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Dominance (genetics)
Dominance (genetics)
Dominance (genetics)
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Autosomal dominant and autosomal recessive inheritance, the two most common Mendelian inheritance patterns. An autosome is any chromosome other than a sex chromosome.

In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome.[1][2] The first variant is termed dominant and the second is called recessive. This state of having two different variants of the same gene on each chromosome is originally caused by a mutation in one of the genes, either new (de novo) or inherited. The terms autosomal dominant or autosomal recessive are used to describe gene variants on non-sex chromosomes (autosomes) and their associated traits, while those on sex chromosomes (allosomes) are termed X-linked dominant, X-linked recessive or Y-linked; these have an inheritance and presentation pattern that depends on the sex of both the parent and the child (see Sex linkage). Since there is only one Y chromosome, Y-linked traits cannot be dominant or recessive. Additionally, there are other forms of dominance, such as incomplete dominance, in which a gene variant has a partial effect compared to when it is present on both chromosomes, and co-dominance, in which different variants on each chromosome both show their associated traits.

Dominance is a key concept in Mendelian inheritance and classical genetics.[3] Letters and Punnett squares are used to demonstrate the principles of dominance in teaching, and the upper-case letters are used to denote dominant alleles and lower-case letters are used for recessive alleles. An often quoted example of dominance is the inheritance of seed 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, and rr. The RR (homozygous) individuals have round peas, and the rr (homozygous) individuals have wrinkled peas. In Rr (heterozygous) individuals, the R allele masks the presence of the r allele, so these individuals also have round peas. Thus, allele R is dominant over allele r, and allele r is recessive to allele R.

Dominance is not inherent to an allele or its traits (phenotype). It is a strictly relative effect between two alleles of a given gene of any function; one allele can be dominant over a second allele of the same gene, recessive to a third, and co-dominant with a fourth. Additionally, one allele may be dominant for one trait but not others. Dominance differs from epistasis, the phenomenon of an allele of one gene masking the effect of alleles of a different gene.

Background

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See also: Introduction to genetics
Inheritance of dwarfing in maize. Demonstrating the heights of plants from the two parent variations and their F1 heterozygous hybrid (centre)

Gregor Johann Mendel, "The Father of Genetics", promulgated the idea of dominance in the 1860s. However, 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.[3] 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, round, red, or tall).[3] 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.[3]

Mendel did not use the terms gene, allele, phenotype, genotype, homozygote, and heterozygote, all of which were introduced later.[4][5] 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 geneticist Sewall Wright responded by stating that dominance is simply a physiological consequence of metabolic pathways and the relative necessity of the gene involved.[4][6][5]

Types of dominance

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Complete dominance (Mendelian)

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In complete dominance, the effect of one allele in a heterozygous genotype completely masks the effect of the other. The allele that masks are considered dominant to the other allele, and the masked allele is considered recessive.[7]

When we only look at one trait determined by one pair of genes, we call it monohybrid inheritance.[8] If the crossing is done between parents (P-generation, F0-generation) who are homozygote dominant and homozygote recessive, the offspring (F1-generation) will always have the heterozygote genotype and always present the phenotype associated with the dominant gene.[9]

Monohybrid cross between heterozygotes (Gg), resulting in genotypical ratio 1:2:1 (GG:Gg:gg) and phenotypical ratio 3:1 (G:g).

However, if the F1-generation is further crossed with the F1-generation (heterozygote crossed with heterozygote) the offspring (F2-generation) will present the phenotype associated with the dominant gene ¾ times.[8] Although heterozygote monohybrid crossing can result in two phenotype variants, it can result in three genotype variants -  homozygote dominant, heterozygote and homozygote recessive, respectively.[10][9]

Dihybrid cross between heterozygotes (GgRr), resulting in the phenotypical ratio 9:3:3:1 (G and R: G and r: g and R: g and r)

