Background
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 widely 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, and 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 the same phenotypes, generation after generation. However, when lines with different phenotypes were crossed (interbred), one and only one of the parental phenotypes showed up in the offspring (green, or round, or red, or tall). 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 (one parent AA and the other parent aa), that each contributed one allele to the offspring, with the result that all of these hybrids were heterozygotes (Aa), and that one of the two alleles in the hybrid cross dominated expression of the other: A masked a. The final cross between two heterozygotes (Aa X Aa) would produce AA, Aa, and aa offspring in a 1:2:1 genotype ratio with the first two classes showing the (A) phenotype, and the last showing the (a) phenotype, thereby producing the 3:1 phenotype ratio. Mendel did not use the terms gene, allele, phenotype, genotype, homozygote, and heterozygote, all of which were introduced later. He did introduce the notation of capital and lowercase letters for dominant and recessive alleles, respectively, still in use today. In 1928, British population geneticist Ronald Fisher proposed that dominance acted based on natural selection through the contribution of modifier genes. In 1929, American geneticistChromosomes, genes, and alleles
Most animals and some plants have paired chromosomes, and are described as diploid. They have two versions of each chromosome, one contributed by the mother's ovum, and the other by the father's sperm, known as gametes, described as haploid, and created through meiosis. These gametes then 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 (non-gamete) cell as its parents. In mammalian genetics, autosomal dominant disorders haveTypes of Dominance
Complete dominance
In complete dominance, the effect of one allele in a heterozygous genotype completely masks the effect of the other. The allele that masks is considered ''dominant'' to the other allele, and the masked allele is considered ''recessive''. Complete dominance in a heterozygote's phenotype is indistinguishable from a dominant homozygote's phenotype. A classic example of complete dominance is the inheritance of seed shape (pea shape) in peas. Peas may be round (associated with allele ''R'') or wrinkled (associated with allele ''r''). In this case, three combinations of alleles ( genotypes) are possible: ''RR, rr, Rr.'' ''RR'' and ''rr'' are homozygous, and ''Rr'' is heterozygous. 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 also have round peas. Thus, allele ''R'' is completely dominant to allele ''r'', and allele ''r'' is recessive to allele ''R''.Incomplete dominance
Incomplete dominance (also called ''partial dominance'', ''semi-dominance'' or ''intermediate inheritance'') occurs when the phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes. The phenotypic result often appears as a blended form of characteristics in heterozygous state. For example, the snapdragon flower color is homozygous for either red or white. When the red homozygous flower is paired with the white homozygous flower, the result yields a pink snapdragon flower. The pink snapdragon is the result of incomplete dominance. A similar type of incomplete dominance is found in the four o'clock plant wherein pink color is produced when true-bred parents of white and red flowers are crossed. In quantitative genetics, where phenotypes are measured and treated numerically, if a heterozygote's phenotype is exactly between (numerically) that of the two homozygotes, the phenotype is said to exhibit ''no dominance'' at all, i.e. dominance exists only when the heterozygote's phenotype measure lies closer to one homozygote than the other. When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1:2:1 (Red:Pink:White). SeeCo-dominance
Co-dominance occurs when the contributions of both alleles are visible in the phenotype and neither allele masks another. For example, in the ABO blood group system, chemical modifications to a glycoprotein (the H antigen) on the surfaces of blood cells are controlled by three alleles, two of which are co-dominant to each other (''IA'', ''IB'') and dominant over the recessive ''i'' at the ABO locus. The ''IA'' and ''IB'' alleles produce different modifications. The enzyme coded for by ''IA'' adds an N-acetylgalactosamine to a membrane-bound H antigen. The ''IB'' enzyme adds a galactose. The ''i'' allele produces no modification. Thus the ''IA'' and ''IB'' alleles are each dominant to ''i'' (''IAIA'' and ''IAi'' individuals both have type A blood, and ''IBIB'' and ''IBi'' individuals both have type B blood), but ''IAIB'' individuals have both modifications on their blood cells and thus have type AB blood, so the ''IA'' and ''IB'' alleles are said to be co-dominant. Another example occurs at the locus for theAddressing common misconceptions
