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104-576: 58800 ENSG00000092439 ENSMUSG00000027365 Q96QT4 Q923J1 NM_001301212 NM_017672 NM_001164325 NM_021450 NP_001288141 NP_060142 NP_001157797 NP_067425 Transient receptor potential cation channel, subfamily M, member 7 , also known as TRPM7 , is a human gene encoding a protein of the same name. TRPs, mammalian homologs of the Drosophila transient receptor potential (trp) protein, are ion channels that are thought to mediate capacitative calcium entry into
208-584: A promoter sequence. The promoter is recognized and bound by transcription factors that recruit and help RNA polymerase bind to the region to initiate transcription. The recognition typically occurs as a consensus sequence like the TATA box . A gene can have more than one promoter, resulting in messenger RNAs ( mRNA ) that differ in how far they extend in the 5' end. Highly transcribed genes have "strong" promoter sequences that form strong associations with transcription factors, thereby initiating transcription at
312-487: A " start codon ", and three " stop codons " indicate the beginning and end of the protein coding region . There are 64 possible codons (four possible nucleotides at each of three positions, hence 4 possible codons) and only 20 standard amino acids; hence the code is redundant and multiple codons can specify the same amino acid. The correspondence between codons and amino acids is nearly universal among all known living organisms. Eye color Eye color
416-401: A brown to a green. Although hazel mostly consists of brown and green, the dominant color in the eye can either be brown/gold or green. This is why hazel eyes can be mistaken as amber, and why amber is often counted as hazel in studies, and vice versa. The combination can sometimes produce a multicolored iris, i.e. an eye that is light brown/amber near the pupil and charcoal or dark green on
520-445: A continuous messenger RNA , referred to as a polycistronic mRNA . The term cistron in this context is equivalent to gene. The transcription of an operon's mRNA is often controlled by a repressor that can occur in an active or inactive state depending on the presence of specific metabolites. When active, the repressor binds to a DNA sequence at the beginning of the operon, called the operator region , and represses transcription of
624-425: A darkened, occasionally black region encircling the iris resulting from a manifestation of the optical properties of the corneal limbus . Limbal rings are not present in all individuals, and their thickness and prominence may correlate with health or youthfulness, and contributes to facial attractiveness . Although people with lighter eye color are generally more sensitive to light because they have less pigment in
728-495: A double-helix run in opposite directions. Nucleic acid synthesis, including DNA replication and transcription occurs in the 5'→3' direction, because new nucleotides are added via a dehydration reaction that uses the exposed 3' hydroxyl as a nucleophile . The expression of genes encoded in DNA begins by transcribing the gene into RNA , a second type of nucleic acid that is very similar to DNA, but whose monomers contain
832-488: A few genes and are transferable between individuals. For example, the genes for antibiotic resistance are usually encoded on bacterial plasmids and can be passed between individual cells, even those of different species, via horizontal gene transfer . Whereas the chromosomes of prokaryotes are relatively gene-dense, those of eukaryotes often contain regions of DNA that serve no obvious function. Simple single-celled eukaryotes have relatively small amounts of such DNA, whereas
936-434: A gene - surprisingly, there is no definition that is entirely satisfactory. A gene is a DNA sequence that codes for a diffusible product. This product may be protein (as is the case in the majority of genes) or may be RNA (as is the case of genes that code for tRNA and rRNA). The crucial feature is that the product diffuses away from its site of synthesis to act elsewhere. The important parts of such definitions are: (1) that
1040-565: A gene corresponds to a transcription unit; (2) that genes produce both mRNA and noncoding RNAs; and (3) regulatory sequences control gene expression but are not part of the gene itself. However, there's one other important part of the definition and it is emphasized in Kostas Kampourakis' book Making Sense of Genes . Therefore in this book I will consider genes as DNA sequences encoding information for functional products, be it proteins or RNA molecules. With 'encoding information', I mean that
1144-410: A gene may be split across chromosomes but those transcripts are concatenated back together into a functional sequence by trans-splicing . It is also possible for overlapping genes to share some of their DNA sequence, either on opposite strands or the same strand (in a different reading frame, or even the same reading frame). In all organisms, two steps are required to read the information encoded in
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#17328838173201248-404: A gene's DNA and produce the protein it specifies. First, the gene's DNA is transcribed to messenger RNA ( mRNA ). Second, that mRNA is translated to protein. RNA-coding genes must still go through the first step, but are not translated into protein. The process of producing a biologically functional molecule of either RNA or protein is called gene expression , and the resulting molecule
1352-565: A gene: that of bacteriophage MS2 coat protein. The subsequent development of chain-termination DNA sequencing in 1977 by Frederick Sanger improved the efficiency of sequencing and turned it into a routine laboratory tool. An automated version of the Sanger method was used in early phases of the Human Genome Project . The theories developed in the early 20th century to integrate Mendelian genetics with Darwinian evolution are called
