have this pattern of inheritance. Genetics_sentence_72

Diploid organisms with two copies of the same allele of a given gene are called homozygous at that gene locus, while organisms with two different alleles of a given gene are called heterozygous. Genetics_sentence_73

The set of alleles for a given organism is called its genotype, while the observable traits of the organism are called its phenotype. Genetics_sentence_74

When organisms are heterozygous at a gene, often one allele is called dominant as its qualities dominate the phenotype of the organism, while the other allele is called recessive as its qualities recede and are not observed. Genetics_sentence_75

Some alleles do not have complete dominance and instead have incomplete dominance by expressing an intermediate phenotype, or codominance by expressing both alleles at once. Genetics_sentence_76

When a pair of organisms reproduce sexually, their offspring randomly inherit one of the two alleles from each parent. Genetics_sentence_77

These observations of discrete inheritance and the segregation of alleles are collectively known as Mendel's first law or the Law of Segregation. Genetics_sentence_78

Notation and diagrams Genetics_section_6

Geneticists use diagrams and symbols to describe inheritance. Genetics_sentence_79

A gene is represented by one or a few letters. Genetics_sentence_80

Often a "+" symbol is used to mark the usual, non-mutant allele for a gene. Genetics_sentence_81

In fertilization and breeding experiments (and especially when discussing Mendel's laws) the parents are referred to as the "P" generation and the offspring as the "F1" (first filial) generation. Genetics_sentence_82

When the F1 offspring mate with each other, the offspring are called the "F2" (second filial) generation. Genetics_sentence_83

One of the common diagrams used to predict the result of cross-breeding is the Punnett square. Genetics_sentence_84

When studying human genetic diseases, geneticists often use pedigree charts to represent the inheritance of traits. Genetics_sentence_85

These charts map the inheritance of a trait in a family tree. Genetics_sentence_86

Multiple gene interactions Genetics_section_7

Organisms have thousands of genes, and in sexually reproducing organisms these genes generally assort independently of each other. Genetics_sentence_87

This means that the inheritance of an allele for yellow or green pea color is unrelated to the inheritance of alleles for white or purple flowers. Genetics_sentence_88

This phenomenon, known as "Mendel's second law" or the "law of independent assortment," means that the alleles of different genes get shuffled between parents to form offspring with many different combinations. Genetics_sentence_89

(Some genes do not assort independently, demonstrating genetic linkage, a topic discussed later in this article.) Genetics_sentence_90

Often different genes can interact in a way that influences the same trait. Genetics_sentence_91

In the Blue-eyed Mary (Omphalodes verna), for example, there exists a gene with alleles that determine the color of flowers: blue or magenta. Genetics_sentence_92

Another gene, however, controls whether the flowers have color at all or are white. Genetics_sentence_93

When a plant has two copies of this white allele, its flowers are white—regardless of whether the first gene has blue or magenta alleles. Genetics_sentence_94

This interaction between genes is called epistasis, with the second gene epistatic to the first. Genetics_sentence_95

Many traits are not discrete features (e.g. purple or white flowers) but are instead continuous features (e.g. human height and skin color). Genetics_sentence_96

These complex traits are products of many genes. Genetics_sentence_97

The influence of these genes is mediated, to varying degrees, by the environment an organism has experienced. Genetics_sentence_98

The degree to which an organism's genes contribute to a complex trait is called heritability. Genetics_sentence_99

Measurement of the heritability of a trait is relative—in a more variable environment, the environment has a bigger influence on the total variation of the trait. Genetics_sentence_100

For example, human height is a trait with complex causes. Genetics_sentence_101

It has a heritability of 89% in the United States. Genetics_sentence_102

In Nigeria, however, where people experience a more variable access to good nutrition and health care, height has a heritability of only 62%. Genetics_sentence_103

Molecular basis for inheritance Genetics_section_8

DNA and chromosomes Genetics_section_9

Main articles: DNA and Chromosome Genetics_sentence_104

The molecular basis for genes is deoxyribonucleic acid (DNA). Genetics_sentence_105

DNA is composed of a chain of nucleotides, of which there are four types: adenine (A), cytosine (C), guanine (G), and thymine (T). Genetics_sentence_106

Genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence along the DNA chain. Genetics_sentence_107

Viruses are the only exception to this rule—sometimes viruses use the very similar molecule RNA instead of DNA as their genetic material. Genetics_sentence_108

Viruses cannot reproduce without a host and are unaffected by many genetic processes, so tend not to be considered living organisms. Genetics_sentence_109

DNA normally exists as a double-stranded molecule, coiled into the shape of a double helix. Genetics_sentence_110

Each nucleotide in DNA preferentially pairs with its partner nucleotide on the opposite strand: A pairs with T, and C pairs with G. Thus, in its two-stranded form, each strand effectively contains all necessary information, redundant with its partner strand. Genetics_sentence_111

This structure of DNA is the physical basis for inheritance: DNA replication duplicates the genetic information by splitting the strands and using each strand as a template for synthesis of a new partner strand. Genetics_sentence_112

Genes are arranged linearly along long chains of DNA base-pair sequences. Genetics_sentence_113

In bacteria, each cell usually contains a single circular genophore, while eukaryotic organisms (such as plants and animals) have their DNA arranged in multiple linear chromosomes. Genetics_sentence_114

These DNA strands are often extremely long; the largest human chromosome, for example, is about 247 million base pairs in length. Genetics_sentence_115

The DNA of a chromosome is associated with structural proteins that organize, compact, and control access to the DNA, forming a material called chromatin; in eukaryotes, chromatin is usually composed of nucleosomes, segments of DNA wound around cores of histone proteins. Genetics_sentence_116

The full set of hereditary material in an organism (usually the combined DNA sequences of all chromosomes) is called the genome. Genetics_sentence_117

DNA is most often found in the nucleus of cells, but Ruth Sager helped in the discovery of nonchromosomal genes found outside of the nucleus. Genetics_sentence_118

In plants, these are often found in the chloroplasts and in other organisms, in the mitochondria. Genetics_sentence_119

These nonchromosomal genes can still be passed on by either partner in sexual reproduction and they control a variety of hereditary characteristics that replicate and remain active throughout generations. Genetics_sentence_120

While haploid organisms have only one copy of each chromosome, most animals and many plants are diploid, containing two of each chromosome and thus two copies of every gene. Genetics_sentence_121

The two alleles for a gene are located on identical loci of the two homologous chromosomes, each allele inherited from a different parent. Genetics_sentence_122

Many species have so-called sex chromosomes that determine the gender of each organism. Genetics_sentence_123

In humans and many other animals, the Y chromosome contains the gene that triggers the development of the specifically male characteristics. Genetics_sentence_124

In evolution, this chromosome has lost most of its content and also most of its genes, while the X chromosome is similar to the other chromosomes and contains many genes. Genetics_sentence_125

This being said, Mary Frances Lyon discovered that there is X-chromosome inactivation during reproduction to avoid passing on twice as many genes to the offspring. Genetics_sentence_126

Lyon's discovery led to the discovery of other things including X-linked diseases. Genetics_sentence_127

The X and Y chromosomes form a strongly heterogeneous pair. Genetics_sentence_128

Reproduction Genetics_section_10

Main articles: Asexual reproduction and Sexual reproduction Genetics_sentence_129

When cells divide, their full genome is copied and each daughter cell inherits one copy. Genetics_sentence_130

This process, called mitosis, is the simplest form of reproduction and is the basis for asexual reproduction. Genetics_sentence_131

Asexual reproduction can also occur in multicellular organisms, producing offspring that inherit their genome from a single parent. Genetics_sentence_132

Offspring that are genetically identical to their parents are called clones. Genetics_sentence_133

Eukaryotic organisms often use sexual reproduction to generate offspring that contain a mixture of genetic material inherited from two different parents. Genetics_sentence_134

The process of sexual reproduction alternates between forms that contain single copies of the genome (haploid) and double copies (diploid). Genetics_sentence_135

Haploid cells fuse and combine genetic material to create a diploid cell with paired chromosomes. Genetics_sentence_136

Diploid organisms form haploids by dividing, without replicating their DNA, to create daughter cells that randomly inherit one of each pair of chromosomes. Genetics_sentence_137

