Q equals all of the alleles in individuals who are homozygous recessive and the other half of the alleles in people heterozygous.
The Punnett square
is a diagram that is used to predict an outcome of a particular cross or breeding experiment. It is named after Reginald C. Punnett, who devised the approach, and is used by biologists to determine the probability of an offspring's having a particular genotype. The Punnett square is a tabular summary of every possible combination of one maternal allele with one paternal allele for each gene being studied in the cross. These tables give the correct probabilities for the genotype outcomes of independent crosses where the probability of inheriting copies of each parental allele is independent. The Punnett Square is a visual representation of Mendelian inheritance.
In this example, both organisms have the genotype Bb
. They can produce gametes that contain either the B
or the b
allele. (It is conventional in genetics to use capital letters to indicate dominant alleles and lower-case letters to indicate recessive alleles.) The probability of an individual offspring's having the genotype BB
is 25%, Bb
is 50%, and bb
It is important to note that Punnett squares give probabilities only for genotypes
, not phenotypes
. The way in which the B
alleles interact with each other to affect the appearance of the offspring depends on how the gene products (proteins) interact (see Mendelian inheritance). For classical dominant/recessive genes, like that which determines whether a rat has black hair (B
) or white hair (b
), the dominant allele will mask the recessive one. Thus, in the example above, 75% of the offspring will be black (BB
) while only 25% will be white (bb
). The ratio of the phenotypes is 3:1, typical for a monohybrid cross.
More complicated crosses can be made by looking at two or more genes. The Punnett square works, however, only if the genes are independent of each other, which means that having a particular allele of gene A does not alter the probability of possessing an allele of gene B. This is equivalent to stating that the genes are not linked, so that the two genes do not tend to sort together during meiosis.
The following example illustrates a dihybrid cross between two heterozygous pea plants. R
represents the dominant allele for shape (round), while r
represents the recessive allele (wrinkled). A
represents the dominant allele for color (yellow), while a
represents the recessive allele (green). If each plant has the genotype RrAa
, and since the alleles for shape and color genes are independent, then they can produce four types of gametes with all possible combinations: RA
, and ra
Since dominant traits mask recessive traits, there are nine combinations that have the phenotype round yellow, three that are round green, three that are wrinkled yellow, and one that is wrinkled green. The ratio 9:3:3:1 is typical for a dihybrid cross.
Another way to solve dihybrid and multihybrid crosses is to use the tree method, although it does not display the genotypes of the gametes correctly.
This method is particularly advantageous when crossing homozygous organisms.
In genetics, a recessive gene
is an allele that causes a phenotype (visible or detectable characteristic) that is only seen in a homozygous genotype (an organism that has two copies of the same allele) and never in a heterozygous genotype. Every person has two copies of every gene on autosomal chromosomes, one from mother and one from father. If a genetic trait is recessive, a person needs to inherit two copies of the gene for the trait to be expressed. Thus, both parents have to be carriers of a recessive trait in order for a child to express that trait. Note that "expression" in this sense does not refer to expressiongenetic
(i.e., transcription and translation) of the gene. Instead, "expression" here refers to the observance of the gene within the phenotype. If both parents are carriers, there is a 25% chance with each child to show the recessive trait in the phenotype. Thus if the parents are closely related (in-breeding) the probability of both having inherited the same gene is increased and as a result the probability of the children showing the recessive trait is increased as well.
The term "recessive gene" is part of the laws of Mendelian inheritance created by Gregor Mendel. Examples of recessive genes in Mendel's famous pea plant experiments include those that determine the color and shape of seed pods, and plant height.
is a mode of inheritance of genetic traits located on the allosomes (the sex determining chromosomes).
Examples include the gene for hemophilia which is carried on the X chromosome. Diseases carried on the Y chromosome cannot be recessive.
X chromosome diseases are recessive in females unless both of their X chromosomes have the gene. They are not recessive in males because males only have 1 X chromosome so there is not a non-diseased version of the gene to default to.
is a mode of inheritance of genetic traits located on the autosomes (the 22 non-sex determining chromosomes).
In opposition to autosomal dominant trait, a recessive
trait only becomes phenotypically apparent when two copies of a gene (two alleles) are present. In other words, the subject is homozygous for the trait. Recessive genes will also show a horizontal inheritance on a pedigree chart. The frequency of the carrier state can be calculated by the Hardy-Weinberg formula:
is the frequency of one allele, and q
= 1 − p
is the frequency of the other allele. Thus p
is the frequency of the homozygous genotype for the first allele, 2pq
is the frequency of the heterozygous genotype, and qq
is the frequency of the other homozygous genotype.)