In dihybrid inheritance we look at the inheritance of two pairs of genes simultaneous. Assuming here that the two pairs of genes are located at non-homologous chromosomes, such that they are not coupled genes (see genetic linkage) but instead inherited independently.[11] Consider now the cross between parents (P-generation) of genotypes homozygote dominant and recessive, respectively. The offspring (F1-generation) will always heterozygous and present the phenotype associated with the dominant allele variant.[11] However, when crossing the F1-generation there are four possible phenotypic possibilities and the phenotypical ratio for the F2-generation will always be 9:3:3:1.[12][11]

Incomplete dominance (non-Mendelian)

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This Punnett square illustrates incomplete dominance. In this example, the red petal trait associated with the R allele recombines with the white petal trait of the r allele. The plant incompletely expresses the dominant trait (R) causing plants with the Rr genotype to express flowers with less red pigment resulting in pink flowers. The colors are not blended together, the dominant trait is just expressed less strongly.
See also: partial dominance hypothesis

Incomplete dominance (also called partial dominance, semi-dominance, intermediate inheritance, or occasionally incorrectly co-dominance in reptile genetics[13]) 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 the 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).[14]

Co-dominance (non-Mendelian)

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A and B blood types in humans show co-dominance, but the O type is recessive to A and B.

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.[14]

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 distinguishable by protein electrophoresis. (The medical condition produced by the heterozygous genotype is called sickle-cell trait and is a milder condition distinguishable from sickle-cell anemia, thus the alleles show incomplete dominance concerning anemia, see above). For most gene loci at the molecular level, both alleles are expressed co-dominantly, because both are transcribed into RNA.[14]

Co-dominance in a Punnett square. A white bull (WW) mates with a red cow (RR), and their offspring exhibit co-dominance expressing white and red hairs.

Co-dominance, where allelic products co-exist in the phenotype, is different from incomplete dominance, where the quantitative interaction of allele products produces an intermediate phenotype. For example, in co-dominance, a red homozygous flower and a white homozygous flower will produce offspring that have red and white spots. When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1:2:1 (Red:Spotted:White). These ratios are the same as those for incomplete dominance. Again, this classical terminology is inappropriate – in reality, such cases should not be said to exhibit dominance at all.[14]

Relationship to other genetic concepts

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Dominance can be influenced by various genetic interactions and it is essential to evaluate them when determining phenotypic outcomes. Multiple alleles, epistasis, pleiotropic genes, and polygenic characteristics are some factors that might influence the phenotypic outcome.[15]

Multiple alleles

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Main article: Allele § Multiple alleles

Although any individual of a diploid organism has at most two different alleles at a given locus, most genes exist in a large number of allelic versions in the population as a whole. This is called polymorphism, and is caused by mutations. Polymorphism can have an effect on the dominance relationship and phenotype, which is observed in the ABO blood group system. The gene responsible for human blood type have three alleles; A, B, and O, and their interactions result in different blood types based on the level of dominance the alleles expresses towards each other.[15][16]

Epistasis

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Main article: Epistasis

Epistasis is interactions between multiple alleles at different loci. More specifically, epistasis is when one gene can mask the phenotype of a gene at a completely different locus.[17] Therefore, several genes can influence the phenotype expressed. Epistasis is slightly different from dominance in the fact that dominance is an allele-to-allele interaction at one locus while epistasis is a gene-to-gene interaction at different loci.[18] The dominance relationship between alleles involved in epistatic interactions can influence the observed phenotypic ratios in offspring.[19]

An example of epistasis can be seen in Labrador retriever coat colors. One gene at one locus codes for the color of hair but another gene at a different locus determines if the color is even deposited in the hair.[18][17] Recessive epistasis is seen in this example due to recessive alleles for color desposition masking both the dominant black (B) allele and recessive brown (b) allele at the first locus to express a yellow coat in the Labrador retriever.[18][17] The yellow color comes from no pigment being deposited in the hair shaft.[18]

Other examples of epistasis interactions are dominant epistasis and duplicate recessive epistasis.[18] Each type of epistasis is a modification of the dihyrbid ratio of 9:3:3:1.[17]

