The X chromosome is one of the two sex-determining chromosomes in many organisms, including mammals, is found in both males and females. It is a part of the XY sex-determination X0 sex-determination system; the X chromosome was named for its unique properties by early researchers, which resulted in the naming of its counterpart Y chromosome, for the next letter in the alphabet, following its subsequent discovery. It was first noted. Henking was studying the testicles of Pyrrhocoris and noticed that one chromosome did not take part in meiosis. Chromosomes are so named because of their ability to take up staining. Although the X chromosome could be stained just as well as the others, Henking was unsure whether it was a different class of object and named it X element, which became X chromosome after it was established that it was indeed a chromosome; the idea that the X chromosome was named after its similarity to the letter "X" is mistaken. All chromosomes appear as an amorphous blob under the microscope and only take on a well defined shape during mitosis.
This shape is vaguely X-shaped for all chromosomes. It is coincidental that the Y chromosome, during mitosis, has two short branches which can look merged under the microscope and appear as the descender of a Y-shape, it was first suggested that the X chromosome was involved in sex determination by Clarence Erwin McClung in 1901. After comparing his work on locusts with Henking's and others, McClung noted that only half the sperm received an X chromosome, he called this chromosome an accessory chromosome, insisted that it was a proper chromosome, theorized that it was the male-determining chromosome. Luke Hutchison noticed that a number of possible ancestors on the X chromosome inheritance line at a given ancestral generation follows the Fibonacci sequence. A male individual has an X chromosome, which he received from his mother, a Y chromosome, which he received from his father; the male counts as the "origin" of his own X chromosome, at his parents' generation, his X chromosome came from a single parent.
The male's mother received one X chromosome from her mother, one from her father, so two grandparents contributed to the male descendant's X chromosome. The maternal grandfather received his X chromosome from his mother, the maternal grandmother received X chromosomes from both of her parents, so three great-grandparents contributed to the male descendant's X chromosome. Five great-great-grandparents contributed to the male descendant's X chromosome, etc; the X chromosome in humans spans more than 153 million base pairs. It represents about 800 protein-coding genes compared to the Y chromosome containing about 70 genes, out of 20,000–25,000 total genes in the human genome; each person has one pair of sex chromosomes in each cell. Females have two X chromosomes, whereas males have one Y chromosome. Both males and females retain one of their mother's X chromosomes, females retain their second X chromosome from their father. Since the father retains his X chromosome from his mother, a human female has one X chromosome from her paternal grandmother, one X chromosome from her mother.
This inheritance pattern follows the Fibonacci numbers at a given ancestral depth. Genetic disorders that are due to mutations in genes on the X chromosome are described as X linked. If X chromosome has a genetic disease gene, it always causes illness in male patients, since men have only one X chromosome and therefore only one copy of each gene. Females, may stay healthy and only be carrier of genetic illness, since they have another X chromosome and possibility to have healthy gene copy. For example hemophilia and red-green colorblindness run in family this way; the X chromosome carries hundreds of genes but few, if any, of these have anything to do directly with sex determination. Early in embryonic development in females, one of the two X chromosomes is randomly and permanently inactivated in nearly all somatic cells; this phenomenon is called X-inactivation or Lyonization, creates a Barr body. If X-inactivation in the somatic cell meant a complete de-functionalizing of one of the X-chromosomes, it would ensure that females, like males, had only one functional copy of the X chromosome in each somatic cell.
This was assumed to be the case. However, recent research suggests that the Barr body may be more biologically active than was supposed; the partial inactivation of the X-chromosome is due to repressive heterochromatin that compacts the DNA and prevents the expression of most genes. Heterochromatin compaction is regulated by Polycomb Repressive Complex 2; the following are some of the gene count estimates of human X chromosome. Because researchers use different approaches to genome annotation their predictions of the number
A base pair is a unit consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA and RNA. Dictated by specific hydrogen bonding patterns, Watson–Crick base pairs allow the DNA helix to maintain a regular helical structure, subtly dependent on its nucleotide sequence; the complementary nature of this based-paired structure provides a redundant copy of the genetic information encoded within each strand of DNA. The regular structure and data redundancy provided by the DNA double helix make DNA well suited to the storage of genetic information, while base-pairing between DNA and incoming nucleotides provides the mechanism through which DNA polymerase replicates DNA and RNA polymerase transcribes DNA into RNA. Many DNA-binding proteins can recognize specific base-pairing patterns that identify particular regulatory regions of genes. Intramolecular base pairs can occur within single-stranded nucleic acids.