Recessive genetic disorders occur when both parents are carriers and each contributes an allele to the embryo. As both parents are heterozygous for the disorder, the chance of two disease alleles being inherited by one of their offspring is 25% (in autosomal dominant traits this is higher). 50% of the children (or 2/3 of the remaining ones) are carriers. When one of the parents is homozygous, the trait will only show in his/her offspring if the other parent is also a carrier. In that case, the chance of disease in the offspring is 50%.
Technically, the term "recessive gene" is imprecise because it is not the gene that is recessive but the phenotype (or trait). It should also be noted that the concepts of recessiveness and dominance were developed before a molecular understanding of DNA and before molecular biology, thus mapping many newer concepts to "dominant" or "recessive" phenotypes is problematic. Many traits previously thought to be recessive have mild forms or biochemical abnormalities that arise from the presence of the one copy of the allele. This suggests that the dominant phenotype is dependent upon having two dominant genes and the presence of one dominant and one recessive gene creates some blending of both dominant and recessive traits.
Recessive genes are usually represented by a lowercase letter in a Punnett square, as opposed to the uppercase letters of dominant genes (see example at right). Using the letter "T" as an example, only in "tt" (the homozygous recessive genotype, indicated by blue) would the recessive physical trait appear. The form "Tt" is called heterozygous (indicated by magenta with a red border), and, even though a recessive allele is present, the dominant gene is the one that appears (becomes the phenotype). The homozygous dominant genotype is "TT" (indicated by red)
Gregor Mendel performed a multitude of experiments on pea plants (Pisum sativum
) while researching traits, chosen because of the simple and low variety of characteristics, as well as the short period of germination. He experimented with color (green or yellow), size (short or tall), pea texture (smooth or wrinkled), and many others. By good fortune, the characteristics displayed by these plants clearly exhibited a dominant and recessive form. This is not true for many organisms. Few do inherit these traits.
For example, when testing the color of the pea plants, he chose two green plants, since green was more common than yellow. He mated them, and examined the offspring. He continued to mate only those that appeared green, and eventually, the yellow ones would stop being produced. He also mated the yellow ones together and determined that only yellow ones were produced.
Mendel determined that this was because yellow was a recessive trait which only appeared when green, the dominant trait, was not present. Also, he determined that the dominant trait would be displayed whether or not the recessive trait was there.
Dominance/recessiveness refers to phenotype, not genotype. An example to prove the point is sickle cell anemia. The genotype of sickle cell is caused by a single base pair change in the beta-globin gene: normal=GAG (glu), sickle=GTG (val). There are several phenotypes associated with the sickle genotype: -
This example demonstrates that one can only refer to dominance/recessiveness with respect to individual phenotypes.
Other recessive disorders:
Balancing selection refers to a number of selective processes by which multiple alleles (different versions of a gene) are actively maintained in the gene pool of a population at frequencies above that of gene mutation. This usually happens when the heterozygotes for the alleles under consideration have a higher adaptive value than the homozygote. In this way genetic polymorphism is conserved.
Evidence for balancing selection can be found in the number of alleles in a population which are maintained above mutation rate frequencies. All modern research has shown that this significant genetic variation is ubiquitous in panmictic populations. It is a genetic expression of the field experience of Darwin, Wallace and others, that natural populations in the wild are extraordinarily varied (though not all such variation is of genetic origin).
There are several mechanisms (which are not exclusive within any given population) by which balancing selection works to maintain polymorphism. The two major and most studied are heterozygote advantage and frequency-dependent selection.
In heterozygote advantage, or heterotic balancing selection, an individual who is heterozygous at a particular gene locus has a greater fitness than a homozygous individual. Polymorphisms maintained by this mechanism are balanced polymorphisms.
A well-studied case is that of sickle cell anemia in humans, a hereditary disease that damages red blood cells. Sickle cell anemia is caused by the inheritance of an allele (HgbS) of the hemoglobin gene from both parents. In such individuals, the hemoglobin in red blood cells is extremely sensitive to oxygen deprivation, which results in shorter life expectancy. A person who inherits the sickle cell gene from one parent and a normal hemoglobin allele (HgbA) from the other, has a normal life expectancy. However, these heterozygote individuals, known as carriers of the sickle cell trait, may suffer problems from time to time.