Most animals and many plants are diploid for most of their lifespan, with the haploid form reduced to single cell gametes such as sperm or eggs. Genetics_sentence_138

Although they do not use the haploid/diploid method of sexual reproduction, bacteria have many methods of acquiring new genetic information. Genetics_sentence_139

Some bacteria can undergo conjugation, transferring a small circular piece of DNA to another bacterium. Genetics_sentence_140

Bacteria can also take up raw DNA fragments found in the environment and integrate them into their genomes, a phenomenon known as transformation. Genetics_sentence_141

These processes result in horizontal gene transfer, transmitting fragments of genetic information between organisms that would be otherwise unrelated. Genetics_sentence_142

Natural bacterial transformation occurs in many bacterial species, and can be regarded as a sexual process for transferring DNA from one cell to another cell (usually of the same species). Genetics_sentence_143

Transformation requires the action of numerous bacterial gene products, and its primary adaptive function appears to be repair of DNA damages in the recipient cell. Genetics_sentence_144

Recombination and genetic linkage Genetics_section_11

Main articles: Chromosomal crossover and Genetic linkage Genetics_sentence_145

The diploid nature of chromosomes allows for genes on different chromosomes to assort independently or be separated from their homologous pair during sexual reproduction wherein haploid gametes are formed. Genetics_sentence_146

In this way new combinations of genes can occur in the offspring of a mating pair. Genetics_sentence_147

Genes on the same chromosome would theoretically never recombine. Genetics_sentence_148

However, they do, via the cellular process of chromosomal crossover. Genetics_sentence_149

During crossover, chromosomes exchange stretches of DNA, effectively shuffling the gene alleles between the chromosomes. Genetics_sentence_150

This process of chromosomal crossover generally occurs during meiosis, a series of cell divisions that creates haploid cells. Genetics_sentence_151

Meiotic recombination, particularly in microbial eukaryotes, appears to serve the adaptive function of repair of DNA damages. Genetics_sentence_152

The first cytological demonstration of crossing over was performed by Harriet Creighton and Barbara McClintock in 1931. Genetics_sentence_153

Their research and experiments on corn provided cytological evidence for the genetic theory that linked genes on paired chromosomes do in fact exchange places from one homolog to the other. Genetics_sentence_154

The probability of chromosomal crossover occurring between two given points on the chromosome is related to the distance between the points. Genetics_sentence_155

For an arbitrarily long distance, the probability of crossover is high enough that the inheritance of the genes is effectively uncorrelated. Genetics_sentence_156

For genes that are closer together, however, the lower probability of crossover means that the genes demonstrate genetic linkage; alleles for the two genes tend to be inherited together. Genetics_sentence_157

The amounts of linkage between a series of genes can be combined to form a linear linkage map that roughly describes the arrangement of the genes along the chromosome. Genetics_sentence_158

Gene expression Genetics_section_12

Genetic code Genetics_section_13

Main article: Genetic code Genetics_sentence_159

Genes generally express their functional effect through the production of proteins, which are complex molecules responsible for most functions in the cell. Genetics_sentence_160

Proteins are made up of one or more polypeptide chains, each of which is composed of a sequence of amino acids, and the DNA sequence of a gene (through an RNA intermediate) is used to produce a specific amino acid sequence. Genetics_sentence_161

This process begins with the production of an RNA molecule with a sequence matching the gene's DNA sequence, a process called transcription. Genetics_sentence_162

This messenger RNA molecule is then used to produce a corresponding amino acid sequence through a process called translation. Genetics_sentence_163

Each group of three nucleotides in the sequence, called a codon, corresponds either to one of the twenty possible amino acids in a protein or an instruction to end the amino acid sequence; this correspondence is called the genetic code. Genetics_sentence_164

The flow of information is unidirectional: information is transferred from nucleotide sequences into the amino acid sequence of proteins, but it never transfers from protein back into the sequence of DNA—a phenomenon Francis Crick called the central dogma of molecular biology. Genetics_sentence_165

The specific sequence of amino acids results in a unique three-dimensional structure for that protein, and the three-dimensional structures of proteins are related to their functions. Genetics_sentence_166