This is important in RNA molecules, where Watson–Crick base pairs permit the formation of short double-stranded helices, a wide variety of non-Watson–Crick interactions allow RNAs to fold into a vast range of specific three-dimensional structures. In addition, base-pairing between transfer RNA and messenger RNA forms the basis for the molecular recognition events that result in the nucleotide sequence of mRNA becoming translated into the amino acid sequence of proteins via the genetic code; the size of an individual gene or an organism's entire genome is measured in base pairs because DNA is double-stranded. Hence, the number of total base pairs is equal to the number of nucleotides in one of the strands; the haploid human genome is estimated to be about 3.2 billion bases long and to contain 20,000–25,000 distinct protein-coding genes. A kilobase is a unit of measurement in molecular biology equal to 1000 base pairs of DNA or RNA; the total amount of related DNA base pairs on Earth is estimated at 5.0×1037 and weighs 50 billion tonnes.
In comparison, the total mass of the biosphere has been estimated to be as much as 4 TtC. Hydrogen bonding is the chemical interaction. Appropriate geometrical correspondence of hydrogen bond donors and acceptors allows only the "right" pairs to form stably. DNA with high GC-content is more stable than DNA with low GC-content. But, contrary to popular belief, the hydrogen bonds do not stabilize the DNA significantly; the larger nucleobases and guanine, are members of a class of double-ringed chemical structures called purines. Purines are complementary only with pyrimidines: pyrimidine-pyrimidine pairings are energetically unfavorable because the molecules are too far apart for hydrogen bonding to be established. Purine-pyrimidine base-pairing of AT or GC or UA results in proper duplex structure; the only other purine-pyrimidine pairings would be AC and GT and UG. The GU pairing, with two hydrogen bonds, does occur often in RNA. Paired DNA and RNA molecules are comparatively stable at room temperature, but the two nucleotide strands will separate above a melting point, determined by the length of the molecules, the extent of mispairing, the GC content.
Higher GC content results in higher melting temperatures. On the converse, regions of a genome that need to separate — for example, the promoter regions for often-transcribed genes — are comparatively GC-poor. GC content and melting temperature must be taken into account when designing primers for PCR reactions; the following DNA sequences illustrate pair double-stranded patterns. By convention, the top strand is written from the 5' end to the 3' end. A base-paired DNA sequence: ATCGATTGAGCTCTAGCG TAGCTAACTCGAGATCGCThe corresponding RNA sequence, in which uracil is substituted for thymine in the RNA strand: AUCGAUUGAGCUCUAGCG UAGCUAACUCGAGAUCGC Chemical analogs of nucleotides can take the place of proper nucleotides and establish non-canonical base-pairing, leading to errors in DNA replication and DNA transcription; this is due to their isosteric chemistry. One common mutagenic base analog is 5-bromouracil, which resembles thymine but can base-pair to guanine in its enol form. Other chemicals, known as DNA intercalators, fit into the gap between adjacent bases on a single strand and induce frameshift mutations by "masquerading" as a base, causing the DNA replication machinery to skip or insert additional nucleotides at the intercalated site.