The heterozygote is resistant to the malarial parasite which kills a large number of people each year. This is an example of balancing selection between the fierce selection against homozygous sickle-cell sufferers, and the selection against the standard HgbA homozygotes by malaria. The heterozygote has a permanent advantage (a higher fitness) wherever malaria exists.
Frequency-dependent selection occurs when the fitness of a phenotype is dependent on its frequency relative to other phenotypes in a given population. In positive frequency-dependent selection the fitness of a phenotype increases as it becomes more common. In negative frequency-dependent selection the fitness of a phenotype increases as it becomes less common. For example in prey switching, rare morphs of prey are actually fitter due to predators concentrating on the more frequent morphs.
The fitness of a genotype may vary greatly between larval and adult stages, or between parts of a habitat range.
The fitness of a genotype may depend on the fitness of other genotypes in the population: this covers many natural situations where the best thing to do (from the point of view of survival and reproduction) depends on what other members of the population are doing at the time.
Species in their natural habitat are often far more complex than the typical text-book examples.
The grove snail, Cepaea nemoralis, is famous for the rich polymorphism of its shell. The system is controlled by a series of multiple alleles. Unbanded is the top dominant trait, and the forms of banding are controlled by modifier genes (see epistasis).
In England the snail is regularly preyed upon by the song thrush Turdus philomelos, which breaks them open on thrush anvils (large stones). Here fragments accumulate, permitting researchers to analyse the snails taken. The thrushes hunt by sight, and capture selectively those forms which match the habitat least well. Snail colonies are found in woodland, hedgerows and grassland, and the predation determines the proportion of phenotypes (morphs) found in each colony.
A second kind of selection also operates on the snail, whereby certain heterozygotes have a physiological advantage over the homozygotes. Thirdly, apostatic selection is likely, with the birds preferentially taking the most common morph. This is the 'search pattern' effect, where a predominantly visual predator persists in targeting the morph which gave a good result, even though other morphs are available.
The polymorphism survives in almost all habitats, though the proportions of morphs varies considerably. The alleles controlling the polymorphism form a supergene with linkage so close as to be nearly absolute. This control saves the population from a high proportion of undesirable recombinants.
In this species predation by birds appears to be the main (but not the only) selective force driving the polymorphism. The snails live on heterogeneous backgrounds, and thrush are adept at detecting poor matches. The inheritance of physiological and cryptic diversity is preserved also by heterozygous advantage in the supergene. Recent work has included the effect of shell colour on thermoregulation, and a wider selection of possible genetic influences is also considered.
In the 1930s Theodosius Dobzhansky and his co-workers collected Drosophila pseudoobscura and D. persimilis from wild populations in California and neighbouring states. Using Painter's technique they studied the polytene chromosomes and discovered that all the wild populations were polymorphic for chromosomal inversions. All the flies look alike whatever inversions they carry, so this is an example of a cryptic polymorphism. Evidence accumulated to show that natural selection was responsible:
1. Values for heterozygote inversions of the third chromosome were often much higher than they should be under the null assumption: if no advantage for any form the number of heterozygotes should conform to Ns (number in sample) = p2+2pq+q2 where 2pq is the number of heterozygotes (see Hardy-Weinberg equilibrium).
2. Using a method invented by L'Heretier and Teissier, Dobzhansky bred populations in population cages, which enabled feeding, breeding and sampling whilst preventing escape. This had the benefit of eliminating migration as a possible explanation of the results. Stocks containing inversions at a known initial frequency can be maintained in controlled conditions. It was found that the various chromosome types do not fluctuate at random, as they would if selectively neutral, but adjust to certain frequencies at which they become stabilised.
3. Different proportions of chromosome morphs were found in different areas. There is, for example, a polymorph-ratio cline in D. robusta along an 18-mile (29 km) transect near Gatlinburg, TN passing from 1,000 feet (300 m) to 4,000 feet. Also, the same areas sampled at different times of year yielded significant differences in the proportions of forms. This indicates a regular cycle of changes which adjust the population to the seasonal conditions. For these results selection is by far the most likely explanation.
4. Lastly, morphs cannot be maintained at the high levels found simply by mutation, nor is drift a possible explanation when population numbers are high.