1456-439: A gene; however, members of a population may have different alleles at the locus, each with a slightly different gene sequence. The majority of eukaryotic genes are stored on a set of large, linear chromosomes. The chromosomes are packed within the nucleus in complex with storage proteins called histones to form a unit called a nucleosome . DNA packaged and condensed in this way is called chromatin . The manner in which DNA
1560-448: A high rate. Others genes have "weak" promoters that form weak associations with transcription factors and initiate transcription less frequently. Eukaryotic promoter regions are much more complex and difficult to identify than prokaryotic promoters. Additionally, genes can have regulatory regions many kilobases upstream or downstream of the gene that alter expression. These act by binding to transcription factors which then cause
1664-462: A highly sexually dimorphic eye color. Studies from various populations in Europe have shown that men are substantially more likely to have blue eyes than women. The inheritance pattern followed by blue eyes was previously assumed to be a Mendelian recessive trait, though this has been shown to be incorrect. Eye color inheritance is now recognized as a polygenic trait , meaning that it is controlled by
1768-417: A lightening of their eye color by adulthood. Children with green eyes often experienced a darkening of their eye color. It was also found that 11% of the children's mothers experienced an eye color change during the same period, with most developing lighter eyes, relative to their original color at the time of their child's birth. Almost all mammals have brown or darkly-pigmented irises. In humans, brown
1872-492: A mutation of the gene coding for the enzyme ATPase 7B , which prevents copper within the liver from entering the Golgi apparatus in cells. Instead, the copper accumulates in the liver and in other tissues, including the iris of the eye. This results in the formation of Kayser–Fleischer rings , which are dark rings that encircle the periphery of the iris. Eye color outside of the iris may also be symptomatic of disease. Yellowing of
1976-572: A new expanded definition that includes noncoding genes. However, some modern writers still do not acknowledge noncoding genes although this so-called "new" definition has been recognised for more than half a century. Although some definitions can be more broadly applicable than others, the fundamental complexity of biology means that no definition of a gene can capture all aspects perfectly. Not all genomes are DNA (e.g. RNA viruses ), bacterial operons are multiple protein-coding regions transcribed into single large mRNAs, alternative splicing enables
2080-644: A part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia). It is a result of the relative excess or lack of pigment within an iris or part of an iris, which may be inherited or acquired by disease or injury . This uncommon condition usually results due to uneven melanin content. A number of causes are responsible, including genetic, such as chimerism , Horner's syndrome and Waardenburg syndrome . A chimera can have two different colored eyes just like any two siblings can—because each cell has different eye color genes. A mosaic can have two different colored eyes if
2184-400: A process known as RNA splicing . Finally, the ends of gene transcripts are defined by cleavage and polyadenylation (CPA) sites , where newly produced pre-mRNA gets cleaved and a string of ~200 adenosine monophosphates is added at the 3' end. The poly(A) tail protects mature mRNA from degradation and has other functions, affecting translation, localization, and transport of the transcript from
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#17328838173202288-419: A protein-coding gene consists of many elements of which the actual protein coding sequence is often only a small part. These include introns and untranslated regions of the mature mRNA. Noncoding genes can also contain introns that are removed during processing to produce the mature functional RNA. All genes are associated with regulatory sequences that are required for their expression. First, genes require
2392-406: A series of 221 photographs of Spanish subjects, 16.3% of the subjects were determined to have blue-gray eyes. Like blue eyes, gray eyes have a dark epithelium at the back of the iris and a relatively clear stroma at the front. One possible explanation for the difference in appearance between gray and blue eyes is that gray eyes have larger deposits of collagen in the stroma, so that the light that
2496-412: A single genomic region to encode multiple district products and trans-splicing concatenates mRNAs from shorter coding sequence across the genome. Since molecular definitions exclude elements such as introns, promotors, and other regulatory regions , these are instead thought of as "associated" with the gene and affect its function. An even broader operational definition is sometimes used to encompass
2600-409: A solid gold hue. Even though amber is similar to gold, some people have russet- or copper-colored amber eyes that are mistaken for hazel, though hazel tends to be duller and contains green with red/gold flecks, as mentioned above. Amber eyes may also contain amounts of very light gold-ish gray. The eyes of some pigeons contain yellow fluorescing pigments known as pteridines . The bright yellow eyes of
2704-472: A strict definition of the word "gene" with which nearly every expert can agree. First, in order for a nucleotide sequence to be considered a true gene, an open reading frame (ORF) must be present. The ORF can be thought of as the "gene itself"; it begins with a starting mark common for every gene and ends with one of three possible finish line signals. One of the key enzymes in this process, the RNA polymerase, zips along