Most intercalators are known or suspected carcinogens. Examples include ethidium acridine. An unnatural base pair is a designed subunit of DNA, created in a laboratory and does not occur in nature. DNA sequences have been described which use newly created nucleobases to form a third base pair, in addition to the two ba
A cofactor is a non-protein chemical compound or metallic ion, required for an enzyme's activity. Cofactors can be considered "helper molecules"; the rates at which these happen are characterized by enzyme kinetics. Cofactors can be subclassified as either inorganic ions or complex organic molecules called coenzymes, the latter of, derived from vitamins and other organic essential nutrients in small amounts. A coenzyme, or covalently bound is termed a prosthetic group. Cosubstrates are transiently bound to the protein and will be released at some point get back in; the prosthetic groups, on the other hand, are bound permanently to the protein. Both of them have the same function, to facilitate the reaction of enzymes and protein. Additionally, some sources limit the use of the term "cofactor" to inorganic substances. An inactive enzyme without the cofactor is called an apoenzyme, while the complete enzyme with cofactor is called a holoenzyme; some enzymes or enzyme complexes require several cofactors.
For example, the multienzyme complex pyruvate dehydrogenase at the junction of glycolysis and the citric acid cycle requires five organic cofactors and one metal ion: loosely bound thiamine pyrophosphate, covalently bound lipoamide and flavin adenine dinucleotide, cosubstrates nicotinamide adenine dinucleotide and coenzyme A, a metal ion. Organic cofactors are vitamins or made from vitamins. Many contain the nucleotide adenosine monophosphate as part of their structures, such as ATP, coenzyme A, FAD, NAD+; this common structure may reflect a common evolutionary origin as part of ribozymes in an ancient RNA world. It has been suggested that the AMP part of the molecule can be considered to be a kind of "handle" by which the enzyme can "grasp" the coenzyme to switch it between different catalytic centers. Cofactors can be divided into two major groups: organic Cofactors, such as flavin or heme, inorganic cofactors, such as the metal ions Mg2+, Cu+, Mn2+, or iron-sulfur clusters. Organic cofactors are sometimes further divided into prosthetic groups.
The term coenzyme refers to enzymes and, as such, to the functional properties of a protein. On the other hand, "prosthetic group" emphasizes the nature of the binding of a cofactor to a protein and, refers to a structural property. Different sources give different definitions of coenzymes and prosthetic groups; some consider bound organic molecules as prosthetic groups and not as coenzymes, while others define all non-protein organic molecules needed for enzyme activity as coenzymes, classify those that are bound as coenzyme prosthetic groups. These terms are used loosely. A 1980 letter in Trends in Biochemistry Sciences noted the confusion in the literature and the arbitrary distinction made between prosthetic groups and coenzymes group and proposed the following scheme. Here, cofactors were defined as an additional substance apart from protein and substrate, required for enzyme activity and a prosthetic group as a substance that undergoes its whole catalytic cycle attached to a single enzyme molecule.
However, the author could not arrive at a single all-encompassing definition of a "coenzyme" and proposed that this term be dropped from use in the literature. Metal ions are common cofactors; the study of these cofactors falls under the area of bioinorganic chemistry. In nutrition, the list of essential trace elements reflects their role as cofactors. In humans this list includes iron, manganese, copper and molybdenum. Although chromium deficiency causes impaired glucose tolerance, no human enzyme that uses this metal as a cofactor has been identified. Iodine is an essential trace element, but this element is used as part of the structure of thyroid hormones rather than as an enzyme cofactor. Calcium is another special case, in that it is required as a component of the human diet, it is needed for the full activity of many enzymes, such as nitric oxide synthase, protein phosphatases, adenylate kinase, but calcium activates these enzymes in allosteric regulation binding to these enzymes in a complex with calmodulin.
Calcium is, therefore, a cell signaling molecule, not considered a cofactor of the enzymes it regulates. Other organisms require additional metals as enzyme cofactors, such as vanadium in the nitrogenase of the nitrogen-fixing bacteria of the genus Azotobacter, tungsten in the aldehyde ferredoxin oxidoreductase of the thermophilic archaean Pyrococcus furiosus, cadmium in the carbonic anhydrase from the marine diatom Thalassiosira weissflogii. In many cases, the cofactor includes both an organic component. One diverse set of examples is the heme proteins, which consist of a porphyrin ring coordinated to iron. Iron-sulfur clusters are complexes of iron and sulfur atoms held within proteins by cysteinyl residues, they play both structural and functional roles, including electron transfer, redox sensing, as structural modules. Organic cofactors are small organic molecules that can be either loosely or bound to the enzyme and directly participate in the reaction. In the latter case, when it is difficult to remove without denaturing the enzyme, it can be called a prosthetic group.