By 1951 Dobzhansky was persuaded that the chromosome morphs were being maintained in the population by the selective advantage of the heterozygotes, as with most polymorphisms.
An allele ( or ), or allel, is one of a number of alternative forms of the same gene or same genetic locus (generally a group of genes). It is the alternative form of a gene for a character producing different effects. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation. However, many variations at the genetic level result in little or no observable variation.
Most multicellular organisms have two sets of chromosomes, that is, they are diploid. These chromosomes are referred to as homologous chromosomes. Diploid organisms have one copy of each gene (and therefore one allele) on each chromosome. If both alleles are the same, they are homozygotes. If the alleles are different, they are heterozygotes.
A population or species of organisms typically includes multiple alleles at each locus among various individuals. Allelic variation at a locus is measurable as the number of alleles (polymorphism) present, or the proportion of heterozygotes in the population.
For example, at the gene locus for the ABO blood type carbohydrate antigens in humans, classical genetics recognizes three alleles, IA, IB, and IO, that determine compatibility of blood transfusions. Any individual has one of six possible genotypes (AA, AO, BB, BO, AB, and OO) that produce one of four possible phenotypes: "A" (produced by AA homozygous and AO heterozygous genotypes), "B" (produced by BB homozygous and BO heterozygous genotypes), "AB" heterozygotes, and ″O″ homozygotes. It is now known that each of the A, B, and O alleles is actually a class of multiple alleles with different DNA sequences that produce proteins with identical properties: more than 70 alleles are known at the ABO locus. An individual with "Type A" blood may be an AO heterozygote, an AA homozygote, or an AA heterozygote with two different ″A″ alleles.
The word "allele" is a short form of allelomorph (″other form″), which was used in the early days of genetics to describe variant forms of a gene detected as different phenotypes. It derives from the Greek prefix ἀλλήλ- [″allel-″], meaning "reciprocal" or "each other", which itself is related to the Greek adjective ἄλλος (allos; cognate with Latin ″alius″), meaning "other".
In many cases, genotypic interactions between the two alleles at a locus can be described as dominant or recessive, according to which of the two homozygous genotypes the phenotype of the heterozygote most resembles. Where the heterozygote is indistinguishable from one of the homozygotes, the allele involved is said to be dominant to the other, which is said to be recessive to the former. The degree and pattern of dominance varies among loci. For a further discussion see Dominance (genetics). This type of interaction was first formally described by Gregor Mendel. However, many traits defy this simple categorization and the phenotypes are modeled by polygenic inheritance.
The term "wild type" allele is sometimes used to describe an allele that is thought to contribute to the typical phenotypic character as seen in "wild" populations of organisms, such as fruit flies (″Drosophila melanogaster″). Such a ″wild type" allele was historically regarded as dominant, common, and normal, in contrast to "mutant" alleles regarded as recessive, rare, and frequently deleterious. It was commonly thought that most individuals were homozygous for the "wild type" allele at most gene loci, and that any alternative ″mutant″ allele was found in homozygous form in a small minority of "affected" individuals, often as genetic diseases, and more frequently in heterozygous form in "carriers" for the mutant allele. It is now appreciated that most or all gene loci are highly polymorphic, with multiple alleles, whose frequencies vary from population to population, and that a great deal of genetic variation is hidden in the form of alleles that do not produce obvious phenotypic differences.
The frequency of alleles in a diploid population can be used to predict the frequencies of the corresponding genotypes (see Hardy-Weinberg principle). For a simple model, with two alleles:
where ″p″ is the frequency of one allele and ″q″ is the frequency of the alternative allele, which necessarily sum to unity. Then, ″p″2 is the fraction of the population homozygous for the first allele, 2 ″pq″ is the fraction of heterozygotes, and ″q″2 is the fraction homozygous for the alternative allele. If the first allele is dominant to the second, then the fraction of the population that will show the dominant phenotype is ″p″2 + 2 ″pq″, and the fraction with the recessive phenotype is ″q″2.
With three alleles:
In the case of multiple alleles at a diploid locus, the number of possible genotypes (G) with a number of alleles (a) is given by the expression:
A number of genetic disorders are caused when an individual inherits two recessive alleles for a single-gene trait. Recessive genetic disorders include Albinism, Cystic Fibrosis, Galactosemia, Phenylketonuria (PKU), and Tay-Sachs Disease. Other disorders are also due to recessive alleles, but because the gene locus is located on the X chromosome, so that males have only one copy (that is, they are hemizygous), they are more frequent in males than in females. Examples include red-green color blindness and Fragile X syndrome.