Some are simple structural molecules, like the fibers formed by the protein collagen. Genetics_sentence_167

Proteins can bind to other proteins and simple molecules, sometimes acting as enzymes by facilitating chemical reactions within the bound molecules (without changing the structure of the protein itself). Genetics_sentence_168

Protein structure is dynamic; the protein hemoglobin bends into slightly different forms as it facilitates the capture, transport, and release of oxygen molecules within mammalian blood. Genetics_sentence_169

A single nucleotide difference within DNA can cause a change in the amino acid sequence of a protein. Genetics_sentence_170

Because protein structures are the result of their amino acid sequences, some changes can dramatically change the properties of a protein by destabilizing the structure or changing the surface of the protein in a way that changes its interaction with other proteins and molecules. Genetics_sentence_171

For example, sickle-cell anemia is a human genetic disease that results from a single base difference within the coding region for the β-globin section of hemoglobin, causing a single amino acid change that changes hemoglobin's physical properties. Genetics_sentence_172

Sickle-cell versions of hemoglobin stick to themselves, stacking to form fibers that distort the shape of red blood cells carrying the protein. Genetics_sentence_173

These sickle-shaped cells no longer flow smoothly through blood vessels, having a tendency to clog or degrade, causing the medical problems associated with this disease. Genetics_sentence_174

Some DNA sequences are transcribed into RNA but are not translated into protein products—such RNA molecules are called non-coding RNA. Genetics_sentence_175

In some cases, these products fold into structures which are involved in critical cell functions (e.g. ribosomal RNA and transfer RNA). Genetics_sentence_176

RNA can also have regulatory effects through hybridization interactions with other RNA molecules (e.g. microRNA). Genetics_sentence_177

Nature and nurture Genetics_section_14

Main article: Nature and nurture Genetics_sentence_178

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. Genetics_sentence_179

The phrase "nature and nurture" refers to this complementary relationship. Genetics_sentence_180

The phenotype of an organism depends on the interaction of genes and the environment. Genetics_sentence_181

An interesting example is the coat coloration of the Siamese cat. Genetics_sentence_182

In this case, the body temperature of the cat plays the role of the environment. Genetics_sentence_183

The cat's genes code for dark hair, thus the hair-producing cells in the cat make cellular proteins resulting in dark hair. Genetics_sentence_184

But these dark hair-producing proteins are sensitive to temperature (i.e. have a mutation causing temperature-sensitivity) and denature in higher-temperature environments, failing to produce dark-hair pigment in areas where the cat has a higher body temperature. Genetics_sentence_185

In a low-temperature environment, however, the protein's structure is stable and produces dark-hair pigment normally. Genetics_sentence_186

The protein remains functional in areas of skin that are colder—such as its legs, ears, tail and face—so the cat has dark hair at its extremities. Genetics_sentence_187

Environment plays a major role in effects of the human genetic disease phenylketonuria. Genetics_sentence_188

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. Genetics_sentence_189

However, if someone with the phenylketonuria mutation follows a strict diet that avoids this amino acid, they remain normal and healthy. Genetics_sentence_190

A common method for determining how genes and environment ("nature and nurture") contribute to a phenotype involves studying identical and fraternal twins, or other siblings of multiple births. Genetics_sentence_191

Identical siblings are genetically the same since they come from the same zygote. Genetics_sentence_192

Meanwhile, fraternal twins are as genetically different from one another as normal siblings. Genetics_sentence_193

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. Genetics_sentence_194

One famous example involved the study of the Genain quadruplets, who were identical quadruplets all diagnosed with schizophrenia. Genetics_sentence_195

However, such tests cannot separate genetic factors from environmental factors affecting fetal development. Genetics_sentence_196

Gene regulation Genetics_section_15

Main article: Regulation of gene expression Genetics_sentence_197

The genome of a given organism contains thousands of genes, but not all these genes need to be active at any given moment. Genetics_sentence_198

A gene is expressed when it is being transcribed into mRNA and there exist many cellular methods of controlling the expression of genes such that proteins are produced only when needed by the cell. Genetics_sentence_199

Transcription factors are regulatory proteins that bind to DNA, either promoting or inhibiting the transcription of a gene. Genetics_sentence_200