2808-431: A strong yellowish/golden or russet/coppery tint, which may be due to a yellow pigment called lipochrome (also found in green eyes). Amber eyes should not be confused with hazel eyes. Although hazel eyes may contain specks of amber or gold, they usually tend to have many other colors, including green, brown, and orange. Also, hazel eyes may appear to shift in color and consist of flecks and ripples, while amber eyes are of
2912-592: A study published in Human Genetics that a mutation in the 86th intron of the HERC2 gene, which is hypothesized to interact with the OCA2 gene promoter , reduced expression of OCA2 with subsequent reduction in melanin production. It has been proposed that blue eyes may have been adaptive to shorter day lengths at higher latitudes, as blue eyes increase intraocular light scattering, which suppresses melatonin release from
3016-574: A total of ten genes, allowing the explanation of about 50% of eye color variation. Blue eyes with a brown spot, green eyes, and gray eyes are caused by an entirely different part of the genome. A 1997 study of White Americans found that eye color may be subject to change in infancy, and from adolescence to adulthood. 17% of children experienced a change of eye color by adulthood. Of those children, 50% of developed lighter eyes as they got older. The other 50% developed darker eyes. Generally, children with hazel and light brown eyes tended to experience
3120-409: A true gene, by this definition, one has to prove that the transcript has a biological function. Early speculations on the size of a typical gene were based on high-resolution genetic mapping and on the size of proteins and RNA molecules. A length of 1500 base pairs seemed reasonable at the time (1965). This was based on the idea that the gene was the DNA that was directly responsible for production of
3224-429: A variant form, causes the pink eye color and hypopigmentation common in human albinism . (The name of the gene is derived from the disorder it causes, oculocutaneous albinism type II.) Different SNPs within OCA2 are strongly associated with blue and green eyes as well as variations in freckling , mole counts, hair and skin tone . The polymorphisms may be in an OCA2 regulatory sequence , where they may influence
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3328-400: Is X-inactivation , which can result in a number of heterochromatic traits, such as calico cats . Trauma and certain medications, such as some prostaglandin analogues , can also cause increased pigmentation in one eye. On occasion, a difference in eye color is caused by blood staining the iris after injury. The limbal ring is also a feature of the iris contributing to eye color, appearing as
3432-407: Is a polygenic phenotypic trait determined by two factors: the pigmentation of the eye 's iris and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris . In humans, the pigmentation of the iris varies from light brown to black, depending on the concentration of melanin in the iris pigment epithelium (located on the back of the iris),
3536-436: Is also associated with an increased risk of ARMD progression. A gray iris may indicate the presence of a uveitis , and an increased risk of uveal melanoma has been found in those with blue, green or gray eyes. However, a study in 2000 suggests that people with dark brown eyes are at increased risk of developing cataracts and therefore should protect their eyes from direct exposure to sunlight. Wilson's disease involves
3640-521: Is by far the most common eye color, with approximately 79% of people in the world having it. Brown eyes result from a relatively high concentration of melanin in the stroma of the iris, which causes light of both shorter and longer wavelengths to be absorbed. In many parts of the world, it is nearly the only iris color present. Brown eyes are common in Europe , East Asia , Southeast Asia , Central Asia , South Asia , West Asia , Oceania , West Africa and
3744-456: Is called a gene product . The nucleotide sequence of a gene's DNA specifies the amino acid sequence of a protein through the genetic code . Sets of three nucleotides, known as codons , each correspond to a specific amino acid. The principle that three sequential bases of DNA code for each amino acid was demonstrated in 1961 using frameshift mutations in the rIIB gene of bacteriophage T4 (see Crick, Brenner et al. experiment ). Additionally,
3848-400: Is nearly the same for all known organisms. The total complement of genes in an organism or cell is known as its genome , which may be stored on one or more chromosomes . A chromosome consists of a single, very long DNA helix on which thousands of genes are encoded. The region of the chromosome at which a particular gene is located is called its locus . Each locus contains one allele of
3952-480: Is reflected from the epithelium undergoes Mie scattering (which is not strongly frequency-dependent) rather than Rayleigh scattering (in which shorter wavelengths of light are scattered more). This would be analogous to the change in the color of the sky, from the blue given by the Rayleigh scattering of sunlight by small gas molecules when the sky is clear, to the gray caused by Mie scattering of large water droplets when
4056-403: Is still part of the definition of a gene in most textbooks. For example, The primary function of the genome is to produce RNA molecules. Selected portions of the DNA nucleotide sequence are copied into a corresponding RNA nucleotide sequence, which either encodes a protein (if it is an mRNA) or forms a 'structural' RNA, such as a transfer RNA (tRNA) or ribosomal RNA (rRNA) molecule. Each region of
4160-399: Is stored on the histones, as well as chemical modifications of the histone itself, regulate whether a particular region of DNA is accessible for gene expression . In addition to genes, eukaryotic chromosomes contain sequences involved in ensuring that the DNA is copied without degradation of end regions and sorted into daughter cells during cell division: replication origins , telomeres , and