Other disorders, such as Huntington disease, occur when an individual inherits only one dominant allele.
is the degree of similarity of the alleles for a trait in an organism.
Most eukaryotes have two matching sets of chromosomes; that is, they are diploid. Diploid organisms have the same loci on each of their two sets of homologous chromosomes, except that the sequences at these loci may differ between the two chromosomes in a matching pair and that a few chromosomes may be mismatched as part of a chromosomal sex-determination system. If both alleles of a diploid organism are the same, the organism is homozygous at that locus. If they are different, the organism is heterozygous at that locus. If one allele is missing, it is hemizygous, and, if both alleles are missing, it is nullizygous.
The DNA sequence of a gene often varies from one individual to another. Those variations are called alleles. While some genes have only one allele because there is low variation, others have only one allele because only that allele can function properly. Any variation from the DNA sequence of that allele will be fatal in the embryo, and the organism will never survive to be born. But most genes have two or more alleles. The frequency of different alleles varies throughout the population. Some genes may have two alleles with equal distribution. For other genes, one allele may be common, and another allele may be rare. Sometimes, one allele is a disease-causing variation while the other allele is healthy. Sometimes, the different variations in the alleles make no difference at all in the function of the organism.
In diploid organisms, one allele is inherited from the male parent and one from the female parent. Zygosity is a description of whether those two alleles have identical or different DNA sequences. In some cases the term "zygosity" is used in the context of a single chromosome.
The words homozygous, heterozygous, and hemizygous are used to describe the genotype of a diploid organism at a single locus on the DNA. Homozygous
describes a genotype consisting of two identical alleles at a given locus, heterozygous
describes a genotype consisting of two different alleles at a locus, hemizygous
describes a genotype consisting of only a single copy of a particular gene in an otherwise diploid organism, and nullizygous
refers to an otherwise-diploid organism in which both copies of the gene are missing.
A cell is said to be homozygous for a particular gene when identical alleles of the gene are present on both homologous chromosomes. The cell or organism in question is called a homozygote
. True breeding organisms are always homozygous for the traits that are to be held constant.
An individual that is homozygous-dominant
for a particular trait carries two copies of the allele that codes for the dominant trait. This allele, often called the "dominant allele", is normally represented by a capital letter (such as "P" for the dominant allele producing purple flowers in pea plants). When an organism is homozygous-dominant for a particular trait, the genotype is represented by a doubling of the symbol for that trait, such as "PP".
An individual that is homozygous-recessive
for a particular trait carries two copies of the allele that codes for the recessive trait. This allele, often called the "recessive allele", is usually represented by the lowercase form of the letter used for the corresponding dominant trait (such as, with reference to the example above, "p" for the recessive allele producing white flowers in pea plants). The genotype of an organism that is homozygous-recessive for a particular trait is represented by a doubling of the appropriate letter, such as "pp".
A diploid organism is heterozygous at a gene locus when its cells contain two different alleles of a gene. Heterozygous genotypes are represented by a capital letter (representing the dominant allele) and a lowercase letter (representing the recessive allele), such as "Rr" or "Ss". The capital letter is usually written first.
If the trait in question is determined by simple (complete) dominance, a heterozygote will express only the trait coded by the dominant allele, and the trait coded by the recessive allele will not be present. In more complex dominance schemes the results of heterozygosity can be more complex.
A chromosome in a diploid organism is hemizygous when only one copy is present. The cell or organism is called a hemizygote
. Hemizygosity is also observed when one copy of a gene is deleted, or in the heterogametic sex when a gene is located on a sex chromosome. For organisms in which the male is heterogametic, such as humans, almost all X-linked genes are hemizygous in males with normal chromosomes because they have only one X chromosome and few of the same genes are on the Y chromosome. Transgenic mice generated through exogenous DNA microinjection of an embryo's pronucleus are also considered to be hemizygous because the introduced allele is expected to be incorporated into only one copy of any locus. A transgenic can later be bred to homozygosity and maintained as an inbred line to reduce the need to confirm the genotypes of each litter.