Within the genome of Escherichia coli bacteria, for example, there exists a series of genes necessary for the synthesis of the amino acid tryptophan. Genetics_sentence_201

However, when tryptophan is already available to the cell, these genes for tryptophan synthesis are no longer needed. Genetics_sentence_202

The presence of tryptophan directly affects the activity of the genes—tryptophan molecules bind to the tryptophan repressor (a transcription factor), changing the repressor's structure such that the repressor binds to the genes. Genetics_sentence_203

The tryptophan repressor blocks the transcription and expression of the genes, thereby creating negative feedback regulation of the tryptophan synthesis process. Genetics_sentence_204

Differences in gene expression are especially clear within multicellular organisms, where cells all contain the same genome but have very different structures and behaviors due to the expression of different sets of genes. Genetics_sentence_205

All the cells in a multicellular organism derive from a single cell, differentiating into variant cell types in response to external and intercellular signals and gradually establishing different patterns of gene expression to create different behaviors. Genetics_sentence_206

As no single gene is responsible for the development of structures within multicellular organisms, these patterns arise from the complex interactions between many cells. Genetics_sentence_207

Within eukaryotes, there exist structural features of chromatin that influence the transcription of genes, often in the form of modifications to DNA and chromatin that are stably inherited by daughter cells. Genetics_sentence_208

These features are called "epigenetic" because they exist "on top" of the DNA sequence and retain inheritance from one cell generation to the next. Genetics_sentence_209

Because of epigenetic features, different cell types grown within the same medium can retain very different properties. Genetics_sentence_210

Although epigenetic features are generally dynamic over the course of development, some, like the phenomenon of paramutation, have multigenerational inheritance and exist as rare exceptions to the general rule of DNA as the basis for inheritance. Genetics_sentence_211

Genetic change Genetics_section_16

Mutations Genetics_section_17

Main article: Mutation Genetics_sentence_212

During the process of DNA replication, errors occasionally occur in the polymerization of the second strand. Genetics_sentence_213

These errors, called mutations, can affect the phenotype of an organism, especially if they occur within the protein coding sequence of a gene. Genetics_sentence_214

Error rates are usually very low—1 error in every 10–100 million bases—due to the "proofreading" ability of DNA polymerases. Genetics_sentence_215

Processes that increase the rate of changes in DNA are called mutagenic: mutagenic chemicals promote errors in DNA replication, often by interfering with the structure of base-pairing, while UV radiation induces mutations by causing damage to the DNA structure. Genetics_sentence_216

Chemical damage to DNA occurs naturally as well and cells use DNA repair mechanisms to repair mismatches and breaks. Genetics_sentence_217

The repair does not, however, always restore the original sequence. Genetics_sentence_218

A particularly important source of DNA damages appears to be reactive oxygen species produced by cellular aerobic respiration, and these can lead to mutations. Genetics_sentence_219

In organisms that use chromosomal crossover to exchange DNA and recombine genes, errors in alignment during meiosis can also cause mutations. Genetics_sentence_220

Errors in crossover are especially likely when similar sequences cause partner chromosomes to adopt a mistaken alignment; this makes some regions in genomes more prone to mutating in this way. Genetics_sentence_221

These errors create large structural changes in DNA sequence – duplications, inversions, deletions of entire regions – or the accidental exchange of whole parts of sequences between different chromosomes (chromosomal translocation). Genetics_sentence_222

Natural selection and evolution Genetics_section_18

Main article: Evolution Genetics_sentence_223

Further information: Natural selection Genetics_sentence_224

Mutations alter an organism's genotype and occasionally this causes different phenotypes to appear. Genetics_sentence_225

Most mutations have little effect on an organism's phenotype, health, or reproductive fitness. Genetics_sentence_226

Mutations that do have an effect are usually detrimental, but occasionally some can be beneficial. Genetics_sentence_227

Studies in the fly Drosophila melanogaster suggest that if a mutation changes a protein produced by a gene, about 70 percent of these mutations will be harmful with the remainder being either neutral or weakly beneficial. Genetics_sentence_228

Population genetics studies the distribution of genetic differences within populations and how these distributions change over time. Genetics_sentence_229