4264-472: Is the third-rarest natural eye color after green and gray, occurring in 5% of the world's population. People with amber-colored eyes are found in Europe , and in fewer numbers in the Middle East , North Africa , and South America . Hazel eyes are due to a combination of Rayleigh scattering and a moderate amount of melanin in the iris' anterior border layer. Hazel eyes often appear to shift in color from
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4368-413: Is therefore a major factor in the expression of eye color genotypes. One study suggested that women's higher levels of the sex hormone estrogen may explain why women tend to have darker eyes than men. People of European descent show the greatest variety in eye color of any population worldwide. Recent advances in ancient DNA technology have revealed some of the history of eye color in Europe. Through
4472-487: Is unknown, but there are a few likely candidates. A study in Rotterdam (2009) found that it was possible to predict eye color with more than 90% accuracy for brown and blue using just six SNPs. In humans, eye color is a highly sexually dimorphic trait. Several studies have shown that men are more likely to have blue eyes than women, while women are more likely to have darker eye colors (green and brown eyes) than men. Sex
4576-508: The Americas . Light or medium-pigmented brown eyes can also be commonly found in Europe , among the Americas , and parts of Central Asia , West Asia , South Asia , and East Africa . Light brown eyes bordering amber and hazel coloration are more common in Europe , but can also be observed in East Asia , Southeast Asia , North Africa and East Africa . Amber eyes are a solid color with
4680-815: The Netherlands and the United Kingdom , and have also been observed to be very common among the Low Saxon -speaking populations of northern Germany. Green eyes probably result from the interaction of multiple allelic variants of OCA2 and other genes. They may have been present in southern Siberia during the Bronze Age . Green eyes are most common in Northern , Western , and Central Europe . Around 8–10% of men and 18–21% of women in Iceland and 6% of men and 17% of women in
4784-408: The Netherlands have green eyes. Among European Americans , green eyes are most common among those of recent Celtic and Germanic ancestry, occurring in about 16% of people with those backgrounds. The green color is caused by the combination of: 1) an amber or light brown pigmentation in the stroma of the iris (which has a low or moderate concentration of melanin), and 2) a blue shade created by
4888-511: The aging process. The centromere is required for binding spindle fibres to separate sister chromatids into daughter cells during cell division . Prokaryotes ( bacteria and archaea ) typically store their genomes on a single, large, circular chromosome . Similarly, some eukaryotic organelles contain a remnant circular chromosome with a small number of genes. Prokaryotes sometimes supplement their chromosome with additional small circles of DNA called plasmids , which usually encode only
4992-401: The central dogma of molecular biology , which states that proteins are translated from RNA , which is transcribed from DNA . This dogma has since been shown to have exceptions, such as reverse transcription in retroviruses . The modern study of genetics at the level of DNA is known as molecular genetics . In 1972, Walter Fiers and his team were the first to determine the sequence of
5096-419: The centromere . Replication origins are the sequence regions where DNA replication is initiated to make two copies of the chromosome. Telomeres are long stretches of repetitive sequences that cap the ends of the linear chromosomes and prevent degradation of coding and regulatory regions during DNA replication . The length of the telomeres decreases each time the genome is replicated and has been implicated in
5200-453: The great horned owl are thought to be due to the presence of the pteridine pigment xanthopterin within certain chromatophores (called xanthophores) located in the iris stroma. In humans, yellowish specks or patches are thought to be due to the pigment lipofuscin , also known as lipochrome. Many animals such as canines, domestic cats, owls, eagles, pigeons, and fish have amber eyes, whereas in humans this color occurs less frequently. Amber
5304-549: The modern synthesis , a term introduced by Julian Huxley . This view of evolution was emphasized by George C. Williams ' gene-centric view of evolution . He proposed that the Mendelian gene is a unit of natural selection with the definition: "that which segregates and recombines with appreciable frequency." Related ideas emphasizing the centrality of Mendelian genes and the importance of natural selection in evolution were popularized by Richard Dawkins . The development of
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#17328838173205408-475: The neutral theory of evolution in the late 1960s led to the recognition that random genetic drift is a major player in evolution and that neutral theory should be the null hypothesis of molecular evolution. This led to the construction of phylogenetic trees and the development of the molecular clock , which is the basis of all dating techniques using DNA sequences. These techniques are not confined to molecular gene sequences but can be used on all DNA segments in
5512-750: The operon ; when the repressor is inactive transcription of the operon can occur (see e.g. Lac operon ). The products of operon genes typically have related functions and are involved in the same regulatory network . Though many genes have simple structures, as with much of biology, others can be quite complex or represent unusual edge-cases. Eukaryotic genes often have introns that are much larger than their exons, and those introns can even have other genes nested inside them . Associated enhancers may be many kilobase away, or even on entirely different chromosomes operating via physical contact between two chromosomes. A single gene can encode multiple different functional products by alternative splicing , and conversely