Dominance relates to the relationship between two versions of a gene. A dominant trait is usually in correspondence to inheritance patterns that can be seen in Punnett Squares. If an individual has two versions of a gene, then the gene that is frequently observed in further generations is considered "dominant". In genetics, there are a few misconceptions that are fairly common. It is thought that a dominant trait is "stronger" and "overpowers" a recessive trait. Dominant traits are also assumed more likely to be inherited as well as more prevalent in a population. The idea of dominant traits being male or masculine is another common misconception. The emergence of these different ideas is due to the various concepts of dominance in non-genetic settings; such as being strong, powerful and controlling; which differs from the genetic concept of dominance. Dominance does not determine whether an allele is deleterious, neutral, or advantageous. However, selection must operate on genes indirectly through phenotypes and dominance affects the exposure of alleles in phenotypes, hence the rate of change in allele frequencies under selection. Deleterious recessive alleles may persist in a population at low frequencies, with most copies carried in heterozygotes, at no cost to those individuals. These rare recessives are the basis for many hereditary genetic disorders.Nomenclature
In genetics, symbols began as algebraic placeholders. When one allele is dominant to another, the oldest convention is to symbolize the dominant allele with a capital letter. The recessive allele is assigned the same letter in lower case. In the pea example, once the dominance relationship between the two alleles is known, it is possible to designate the dominant allele that produces a round shape by a capital-letter symbol R, and the recessive allele that produces a wrinkled shape by a lower-case symbol r. The homozygous dominant, heterozygous, and homozygous recessive genotypes are then written RR, Rr, and rr, respectively. It would also be possible to designate the two alleles as W and w, and the three genotypes WW, Ww, and ww, the first two of which produced round peas and the third wrinkled peas. The choice of "R" or "W" as the symbol for the dominant allele does not pre-judge whether the allele causing the "round" or "wrinkled" phenotype when homozygous is the dominant one. A gene may have several alleles. Each allele is symbolized by the locus symbol followed by a unique superscript. In many species, the most common allele in the wild population is designated the wild type allele. It is symbolized with a + character as a superscript. Other alleles are dominant or recessive to the wild type allele. For recessive alleles, the locus symbol is in lower case letters. For alleles with any degree of dominance to the wild type allele, the first letter of the locus symbol is in upper case. For example, here are some of the alleles at the ''a'' locus of the laboratory mouse, ''Mus musculus'': ''Ay'', dominant yellow; ''a+'', wild type; and ''abt'', black and tan. The ''abt'' allele is recessive to the wild type allele, and the ''Ay'' allele is codominant to the wild type allele. The ''Ay'' allele is also codominant to the ''abt'' allele, but showing that relationship is beyond the limits of the rules for mouse genetic nomenclature. Rules of genetic nomenclature have evolved as genetics has become more complex. Committees have standardized the rules for some species, but not for all. Rules for one species may differ somewhat from the rules for a different species.Relationship to other genetic concepts
Multiple alleles
Although any individual of a diploid organism has at most two different alleles at any one locus (barringAutosomal ''versus'' sex-linked dominance
In humans and other mammal species, sex is determined by two sex chromosomes called the X chromosome and the Y chromosome. Human females are XX; males are XY. The remaining pairs of chromosome are found in both sexes and are called autosomes; genetic traits associated with loci on these chromosomes are described as autosomal, and may be dominant or recessive. Genetic traits on the X and Y chromosomes are called sex-linked, because they are linked to sex chromosomes, not because they are characteristic of one sex or the other. In practice, the term almost always refers to X-linked traits and a great many such traits (such as red-green colour vision deficiency) are not affected by sex. Females have two copies of every gene locus found on the X chromosome, just as for the autosomes, and the same dominance relationships apply. Males, however, have only one copy of each X chromosome gene locus, and are described as hemizygous for these genes. The Y chromosome is much smaller than the X, and contains a much smaller set of genes, including, but not limited to, those that influence 'maleness', such as the SRY gene for testis determining factor. Dominance rules for sex-linked gene loci are determined by their behavior in the female: because the male has only one allele (except in the case of certain types of Y chromosome aneuploidy), that allele is always expressed regardless of whether it is dominant or recessive. Birds have opposite sex chromosomes: male birds have ZZ and female birds ZW chromosomes. However, inheritance of traits reminds XY-system otherwise; male zebra finches may carry white colouring gene in their one of two Z chromosome, but females develop white colouring always. Grasshoppers have XO-system. Females have XX, but males only X. There is no Y chromosome at all.Epistasis