Changes in the frequency of an allele in a population are mainly influenced by natural selection, where a given allele provides a selective or reproductive advantage to the organism, as well as other factors such as mutation, genetic drift, genetic hitchhiking, artificial selection and migration. Genetics_sentence_230

Over many generations, the genomes of organisms can change significantly, resulting in evolution. Genetics_sentence_231

In the process called adaptation, selection for beneficial mutations can cause a species to evolve into forms better able to survive in their environment. Genetics_sentence_232

New species are formed through the process of speciation, often caused by geographical separations that prevent populations from exchanging genes with each other. Genetics_sentence_233

By comparing the homology between different species' genomes, it is possible to calculate the evolutionary distance between them and when they may have diverged. Genetics_sentence_234

Genetic comparisons are generally considered a more accurate method of characterizing the relatedness between species than the comparison of phenotypic characteristics. Genetics_sentence_235

The evolutionary distances between species can be used to form evolutionary trees; these trees represent the common descent and divergence of species over time, although they do not show the transfer of genetic material between unrelated species (known as horizontal gene transfer and most common in bacteria). Genetics_sentence_236

Model organisms Genetics_section_19

Although geneticists originally studied inheritance in a wide range of organisms, researchers began to specialize in studying the genetics of a particular subset of organisms. Genetics_sentence_237

The fact that significant research already existed for a given organism would encourage new researchers to choose it for further study, and so eventually a few model organisms became the basis for most genetics research. Genetics_sentence_238

Common research topics in model organism genetics include the study of gene regulation and the involvement of genes in development and cancer. Genetics_sentence_239

Organisms were chosen, in part, for convenience—short generation times and easy genetic manipulation made some organisms popular genetics research tools. Genetics_sentence_240

Widely used model organisms include the gut bacterium Escherichia coli, the plant Arabidopsis thaliana, baker's yeast (Saccharomyces cerevisiae), the nematode Caenorhabditis elegans, the common fruit fly (Drosophila melanogaster), and the common house mouse (Mus musculus). Genetics_sentence_241

Medicine Genetics_section_20

Medical genetics seeks to understand how genetic variation relates to human health and disease. Genetics_sentence_242

When searching for an unknown gene that may be involved in a disease, researchers commonly use genetic linkage and genetic pedigree charts to find the location on the genome associated with the disease. Genetics_sentence_243

At the population level, researchers take advantage of Mendelian randomization to look for locations in the genome that are associated with diseases, a method especially useful for multigenic traits not clearly defined by a single gene. Genetics_sentence_244

Once a candidate gene is found, further research is often done on the corresponding (or homologous) genes of model organisms. Genetics_sentence_245

In addition to studying genetic diseases, the increased availability of genotyping methods has led to the field of pharmacogenetics: the study of how genotype can affect drug responses. Genetics_sentence_246

Individuals differ in their inherited tendency to develop cancer, and cancer is a genetic disease. Genetics_sentence_247

The process of cancer development in the body is a combination of events. Genetics_sentence_248

Mutations occasionally occur within cells in the body as they divide. Genetics_sentence_249

Although these mutations will not be inherited by any offspring, they can affect the behavior of cells, sometimes causing them to grow and divide more frequently. Genetics_sentence_250

There are biological mechanisms that attempt to stop this process; signals are given to inappropriately dividing cells that should trigger cell death, but sometimes additional mutations occur that cause cells to ignore these messages. Genetics_sentence_251

An internal process of natural selection occurs within the body and eventually mutations accumulate within cells to promote their own growth, creating a cancerous tumor that grows and invades various tissues of the body. Genetics_sentence_252

Normally, a cell divides only in response to signals called growth factors and stops growing once in contact with surrounding cells and in response to growth-inhibitory signals. Genetics_sentence_253

It usually then divides a limited number of times and dies, staying within the epithelium where it is unable to migrate to other organs. Genetics_sentence_254

To become a cancer cell, a cell has to accumulate mutations in a number of genes (three to seven). Genetics_sentence_255

A cancer cell can divide without growth factor and ignores inhibitory signals. Genetics_sentence_256