A nullizygous organism carries two mutant alleles for the same gene. The mutant alleles are both complete loss-of-function or 'null' alleles, so homozygous null and nullizygous are synonymous. The mutant cell or organism is called a nullizygote
Zygosity may also refer to the origin(s) of the alleles in a genotype. When the two alleles at a locus originate from a common ancestor by way of nonrandom mating (inbreeding), the genotype is said to be autozygous
. This is also known as being "identical by descent", or IBD. When the two alleles come (at least to the extent that the descent can be traced) from completely different sources, as is the case in most normal, random mating, the genotype is called allozygous
. This is known as being "identical by state", or IBS.
Because the alleles of autozygous genotypes come from the same source, they are always homozygous, but allozygous genotypes may be homozygous too. All heterozygous genotypes are, by definition, allozygous because they contain two completely different alleles. Hemizygous and nullizygous genotypes do not contain enough alleles to allow for comparison of sources, so this classification is irrelevant for them.
As discussed above, "zygosity" can be used in the context of a specific genetic locus (example). In addition, the word "zygosity" may also be used to describe the genetic similarity or dissimilarity of twins. Identical twins are monozygotic
, meaning that they develop from one zygote that splits and forms two embryos. Fraternal twins are dizygotic
because they develop from two separate eggs that are fertilized by two separate sperm.
In population genetics, the concept of heterozygosity is commonly extended to refer to the population as a whole, i.e., the fraction of individuals in a population that are heterozygous for a particular locus. It can also refer to the fraction of loci within an individual that are heterozygous.
Typically, the observed (
) and expected (
) heterozygosities are compared, defined as follows for diploid individuals in a population:
is the number of individuals in the population, and
are the alleles of individual
at the target locus.
is the number of alleles at the target locus, and
is the allele frequency of the
allele at the target locus.
In genetics, codominance is a phenomenon in which a single gene has more than one dominant allele. An individual who is heterozygous for two codominant alleles will express the phenotypes associated with both alleles.
Codominance and incomplete or semidominance are not the same thing. For example, in some plant species, white and red spotted flowers may be the product of codominance between the red allele for the gene and the white allele for the gene (co-dominance on the pigment level, no dominance on the color level), or the result of one allele that produces the usual amount of red pigment and another non-functional allele that produces no pigment, so as to produce a dilute, intermediate pink color (no dominance at either level).
In the ABO blood group system, the IA and IB alleles are codominant to each other. Individuals who are heterozygous for the IA and IB alleles express the AB blood group phenotype, in which both A- and B-type antigens are present on the surface of red blood cells.
Another example occurs at the locus for the beta globin component of hemoglobin, where the three molecular phenotypes of HbA/HbA, HbA/HbS, and HbS/HbS are all equally detectable by protein electrophoresis. For most gene loci at the level, both alleles are expressed co-dominantly, because both are transcribed into RNA.
Haploinsufficiency occurs when a diploid organism has only a single functional copy of a gene (with the other copy inactivated by mutation) and the single functional copy does not produce enough of a gene product (typically a protein) to bring about a wild-type condition, leading to an abnormal or diseased state. It is responsible for some but not all autosomal dominant disorders.
Haplosufficiency is the opposite case: when a diploid organism only has a single functional copy of a gene (with the other copy inactivated by mutation) and the single functional copy produces enough of a gene product (typically a protein) to bring about a wild-type condition.
The wild-type allele (i.e. version) of a haplosufficient gene is dominant over the mutant allele, since a heterozygote (with one mutant and one normal allele) displays the normal wild-type phenotype (i.e. is not diseased). On the other hand, the wild-type allele of a haploinsufficient gene is recessive to the mutant allele, since a heterozygote (with one mutant and one normal allele) displays the mutant (disease) phenotype. It is also possible that the heterozygote will display a third phenotype (such as diseased but of lesser severity) and in that case, the mutant allele is incompletely dominant to the recessive wild-type allele.
Haploinsufficiency can occur through a number of ways. A mutation in the gene may have erased the production message. One of the two copies of the gene may be missing due to a deletion. The message or protein produced by the cell may be unstable or degraded by the cell.
A haploinsufficient gene is described as needing both alleles to be functional in order to express the wild type. A mutation is not haploinsufficient, but dominant loss of function mutations are the result of mutations in haploinsufficient genes.