5616-508: The phenotype of the individual. Most biological traits occur under the combined influence of polygenes (a set of different genes) and gene–environment interactions . Some genetic traits are instantly visible, such as eye color or the number of limbs, others are not, such as blood type , the risk for specific diseases, or the thousands of basic biochemical processes that constitute life . A gene can acquire mutations in its sequence , leading to different variants, known as alleles , in
5720-529: The pineal gland , perhaps reducing psychological depression (which is linked to the short day length of higher latitudes). Blue eyes are predominant in northern and eastern Europe, particularly around the Baltic Sea . Blue eyes are also found in Southern Europe , Central Asia , South Asia , North Africa , and West Asia . Approximately 8% to 10% of the global population have blue eyes. A 2002 study found
5824-449: The population . These alleles encode slightly different versions of a gene, which may cause different phenotypical traits. Genes evolve due to natural selection or survival of the fittest and genetic drift of the alleles. There are many different ways to use the term "gene" based on different aspects of their inheritance, selection, biological function, or molecular structure but most of these definitions fall into two categories,
5928-407: The sclera (the "whites of the eyes") is associated with jaundice , and may be symptomatic of liver diseases such as cirrhosis or hepatitis . A blue coloration of the sclera may also be symptomatic of disease. Aniridia is a congenital condition characterized by an extremely underdeveloped iris, which appears absent on superficial examination. Normally, there is a thick layer of melanin on
6032-477: The DNA difference happens to be in an eye-color gene. There are many other possible reasons for having two different-colored eyes. For example, the film actor Lee Van Cleef was born with one blue eye and one green eye, a trait that reportedly was common in his family, suggesting that it was a genetic trait. This anomaly, which film producers thought would be disturbing to film audiences, was "corrected" by having Van Cleef wear brown contact lenses. David Bowie , on
6136-404: The DNA helix that produces a functional RNA molecule constitutes a gene. We define a gene as a DNA sequence that is transcribed. This definition includes genes that do not encode proteins (not all transcripts are messenger RNA). The definition normally excludes regions of the genome that control transcription but are not themselves transcribed. We will encounter some exceptions to our definition of
6240-450: The DNA sequence is used as a template for the production of an RNA molecule or a protein that performs some function. The emphasis on function is essential because there are stretches of DNA that produce non-functional transcripts and they do not qualify as genes. These include obvious examples such as transcribed pseudogenes as well as less obvious examples such as junk RNA produced as noise due to transcription errors. In order to qualify as
6344-766: The DNA to loop so that the regulatory sequence (and bound transcription factor) become close to the RNA polymerase binding site. For example, enhancers increase transcription by binding an activator protein which then helps to recruit the RNA polymerase to the promoter; conversely silencers bind repressor proteins and make the DNA less available for RNA polymerase. The mature messenger RNA produced from protein-coding genes contains untranslated regions at both ends which contain binding sites for ribosomes , RNA-binding proteins , miRNA , as well as terminator , and start and stop codons . In addition, most eukaryotic open reading frames contain untranslated introns , which are removed and exons , which are connected together in
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#17328838173206448-506: The Mendelian gene or the molecular gene. The Mendelian gene is the classical gene of genetics and it refers to any heritable trait. This is the gene described in The Selfish Gene . More thorough discussions of this version of a gene can be found in the articles Genetics and Gene-centered view of evolution . The molecular gene definition is more commonly used across biochemistry, molecular biology, and most of genetics —
6552-456: The Rayleigh scattering of reflected light. Green eyes contain the yellowish pigment lipochrome . There is no intrinsically blue pigmentation either in the iris or in the vitreous body . Rather, blue eyes result from structural color in combination with certain concentrations of non-blue pigments. The iris pigment epithelium is brownish black due to the presence of melanin . Unlike brown eyes, blue eyes have low concentrations of melanin in
6656-571: The TRPM7 gene suffer from hypomagnesemia, seizures and developmental delay. Defects in this gene have been associated with magnesium deficiency in human microvascular endothelial cells. This article incorporates text from the United States National Library of Medicine , which is in the public domain . This membrane protein –related article is a stub . You can help Misplaced Pages by expanding it . Gene In biology ,
6760-433: The adenines of one strand are paired with the thymines of the other strand, and so on. Due to the chemical composition of the pentose residues of the bases, DNA strands have directionality. One end of a DNA polymer contains an exposed hydroxyl group on the deoxyribose ; this is known as the 3' end of the molecule. The other end contains an exposed phosphate group; this is the 5' end . The two strands of