Epistasis ''epi'' + ''stasis'' = to sit on top"is an interaction between alleles at two ''different'' gene loci that affect a single trait, which may sometimes resemble a dominance interaction between two ''different'' alleles at the ''same'' locus. Epistasis modifies the characteristic 9:3:3:1 ratio expected for two non-epistatic genes. For two loci, 14 classes of epistatic interactions are recognized. As an example of ''recessive epistasis'', one gene locus may determine whether a flower pigment is yellow (AA or Aa) or green (aa), while another locus determines whether the pigment is produced (BB or Bb) or not (bb). In a bb plant, the flowers will be white, irrespective of the genotype of the other locus as AA, Aa, or aa. The bb combination is ''not'' dominant to the A allele: rather, the B gene shows ''recessive epistasis'' to the A gene, because the B locus when homozygous for the ''recessive'' allele (bb) suppresses phenotypic expression of the A locus. In a cross between two AaBb plants, this produces a characteristic 9:3:4 ratio, in this case of yellow : green : white flowers. In ''dominant epistasis'', one gene locus may determine yellow or green pigment as in the previous example: AA and Aa are yellow, and aa are green. A second locus determines whether a pigment precursor is produced (dd) or not (DD or Dd). Here, in a DD or Dd plant, the flowers will be colorless irrespective of the genotype at the ''A'' locus, because of the epistatic effect of the dominant D allele. Thus, in a cross between two AaDd plants, 3/4 of the plants will be colorless, and the yellow and green phenotypes are expressed only in dd plants. This produces a characteristic 12:3:1 ratio of white : yellow : green plants. ''Supplementary epistasis'' occurs when two loci affect the same phenotype. For example, if pigment color is produced by CC or Cc but not cc, and by DD or Dd but not dd, then pigment is not produced in any genotypic combination with either cc ''or'' dd. That is, ''both'' loci must have at least one dominant allele to produce the phenotype. This produces a characteristic 9:7 ratio of pigmented to unpigmented plants. ''Complementary epistasis'' in contrast produces an unpigmented plant if and only if the genotype is cc ''and'' dd, and the characteristic ratio is 15:1 between pigmented and unpigmented plants. Classical genetics considered epistatic interactions between two genes at a time. It is now evident from molecular genetics that all gene loci are involved in complex interactions with many other genes (e.g., metabolic pathways may involve scores of genes), and that this creates epistatic interactions that are much more complex than the classic two-locus models.Hardy–Weinberg principle (estimation of carrier frequency)
The frequency of the heterozygous state (which is the carrier state for a recessive trait) can be estimated using the Hardy–Weinberg formula: This formula applies to a gene with exactly two alleles and relates the frequencies of those alleles in a large population to the frequencies of their three genotypes in that population. For example, if ''p'' is the frequency of allele A, and ''q'' is the frequency of allele a then the terms ''p''2, 2''pq'', and ''q''2 are the frequencies of the genotypes AA, Aa and aa respectively. Since the gene has only two alleles, all alleles must be either A or a and . Now, if A is completely dominant to a then the frequency of the carrier genotype Aa cannot be directly observed (since it has the same traits as the homozygous genotype AA), however it can be estimated from the frequency of the recessive trait in the population, since this is the same as that of the homozygous genotype aa. i.e. the individual allele frequencies can be estimated: , , and from those the frequency of the carrier genotype can be derived: . This formula relies on a number of assumptions and an accurate estimate of the frequency of the recessive trait. In general, any real-world situation will deviate from these assumptions to some degree, introducing corresponding inaccuracies into the estimate. If the recessive trait is rare, then it will be hard to estimate its frequency accurately, as a very large sample size will be needed.Dominant versus advantageous
The property of "dominant" is sometimes confused with the concept of advantageous and the property of "recessive" is sometimes confused with the concept of deleterious, but the phenomena are distinct. Dominance describes the phenotype of heterozygotes with regard to the phenotypes of the homozygotes and without respect to the degree to which different phenotypes may be beneficial or deleterious. Since many genetic disease alleles are recessive and because the word dominance has a positive connotation, the assumption that the dominant phenotype is superior with respect to fitness is often made. This is not assured however; as discussed below while most genetic disease alleles are deleterious and recessive, not all genetic diseases are recessive. Nevertheless, this confusion has been pervasive throughout the history of genetics and persists to this day. Addressing this confusion was one of the prime motivations for the publication of the Hardy–Weinberg principle.Molecular mechanisms