The alteration in the gene dosage, which is caused by the loss of a functional allele, is also called allelic insufficiency. An example of this is seen in the case of Williams syndrome, a neurodevelopmental disorder caused by the haploinsufficiency of genes at 7q11.23. The haploinsufficiency is caused by the Copy Number Variation (CNV) of 28 genes led by the deletion of ~1.6 Mb. These dosage-sensitive genes are vital for human language and constructive cognition.
Another example is the haploinsufficiency of telomerase reverse transcriptase which leads to anticipation in autosomal dominant dyskeratosis congenita. It is a rare inherited disorder characterized by abnormal skin manifestations, which results in bone marrow failure, pulmonary fibrosis and an increased predisposition to cancer. A null mutation in motif D of the reverse transcriptase domain of the telomerase protein, hTERT, leads to this phenotype. Thus telomerase dosage is important for maintaining tissue proliferation.
A variation of haploinsufficiency exists for mutations in the gene PRPF31, a known cause of autosomal dominant retinitis pigmentosa. There are two wild-type alleles of this gene—a high-expressivity allele and a low-expressivity allele. When the mutant gene is inherited with a high-expressivity allele, there is no disease phenotype. However, if a mutant allele and a low-expressivity allele are inherited, the residual protein levels falls below that required for normal function, and disease phenotype is present.
Copy-number variation (CNV) refers to the differences in the number of copies of a particular region of the genome. This leads to too many or too few of the dosage sensitive genes. The genomic rearrangements, that is, deletions or duplications, are caused by the mechanism of non allelic homologous recombination (NAHR). In the case of the Williams Syndrome, the microdeletion includes the ELN genes. The hemizygosity of the elastinis is responsible for Aortic Stenosis, the obstruction in the left ventricular outflow of blood in the heart.
The Hardy–Weinberg principle (also known as the Hardy–Weinberg equilibrium, model, theorem, or law) states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences. These influences include non-random mating, mutation, selection, genetic drift, gene flow and meiotic drive. Because one or more of these influences are typically present in real populations, the Hardy–Weinberg principle describes an ideal condition against which the effects of these influences can be analyzed.
In the simplest case of a single locus with two alleles denoted A and a with frequencies f(A) = p and f(a) = q, respectively, the expected genotype frequencies are f(AA) = p2 for the AA homozygotes, f(aa) = q2 for the aa homozygotes, and f(Aa) = 2pq for the heterozygotes. The genotype proportions p2, 2pq, and q2 are called the Hardy-Weinberg proportions. [Note that the sum of all genotype frequencies of this case is the binomial expansion of the square of the sum of p and q, and such a sum, as it represents the total of all possibilities, must be equal to 1. Therefore (p + q)2 = p2 + 2pq + q2 = 1. The solution of this equation is q = 1 - p.] Zygosity
Dominance versus overdominance is a scientific controversy in the field of genetics that has persisted for more than a century. These two alternative hypotheses were first stated in 1908.
When a population is small or inbred, it tends to lose genetic diversity. Inbreeding depression is the loss of fitness due to loss of genetic diversity. Inbred strains tend to be homozygous for recessive alleles that are mildly harmful (or produce a trait that is undesirable from the standpoint of the breeder). Heterosis or hybrid vigor, on the other hand, is the tendency of outbred strains to exceed both inbred parents in fitness. Genetics
Classical genetics consists of the technique and methodologies of genetics that predate the advent of molecular biology. A key discovery of classical genetics in eukaryotes was genetic linkage. The observation that some genes do not segregate independently at meiosis broke the laws of Mendelian inheritance, and provided science with a way to map characteristics to a location on the chromosomes. Linkage maps are still used today, especially in breeding for plant improvement.
After the discovery of the genetic code and such tools of cloning as restriction enzymes, the avenues of investigation open to geneticists were greatly broadened. Some classical genetic ideas have been supplanted with the mechanistic understanding brought by molecular discoveries, but many remain intact and in use. Classical genetics is often contrasted with reverse genetics, and aspects of molecular biology are sometimes referred to as molecular genetics. Biology
In journalism, a human interest story is a feature story that discusses a person or people in an emotional way. It presents people and their problems, concerns, or achievements in a way that brings about interest, sympathy or motivation in the reader or viewer.
Human interest stories may be "the story behind the story" about an event, organization, or otherwise faceless historical happening, such as about the life of an individual soldier during wartime, an interview with a survivor of a natural disaster, a random act of kindness or profile of someone known for a career achievement.