6864-648: The analysis of ancient DNA, a 2020 study published in Experimental Dermatology suggested that the common gene for blue eye color likely originated in the Near East and arrived in Europe around 42,000 years ago, after the exodus out of Africa . There is evidence that as many as 16 different genes could be responsible for eye color in humans; however, the main two genes associated with eye color variation are OCA2 and HERC2 , and both are localized in chromosome 15 . The gene OCA2 (OMIM: 203200 ), when in
6968-424: The back of the iris. Even people with the lightest blue eyes, with no melanin on the front of the iris at all, have dark brown coloration on the back of it, to prevent light from scattering around inside the eye. In those with milder forms of albinism , the color of the iris is typically blue but can vary from blue to brown. In severe forms of albinism, there is no pigment on the back of the iris, and light from inside
7072-498: The cell. TRPM7 is a protein that is both an ion channel and a kinase . As a channel, it conducts calcium and monovalent cations to depolarize cells and increase intracellular calcium. As a kinase, it is capable of phosphorylating itself and other substrates. The kinase activity is necessary for channel function, as shown by its dependence on intracellular ATP and by the kinase mutants. TRPM7 has been shown to interact with PLCB1 and PLCB2 . Patients with pathogenic variants in
7176-402: The complexity of these diverse phenomena, where a gene is defined as a union of genomic sequences encoding a coherent set of potentially overlapping functional products. This definition categorizes genes by their functional products (proteins or RNA) rather than their specific DNA loci, with regulatory elements classified as gene-associated regions. The existence of discrete inheritable units
7280-524: The distinction between a heterozygote and homozygote , and the phenomenon of discontinuous inheritance. Prior to Mendel's work, the dominant theory of heredity was one of blending inheritance , which suggested that each parent contributed fluids to the fertilization process and that the traits of the parents blended and mixed to produce the offspring. Charles Darwin developed a theory of inheritance he termed pangenesis , from Greek pan ("all, whole") and genesis ("birth") / genos ("origin"). Darwin used
7384-410: The early 1950s the prevailing view was that the genes in a chromosome acted like discrete entities arranged like beads on a string. The experiments of Benzer using mutants defective in the rII region of bacteriophage T4 (1955–1959) showed that individual genes have a simple linear structure and are likely to be equivalent to a linear section of DNA. Collectively, this body of research established
7488-457: The expression of the gene product, which in turn affects pigmentation. A specific mutation within the HERC2 gene, a gene that regulates OCA2 expression, is partly responsible for blue eyes. Other genes implicated in eye color variation are SLC24A4 and TYR . A 2010 study of eye color variation in hue and saturation values using high-resolution digital full-eye photographs found three new loci for
7592-593: The extremely low quantities of melanin , allowing the blood vessels to show through. In addition, flash photography can sometimes cause a " red-eye effect ", in which the very bright light from a flash reflects off the retina, which is abundantly vascular, causing the pupil to appear red in the photograph. Although the deep blue eyes of some people such as Elizabeth Taylor can appear purple or violet at certain times, "true" violet-colored eyes occur only due to albinism. Eyes that appear red or violet under certain conditions due to albinism occur in less than 1 percent of
7696-448: The eye can pass through the iris to the front. In these cases, the only color seen is the red from the hemoglobin of the blood in the capillaries of the iris. Such albinos have pink eyes, as do albino rabbits, mice, or any other animal with a total lack of melanin. Transillumination defects can almost always be observed during an eye examination due to lack of iridial pigmentation. The ocular albino also lacks normal amounts of melanin in
7800-534: The eye-color genes include OCA2 and HERC2 . The earlier belief that blue eye color is a recessive trait has been shown to be incorrect, and the genetics of eye color are so complex that almost any parent-child combination of eye colors can occur. Eye color is an inherited trait determined by multiple genes . These genes are sought by studying small changes in the genes themselves and in neighboring genes, called single-nucleotide polymorphisms or SNPs. The total number of genes that contribute to eye color
7904-514: The fact that both protein-coding genes and noncoding genes have been known for more than 50 years, there are still a number of textbooks, websites, and scientific publications that define a gene as a DNA sequence that specifies a protein. In other words, the definition is restricted to protein-coding genes. Here is an example from a recent article in American Scientist. ... to truly assess the potential significance of de novo genes, we relied on
8008-413: The functional product. The discovery of introns in the 1970s meant that many eukaryotic genes were much larger than the size of the functional product would imply. Typical mammalian protein-coding genes, for example, are about 62,000 base pairs in length (transcribed region) and since there are about 20,000 of them they occupy about 35–40% of the mammalian genome (including the human genome). In spite of
8112-630: The gene that is described in terms of DNA sequence. There are many different definitions of this gene — some of which are misleading or incorrect. Very early work in the field that became molecular genetics suggested the concept that one gene makes one protein (originally 'one gene - one enzyme'). However, genes that produce repressor RNAs were proposed in the 1950s and by the 1960s, textbooks were using molecular gene definitions that included those that specified functional RNA molecules such as ribosomal RNA and tRNA (noncoding genes) as well as protein-coding genes. This idea of two kinds of genes