It is important to emphasize that there is no sharp division between loosely and bound cofactors. Indeed, many such as NAD+ can be bound in some enzymes, while it is loosely bound in others. Another example is thiamine pyrophosphate, bound in transketolase or pyruvate decarboxylase, while it is less tightly
Ceruloplasmin is a ferroxidase enzyme that in humans is encoded by the CP gene. Ceruloplasmin is the major copper-carrying protein in the blood, in addition plays a role in iron metabolism, it was first described in 1948. Another protein, hephaestin, is noted for its homology to ceruloplasmin, participates in iron and copper metabolism. Ceruloplasmin is an enzyme synthesized in the liver containing 6 atoms of copper in its structure. Ceruloplasmin carries more than 95% of the total copper in healthy human plasma; the rest is accounted for by macroglobulins. Ceruloplasmin exhibits a copper-dependent oxidase activity, associated with possible oxidation of Fe2+ into Fe3+, therefore assisting in its transport in the plasma in association with transferrin, which can carry iron only in the ferric state; the molecular weight of human ceruloplasmin is reported to be 151kDa. A cis-regulatory element called the GAIT element is involved in the selective translational silencing of the Ceruloplasmin transcript.
The silencing requires binding of a cytosolic inhibitor complex called IFN-gamma-activated inhibitor of translation to the GAIT element. Like any other plasma protein, levels drop in patients with hepatic disease due to reduced synthesizing capabilities. Mechanisms of low ceruloplasmin levels: Gene expression genetically low Copper levels are low in general Malnutrition/trace metal deficiency in the food source Copper does not cross the intestinal barrier due to ATP7A deficiency Delivery of copper into the lumen of the ER-Golgi network is absent in hepatocytes due to absent ATP7B Copper availability doesn't affect the translation of the nascent protein. However, the apoenzyme without copper is unstable. Apoceruloplasmin is degraded intracellularly in the hepatocyte and the small amount, released has a short circulation half life of 5 hours as compared to the 5.5 days for the holo-ceruloplasmin. Mutations in the ceruloplasmin gene, which are rare, can lead to the genetic disease aceruloplasminemia, characterized by hyperferritinemia with iron overload.
In the brain, this iron overload may lead to characteristic neurologic signs and symptoms, such as cerebellar ataxia, progressive dementia, extrapyramidal signs. Excess iron may deposit in the liver and retina, leading to cirrhosis, endocrine abnormalities, loss of vision, respectively. Lower-than-normal ceruloplasmin levels may indicate the following: Wilson disease. Menkes disease Copper deficiency Aceruloplasminemia Greater-than-normal ceruloplasmin levels may indicate or be noticed in: copper toxicity / zinc deficiency pregnancy oral contraceptive pill use lymphoma acute and chronic inflammation rheumatoid arthritis Angina Alzheimer's disease Schizophrenia Obsessive-compulsive disorder Normal blood concentration of ceruloplasmin in humans is 20-50 mg/dL. GeneReviews/NCBI/NIH/UW entry on Aceruloplasminemia OMIM entries on Aceruloplasminemia
In biology, homology is the existence of shared ancestry between a pair of structures, or genes, in different taxa. A common example of homologous structures is the forelimbs of vertebrates, where the wings of bats, the arms of primates, the front flippers of whales and the forelegs of dogs and horses are all derived from the same ancestral tetrapod structure. Evolutionary biology explains homologous structures adapted to different purposes as the result of descent with modification from a common ancestor; the term was first applied to biology in a non-evolutionary context by the anatomist Richard Owen in 1843. Homology was explained by Charles Darwin's theory of evolution in 1859, but had been observed before this, from Aristotle onwards, it was explicitly analysed by Pierre Belon in 1555. In developmental biology, organs that developed in the embryo in the same manner and from similar origins, such as from matching primordia in successive segments of the same animal, are serially homologous.