The molecular basis of dominance was unknown to Mendel. It is now understood that a gene locus includes a long series (hundreds to thousands) of bases or nucleotides of deoxyribonucleic acid (DNA) at a particular point on a chromosome. The central dogma of molecular biology states that "'' DNA makes RNA makesZygosity
Historically, Mendel's Law of Independent Assortment assumed that alleles will sort independently, with one allele being "dominant". Zygosity, degree of similarity of an organism's alleles, may affect dominance. Within a diploid organism, these would be defined by the Haplotype interactions of the alleles. Gene haploidy may result in a single, functional allele making sufficient protein to produce a phenotype identical to that of the homozygote. Three general types of haplotype interactions are possible: # Haplosufficiency. In a diploid, a functional allele of a haplosufficient gene would be considered dominant, while a non-functional allele would be considered recessive. For example, suppose the standard amount of enzyme produced in the functional homozygote is 100%, with the two functional alleles contributing 50% each. The single functional allele in the heterozygote produces 50% of the standard amount of enzyme, which is sufficient to produce the standard phenotype. If the heterozygote and the functional-allele homozygote have identical phenotypes, the functional allele is dominant to the non-functional allele. This occurs at the albino gene locus: the heterozygote produces sufficient enzyme to convert the pigment precursor to melanin, and the individual has standard pigmentation. For example, in humans and other organisms, the unpigmented skin of the albino phenotype results when an individual is homozygous for an allele that encodes a non-functional version of an enzyme needed to produce the skin pigmentDominant-negative mutations
Many proteins are normally active in the form of a multimer, an aggregate of multiple copies of the same protein, otherwise known as a homomultimeric protein or homooligomeric protein. In fact, a majority of the 83,000 different enzymes from 9800 different organisms in the BRENDA Enzyme Database represent homooligomers. When the wild-type version of the protein is present along with a mutant version, a mixed multimer can be formed. A mutation that leads to a mutant protein that disrupts the activity of the wild-type protein in the multimer is a dominant-negative mutation. A dominant-negative mutation may arise in a human somatic cell and provide a proliferative advantage to the mutant cell, leading to its clonal expansion. For instance, a dominant-negative mutation in a gene necessary for the normal process of programmed cell death ( Apoptosis) in response to DNA damage can make the cell resistant to apoptosis. This will allow proliferation of the clone even when excessive DNA damage is present. Such dominant-negative mutations occur in the tumor suppressor gene '' p53''. The P53 wild-type protein is normally present as a four-protein multimer (oligotetramer). Dominant-negative ''p53'' mutations occur in a number of different types of cancer and pre-cancerous lesions (e.g. brain tumors, breast cancer, oral pre-cancerous lesions and oral cancer). Dominant-negative mutations also occur in other tumor suppressor genes. For instance two dominant-negative germ line mutations were identified in the Ataxia telangiectasia mutated (ATM) gene which increases susceptibility to breast cancer. Dominant negative mutations of the transcription factor C/EBPα can cause acute myeloid leukemia. Inherited dominant negative mutations can also increase the risk of diseases other than cancer. Dominant-negative mutations in Peroxisome proliferator-activated receptor gamma (PPARγ) are associated with severe insulin resistance, diabetes mellitus and hypertension. Dominant-negative mutations have also been described in organisms other than humans. In fact, the first study reporting a mutant protein inhibiting the normal function of a wild-type protein in a mixed multimer was with the bacteriophage T4 tail fiber protein GP37. Mutations that produce a truncated protein rather than a full-length mutant protein seem to have the strongest dominant-negative effect in the studies of P53, ATM, C/EBPα, and bacteriophage T4 GP37.Dominant and recessive genetic diseases in humans
In humans, many genetic traits or diseases are classified simply as "dominant" or "recessive". Especially with so-called recessive diseases, which are indeed a factor of recessive genes, but can oversimplify the underlying molecular basis and lead to misunderstanding of the nature of dominance. For example, the recessive genetic diseaseSee also
*References
External links