8216-421: The genome. The vast majority of organisms encode their genes in long strands of DNA (deoxyribonucleic acid). DNA consists of a chain made from four types of nucleotide subunits, each composed of: a five-carbon sugar ( 2-deoxyribose ), a phosphate group, and one of the four bases adenine , cytosine , guanine , and thymine . Two chains of DNA twist around each other to form a DNA double helix with
8320-421: The genomes of complex multicellular organisms , including humans, contain an absolute majority of DNA without an identified function. This DNA has often been referred to as " junk DNA ". However, more recent analyses suggest that, although protein-coding DNA makes up barely 2% of the human genome , about 80% of the bases in the genome may be expressed, so the term "junk DNA" may be a misnomer. The structure of
8424-565: The interactions of several genes. In 2008, a team of researchers from the University of Copenhagen located a single mutation that causes the phenomenon of blue eyes. The research was published in the Journal of Human Genetics . The same DNA sequence of the OCA2 gene among blue-eyed people suggests they may have a single common ancestor. The researchers hypothesized that the OCA2 mutation responsible for blue eyes arose in an individual who lived in
8528-477: The intermediate template for the synthesis of a protein. The transmission of genes to an organism's offspring , is the basis of the inheritance of phenotypic traits from one generation to the next. These genes make up different DNA sequences, together called a genotype , that is specific to every given individual, within the gene pool of the population of a given species . The genotype, along with environmental and developmental factors, ultimately determines
8632-433: The iris to protect them from sunlight, there is little to no evidence that eye color has a direct impact on vision qualities such as visual acuity . However, there is a study that found that dark-eyed people perform better at "reactive-type tasks", which suggests they may have better reaction times. People with light-colored eyes, however, performed better at so-called "self-paced tasks", which include activities like hitting
8736-445: The iris. Gray is the second-rarest natural eye color after green, with 3% of the world's population having it. As a result of heterochromia iridum , it is also possible to have two different eye colors. This occurs in humans and certain breeds of domesticated animals and affects less than 1 percent of the world's population. The eyes of people with severe forms of albinism may appear red under certain lighting conditions owing to
8840-441: The lighting conditions, especially for lighter-colored eyes. The brightly colored eyes of many bird species result from the presence of other pigments, such as pteridines , purines , and carotenoids . Humans and other animals have many phenotypic variations in eye color. The genetics and inheritance of eye color in humans is complicated. As of 2010 , as many as 16 genes have been associated with eye color inheritance. Some of
8944-501: The melanin content within the iris stroma (located at the front of the iris), and the cellular density of the stroma. The appearance of blue, green, and hazel eyes results from the Tyndall scattering of light in the stroma, a phenomenon similar to Rayleigh scattering which accounts for the blue sky. Neither blue nor green pigments are present in the human iris or vitreous humour . This is an example of structural color , which depends on
9048-681: The northwestern part of the Black Sea region in Europe sometime between 6,000 and 10,000 years ago, during the Neolithic period. However, more recent ancient DNA research has identified human remains much older than the Neolithic period which possess the OCA2 mutation for blue eyes. It is now believed that the OCA2 allele responsible for blue eyes dates back to the migration of modern humans out of Africa roughly 50,000 years ago, and entered Europe from western Asia. Eiberg and colleagues suggested in
9152-413: The nucleus. Splicing, followed by CPA, generate the final mature mRNA , which encodes the protein or RNA product. Many noncoding genes in eukaryotes have different transcription termination mechanisms and they do not have poly(A) tails. Many prokaryotic genes are organized into operons , with multiple protein-coding sequences that are transcribed as a unit. The genes in an operon are transcribed as
9256-428: The other hand, had the appearance of different eye colors due to an injury that caused one pupil to be permanently dilated. Another hypothesis about heterochromia is that it can result from a viral infection in utero affecting the development of one eye, possibly through some sort of genetic mutation. Occasionally, heterochromia can be a sign of a serious medical condition. A common cause in females with heterochromia
9360-496: The outer part of the iris (or vice versa) when observed in sunlight. Definitions of the eye color "hazel" vary: it is sometimes considered to be synonymous with light brown or gold, as in the color of a hazelnut shell. Around 18% of the US population and 5% of the world population have hazel eyes. 55.2% of Spanish subjects in a series of 221 photographs were judged to have hazel eyes. Hazel eyes are found in Europe , most commonly in
9464-431: The phosphate–sugar backbone spiralling around the outside, and the bases pointing inward with adenine base pairing to thymine and guanine to cytosine. The specificity of base pairing occurs because adenine and thymine align to form two hydrogen bonds , whereas cytosine and guanine form three hydrogen bonds. The two strands in a double helix must, therefore, be complementary , with their sequence of bases matching such that