Also, it is immortal and can grow indefinitely, even after it makes contact with neighboring cells. Genetics_sentence_257

It may escape from the epithelium and ultimately from the primary tumor. Genetics_sentence_258

Then, the escaped cell can cross the endothelium of a blood vessel and get transported by the bloodstream to colonize a new organ, forming deadly metastasis. Genetics_sentence_259

Although there are some genetic predispositions in a small fraction of cancers, the major fraction is due to a set of new genetic mutations that originally appear and accumulate in one or a small number of cells that will divide to form the tumor and are not transmitted to the progeny (somatic mutations). Genetics_sentence_260

The most frequent mutations are a loss of function of p53 protein, a tumor suppressor, or in the p53 pathway, and gain of function mutations in the Ras proteins, or in other oncogenes. Genetics_sentence_261

Research methods Genetics_section_21

DNA can be manipulated in the laboratory. Genetics_sentence_262

Restriction enzymes are commonly used enzymes that cut DNA at specific sequences, producing predictable fragments of DNA. Genetics_sentence_263

DNA fragments can be visualized through use of gel electrophoresis, which separates fragments according to their length. Genetics_sentence_264

The use of ligation enzymes allows DNA fragments to be connected. Genetics_sentence_265

By binding ("ligating") fragments of DNA together from different sources, researchers can create recombinant DNA, the DNA often associated with genetically modified organisms. Genetics_sentence_266

Recombinant DNA is commonly used in the context of plasmids: short circular DNA molecules with a few genes on them. Genetics_sentence_267

In the process known as molecular cloning, researchers can amplify the DNA fragments by inserting plasmids into bacteria and then culturing them on plates of agar (to isolate clones of bacteria cells—"cloning" can also refer to the various means of creating cloned ("clonal") organisms). Genetics_sentence_268

DNA can also be amplified using a procedure called the polymerase chain reaction (PCR). Genetics_sentence_269

By using specific short sequences of DNA, PCR can isolate and exponentially amplify a targeted region of DNA. Genetics_sentence_270

Because it can amplify from extremely small amounts of DNA, PCR is also often used to detect the presence of specific DNA sequences. Genetics_sentence_271

DNA sequencing and genomics Genetics_section_22

DNA sequencing, one of the most fundamental technologies developed to study genetics, allows researchers to determine the sequence of nucleotides in DNA fragments. Genetics_sentence_272

The technique of chain-termination sequencing, developed in 1977 by a team led by Frederick Sanger, is still routinely used to sequence DNA fragments. Genetics_sentence_273

Using this technology, researchers have been able to study the molecular sequences associated with many human diseases. Genetics_sentence_274

As sequencing has become less expensive, researchers have sequenced the genomes of many organisms using a process called genome assembly, which utilizes computational tools to stitch together sequences from many different fragments. Genetics_sentence_275

These technologies were used to sequence the human genome in the Human Genome Project completed in 2003. Genetics_sentence_276

New high-throughput sequencing technologies are dramatically lowering the cost of DNA sequencing, with many researchers hoping to bring the cost of resequencing a human genome down to a thousand dollars. Genetics_sentence_277

Next-generation sequencing (or high-throughput sequencing) came about due to the ever-increasing demand for low-cost sequencing. Genetics_sentence_278

These sequencing technologies allow the production of potentially millions of sequences concurrently. Genetics_sentence_279

The large amount of sequence data available has created the field of genomics, research that uses computational tools to search for and analyze patterns in the full genomes of organisms. Genetics_sentence_280

Genomics can also be considered a subfield of bioinformatics, which uses computational approaches to analyze large sets of biological data. Genetics_sentence_281

A common problem to these fields of research is how to manage and share data that deals with human subject and personally identifiable information. Genetics_sentence_282

Society and culture Genetics_section_23

See also: Genetics in fiction Genetics_sentence_283

On 19 March 2015, a group of leading biologists urged a worldwide ban on clinical use of methods, particularly the use of CRISPR and zinc finger, to edit the human genome in a way that can be inherited. Genetics_sentence_284

In April 2015, Chinese researchers reported results of basic research to edit the DNA of non-viable human embryos using CRISPR. Genetics_sentence_285

See also Genetics_section_24

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