Examples include the legs of a centipede, the maxillary palp and labial palp of an insect, the spinous processes of successive vertebrae in a vertebral column. Male and female reproductive organs are homologous if they develop from the same embryonic tissue, as do the ovaries and testicles of mammals including humans. Sequence homology between protein or DNA sequences is defined in terms of shared ancestry. Two segments of DNA can have shared ancestry because of either a speciation event or a duplication event. Homology among proteins or DNA is inferred from their sequence similarity. Significant similarity is strong evidence that two sequences are related by divergent evolution from a common ancestor. Alignments of multiple sequences are used to discover the homologous regions. Homology remains controversial in animal behaviour, but there is suggestive evidence that, for example, dominance hierarchies are homologous across the primates. Homology was noticed by Aristotle, was explicitly analysed by Pierre Belon in his 1555 Book of Birds, where he systematically compared the skeletons of birds and humans.
The pattern of similarity was interpreted as part of the static great chain of being through the mediaeval and early modern periods: it was not seen as implying evolutionary change. In the German Naturphilosophie tradition, homology was of special interest as demonstrating unity in nature. In 1790, Goethe stated his foliar theory in his essay "Metamorphosis of Plants", showing that flower part are derived from leaves; the serial homology of limbs was described late in the 18th century. The French zoologist Etienne Geoffroy Saint-Hilaire showed in 1818 in his theorie d'analogue that structures were shared between fishes, reptiles and mammals; when Geoffroy went further and sought homologies between Georges Cuvier's embranchements, such as vertebrates and molluscs, his claims triggered the 1830 Cuvier-Geoffroy debate. Geoffroy stated the principle of connections, namely that what is important is the relative position of different structures and their connections to each other; the Estonian embryologist Karl Ernst von Baer stated what are now called von Baer's laws in 1828, noting that related animals begin their development as similar embryos and diverge: thus, animals in the same family are more related and diverge than animals which are only in the same order and have fewer homologies.
Von Baer's theory recognises that each taxon has distinctive shared features, that embryonic development parallels the taxonomic hierarchy: not the same as recapitulation theory. The term "homology" was first used in biology by the anatomist Richard Owen in 1843 when studying the similarities of vertebrate fins and limbs, defining it as the "same organ in different animals under every variety of form and function", contrasting it with the matching term "analogy" which he used to describe different structures with the same function. Owen codified 3 main criteria for determining if features were homologous: position and composition. In 1859, Charles Darwin explained homologous structures as meaning that the organisms concerned shared a body plan from a common ancestor, that taxa were branches of a single tree of life; the word homology, coined in about 1656, is derived from the Greek ὁμόλογος homologos from ὁμός homos "same" and λόγος logos "relation". Biological structures or sequences in different taxa are homologous if they are derived from a common ancestor.
Homology thus implies divergent evolution. For example, many insects possess two pairs of flying wings. In beetles, the first pair of wings has evolved into a pair of hard wing covers, while in Dipteran flies the second pair of wings has evolved into small halteres used for balance; the forelimbs of ancestral vertebrates have evolved into the front flippers of whales, the wings of birds, the running forelegs of dogs and horses, the short forelegs of frogs and lizards, the grasping hands of primates including humans. The same major forearm bones are found in fossils of lobe-finned fish such as Eusthenopteron; the opposite of homologous organs are analogous organs which do similar jobs in two taxa that were not present in their most recent common ancestor but rather evolved separately. For example, the wings of insects and birds evolved independently in separated groups, converged functionally to support powered flight, so they are analogous; the wings of a sycamore maple seed and the wings of a bird are analogous but not homologous, as they develop from quite different structures.