9568-493: The prevalence of blue eye color among the white population in the United States to be 33.8% for those born from 1936 through 1951, compared with 57.4% for those born from 1899 through 1905. As of 2006 , one out of every six Americans, or 16.6% of the total US population, has blue eyes, including 22.3% of whites. The incidence of blue eyes continues to decline among American children. Of Slovenes , 56% have blue/green eyes. In
9672-426: The retina as well, which allows more light than normal to reflect off the retina and out of the eye. Because of this, the pupillary reflex is much more pronounced in albino individuals, and this can emphasize the red eye effect in photographs. Heterochromia ( heterochromia iridum or heterochromia iridis ) is an eye condition in which one iris is a different color from the other (complete heterochromia), or where
9776-770: The sky is cloudy. Alternatively, it has been suggested that gray and blue eyes might differ in the concentration of melanin at the front of the stroma. Gray eyes can also be found among the Algerian Shawia people of the Aurès Mountains in Northwest Africa, in the Middle East / West Asia , Central Asia , and South Asia . In the Iliad , the Greek goddess Athene is said to have gray eyes (γλαυκῶπις). Under magnification, gray eyes exhibit small amounts of yellow and brown color in
9880-467: The strand of DNA like a train on a monorail, transcribing it into its messenger RNA form. This point brings us to our second important criterion: A true gene is one that is both transcribed and translated. That is, a true gene is first used as a template to make transient messenger RNA, which is then translated into a protein. This restricted definition is so common that it has spawned many recent articles that criticize this "standard definition" and call for
9984-457: The stroma of the iris, which lies in front of the dark epithelium. Longer wavelengths of light tend to be absorbed by the dark underlying epithelium, while shorter wavelengths are reflected and undergo Rayleigh scattering in the turbid medium of the stroma. This is the same scattering that accounts for the blue appearance of the sky. The result is a " Tyndall blue" structural color that varies with external lighting conditions. Blue eyes are
10088-461: The sugar ribose rather than deoxyribose . RNA also contains the base uracil in place of thymine . RNA molecules are less stable than DNA and are typically single-stranded. Genes that encode proteins are composed of a series of three- nucleotide sequences called codons , which serve as the "words" in the genetic "language". The genetic code specifies the correspondence during protein translation between codons and amino acids . The genetic code
10192-805: The term gemmule to describe hypothetical particles that would mix during reproduction. Mendel's work went largely unnoticed after its first publication in 1866, but was rediscovered in the late 19th century by Hugo de Vries , Carl Correns , and Erich von Tschermak , who (claimed to have) reached similar conclusions in their own research. Specifically, in 1889, Hugo de Vries published his book Intracellular Pangenesis , in which he postulated that different characters have individual hereditary carriers and that inheritance of specific traits in organisms comes in particles. De Vries called these units "pangenes" ( Pangens in German), after Darwin's 1868 pangenesis theory. Twenty years later, in 1909, Wilhelm Johannsen introduced
10296-436: The term gene , he explained his results in terms of discrete inherited units that give rise to observable physical characteristics. This description prefigured Wilhelm Johannsen 's distinction between genotype (the genetic material of an organism) and phenotype (the observable traits of that organism). Mendel was also the first to demonstrate independent assortment , the distinction between dominant and recessive traits,
10400-412: The term "gene" (inspired by the ancient Greek : γόνος, gonos , meaning offspring and procreation) and, in 1906, William Bateson , that of " genetics " while Eduard Strasburger , among others, still used the term "pangene" for the fundamental physical and functional unit of heredity. Advances in understanding genes and inheritance continued throughout the 20th century. Deoxyribonucleic acid (DNA)
10504-469: The word gene has two meanings. The Mendelian gene is a basic unit of heredity . The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA . There are two types of molecular genes: protein-coding genes and non-coding genes. During gene expression (the synthesis of RNA or protein from a gene), DNA is first copied into RNA . RNA can be directly functional or be
10608-482: The world's population. The most important role of melanin in the iris is to protect the eyes from the sun's harmful rays. People with lighter eye colors, such as blue or green, have lessened protection from the sun, and so need greater protection from the sun's rays than those with darker eye colors. Those with lighter iris color have been found to have a higher prevalence of age-related macular degeneration (ARMD) than those with darker iris color; lighter eye color
10712-446: Was first suggested by Gregor Mendel (1822–1884). From 1857 to 1864, in Brno , Austrian Empire (today's Czech Republic), he studied inheritance patterns in 8000 common edible pea plants , tracking distinct traits from parent to offspring. He described these mathematically as 2 combinations where n is the number of differing characteristics in the original peas. Although he did not use
10816-430: Was shown to be the molecular repository of genetic information by experiments in the 1940s to 1950s. The structure of DNA was studied by Rosalind Franklin and Maurice Wilkins using X-ray crystallography , which led James D. Watson and Francis Crick to publish a model of the double-stranded DNA molecule whose paired nucleotide bases indicated a compelling hypothesis for the mechanism of genetic replication. In
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