A structure can be only analogous at another. Pterosaur and bat wings are analogous as wings
Proteins are large biomolecules, or macromolecules, consisting of one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, providing structure to cells and organisms, transporting molecules from one location to another. Proteins differ from one another in their sequence of amino acids, dictated by the nucleotide sequence of their genes, which results in protein folding into a specific three-dimensional structure that determines its activity. A linear chain of amino acid residues is called a polypeptide. A protein contains at least one long polypeptide. Short polypeptides, containing less than 20–30 residues, are considered to be proteins and are called peptides, or sometimes oligopeptides; the individual amino acid residues are bonded together by peptide bonds and adjacent amino acid residues. The sequence of amino acid residues in a protein is defined by the sequence of a gene, encoded in the genetic code.
In general, the genetic code specifies 20 standard amino acids. Shortly after or during synthesis, the residues in a protein are chemically modified by post-translational modification, which alters the physical and chemical properties, stability and the function of the proteins. Sometimes proteins have non-peptide groups attached, which can be called prosthetic groups or cofactors. Proteins can work together to achieve a particular function, they associate to form stable protein complexes. Once formed, proteins only exist for a certain period and are degraded and recycled by the cell's machinery through the process of protein turnover. A protein's lifespan covers a wide range, they can exist for years with an average lifespan of 1 -- 2 days in mammalian cells. Abnormal or misfolded proteins are degraded more either due to being targeted for destruction or due to being unstable. Like other biological macromolecules such as polysaccharides and nucleic acids, proteins are essential parts of organisms and participate in every process within cells.
Many proteins are enzymes that are vital to metabolism. Proteins have structural or mechanical functions, such as actin and myosin in muscle and the proteins in the cytoskeleton, which form a system of scaffolding that maintains cell shape. Other proteins are important in cell signaling, immune responses, cell adhesion, the cell cycle. In animals, proteins are needed in the diet to provide the essential amino acids that cannot be synthesized. Digestion breaks the proteins down for use in the metabolism. Proteins may be purified from other cellular components using a variety of techniques such as ultracentrifugation, precipitation and chromatography. Methods used to study protein structure and function include immunohistochemistry, site-directed mutagenesis, X-ray crystallography, nuclear magnetic resonance and mass spectrometry. Most proteins consist of linear polymers built from series of up to 20 different L-α- amino acids. All proteinogenic amino acids possess common structural features, including an α-carbon to which an amino group, a carboxyl group, a variable side chain are bonded.
Only proline differs from this basic structure as it contains an unusual ring to the N-end amine group, which forces the CO–NH amide moiety into a fixed conformation. The side chains of the standard amino acids, detailed in the list of standard amino acids, have a great variety of chemical structures and properties; the amino acids in a polypeptide chain are linked by peptide bonds. Once linked in the protein chain, an individual amino acid is called a residue, the linked series of carbon and oxygen atoms are known as the main chain or protein backbone; the peptide bond has two resonance forms that contribute some double-bond character and inhibit rotation around its axis, so that the alpha carbons are coplanar. The other two dihedral angles in the peptide bond determine the local shape assumed by the protein backbone; the end with a free amino group is known as the N-terminus or amino terminus, whereas the end of the protein with a free carboxyl group is known as the C-terminus or carboxy terminus.
The words protein and peptide are a little ambiguous and can overlap in meaning. Protein is used to refer to the complete biological molecule in a stable conformation, whereas peptide is reserved for a short amino acid oligomers lacking a stable three-dimensional structure. However, the boundary between the two is not well defined and lies near 20–30 residues. Polypeptide can refer to any single linear chain of amino acids regardless of length, but implies an absence of a defined conformation. Proteins can interact with many types of molecules, including with other proteins, with lipids, with carboyhydrates, with DNA, it has been estimated. Smaller bacteria, such as Mycoplasma or spirochetes contain fewer molecules, on the order of 50,000 to 1 million. By contrast, eukaryotic cells are larger and thus contain much more pro
Dominance in genetics is a relationship between alleles of one gene, in which the effect on phenotype of one allele masks the contribution of a second allele at the same locus. The first allele is dominant and the second allele is recessive. For genes on an autosome, the alleles and their associated traits are autosomal dominant or autosomal recessive. Dominance is a key concept in Mendelian inheritance and classical genetics; the dominant allele codes for a functional protein whereas the recessive allele does not. A classic example of dominance is the inheritance of seed shape in peas. Peas associated with allele r. In this case, three combinations of alleles are possible: RR, Rr, rr; the RR individuals have round peas and the rr individuals have wrinkled peas. In Rr individuals the R allele masks the presence of the r allele, so these individuals have round peas. Thus, allele R is dominant to allele r, allele r is recessive to allele R; this use of upper case letters for dominant alleles and lower case ones for recessive alleles is a followed convention.
More where a gene exists in two allelic versions, three combinations of alleles are possible: AA, Aa, aa. If AA and aa individuals show different forms of some trait, Aa individuals show the same phenotype as AA individuals allele A is said to dominate, be dominant to or show dominance to allele a, a is said to be recessive to A. Dominance is not inherent to either its phenotype, it is a relationship between two alleles of their associated phenotypes. An allele may be dominant for a particular aspect of phenotype but not for other aspects influenced by the same gene. Dominance differs from epistasis, a relationship in which an allele of one gene affects the expression of another allele at a different gene; the concept of dominance was introduced by Gregor Johann Mendel. Though Mendel, "The Father of Genetics", first used the term in the 1860s, it was not known until the early twentieth century. Mendel observed that, for a variety of traits of garden peas having to do with the appearance of seeds, seed pods, plants, there were two discrete phenotypes, such as round versus wrinkled seeds, yellow versus green seeds, red versus white flowers or tall versus short plants.
When bred separately, the plants always produced generation after generation. However, when lines with different phenotypes were crossed and only one of the parental phenotypes showed up in the offspring. However, when these hybrid plants were crossed, the offspring plants showed the two original phenotypes, in a characteristic 3:1 ratio, the more common phenotype being that of the parental hybrid plants. Mendel reasoned that each parent in the first cross was a homozygote for different alleles, that each contributed one allele to the offspring, with the result that all of these hybrids were heterozygotes, that one of the two alleles in the hybrid cross dominated expression of the other: A masked a; the final cross between two heterozygotes would produce AA, Aa, aa offspring in a 1:2:1 genotype ratio with the first two classes showing the phenotype, the last showing the phenotype, thereby producing the 3:1 phenotype ratio. Mendel did not use the terms gene, phenotype, genotype and heterozygote, all of which were introduced later.
He did introduce the notation of capital and lowercase letters for dominant and recessive alleles still in use today. Most animals and some plants have paired chromosomes, are described as diploid, they have two versions of each chromosome, one contributed by the mother's ovum, the other by the father's sperm, known as gametes, described as haploid, created through meiosis. These gametes fuse during fertilization during sexual reproduction, into a new single cell zygote, which divides multiple times, resulting in a new organism with the same number of pairs of chromosomes in each cell as its parents; each chromosome of a matching pair is structurally similar to the other, has a similar DNA sequence. The DNA in each chromosome functions as a series of discrete genes that influence various traits. Thus, each gene has a corresponding homologue, which may exist in different versions called alleles; the alleles at the same locus on the two homologous chromosomes may be different. The blood type of a human is determined by a gene that creates an A, B, AB or O blood type and is located in the long arm of chromosome nine.
There are three different alleles that could be present at this locus, but only two can be present in any individual, one inherited from their mother and one from their father. If two alleles of a given gene are identical, the organism is called a homozygote and is said to be homozygous with respect to that gene; the genetic makeup of an organism, either at a single locus or over all its genes collectively, is called its genotype. The genotype of an organism directly and indirectly affects its molecular and other traits, which individually or collectively are called its phenotype. At heterozygous gene loci, the two alleles interact to produce the phenotype. In complete dominance, the effect of one allele in a heterozygous genotype masks the effect of the other; the allele that mas