Pleiotropic genes

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Main article: Pleiotropy

Pleiotropic genes are genes where one single gene affects two or more characteristics. An example of this concept is Marfan Syndrome which is a mutation of the FBN1 gene. The effects this causes are a person's appearance being tall and long limbed. They can also have Scoliosis, Ectopia Lentis, and larger than normal aortas.[20] Pleiotropy shares a relationship with Epistasis. While pleiotropy represents one single gene, epistasis is multiple genes interacting with one another to cause different traits to arise.  it is helpful to recognize how Epistasis could affect viewing pleiotropic genes if different traits arise or mask themselves to varying degrees.[21]

Polygenic characteristics

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Main article: Polygene

Polygenic characteristics are those affected by multiple genes at different loci.[22] These different genes interact in a way to produce a quantitative characteristic, which is a characteristic that presents a wide variety phenotypes, such as height in humans.[22] The greater the number of genes that interact to influence this characteristic, the greater the number of different phenotypes possible due to more possible genotypes.[22] Many more characteristics also appear to be affected by more than one gene located on different loci, including diabetes and some autoimmune diseases.[23]  

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Dominance (genetics)

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In , dominance refers to the relationship between two of the same in a heterozygous , where one allele (the dominant) masks the phenotypic expression of the other (the recessive), resulting in the dominant allele determining the observable trait. This phenomenon is a cornerstone of and occurs at diploid loci, where the heterozygote's aligns with that of the homozygous dominant . The concept of dominance was first systematically described by in the mid-19th century through his hybridization experiments with pea plants (Pisum sativum), where he identified seven traits exhibiting clear dominant-recessive patterns, such as tall versus and yellow versus green seed color. Mendel's observations led to the formulation of his laws of segregation and independent assortment, revealing that traits are inherited as discrete units (now known as genes) and that dominance explains why certain traits reappear across generations despite not being visible in some hybrids. His work, published in 1866 but largely overlooked until rediscovered in 1900, laid the foundation for . While complete dominance—where the dominant fully suppresses the recessive one—is the classical model Mendel observed, genetic dominance manifests in varied forms depending on the interaction between alleles. In incomplete dominance, the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes, as seen in snapdragon flower color where and alleles produce flowers. Codominance, by contrast, involves both alleles being fully and simultaneously expressed without blending, such as in the where A and alleles produce distinct antigens on cells. These non-Mendelian patterns highlight that dominance is not absolute but context-dependent on the and . At the molecular level, dominance arises from diverse mechanisms, including haplosufficiency (where one functional produces sufficient for normal function), loss-of-function mutations in the recessive , or gain-of-function effects in the dominant one. For instance, many recessive traits result from null mutations that eliminate protein function, while a single wild-type suffices for dominance to dosage compensation. Understanding these underpinnings has advanced fields like , where dominant mutations often cause diseases like Huntington's through toxic protein aggregates, contrasting with recessive disorders requiring biallelic defects.

Introduction and History

Definition and Basic Principles

In genetics, dominance describes the relationship between alleles—alternative forms of the same located at a specific position, or locus, on a —such that one allele, termed dominant, masks or overrides the expression of another allele, termed recessive, in individuals carrying both (heterozygotes), resulting in a that matches the homozygous dominant . The refers to an organism's complete set of or genetic makeup, including the specific present at each locus, while the encompasses the observable traits or characteristics arising from the interaction of and environment. A homozygote possesses two identical at a given locus (e.g., AA or aa), whereas a heterozygote has two different (e.g., Aa). This concept is illustrated through a simple , such as one involving plant height in pea plants, where the for tall height (T) is dominant to the for short height (t). Consider a between two heterozygous individuals (Tt × Tt); the possible offspring genotypes can be predicted using a , a diagrammatic tool for determining genotypic outcomes based on parental s:
Tt
TTTTt
tTttt
This yields genotypic ratios of 1 TT : 2 Tt : 1 tt, corresponding to phenotypic ratios of 3 tall : 1 short, as both TT and Tt express the dominant tall . Dominance is inherently allele-specific, meaning it depends on the particular pair of alleles at a locus and varies across different genes and traits rather than being an absolute property of alleles themselves. This principle forms a foundational aspect of patterns.

Historical Development

The concept of dominance in genetics originated with Gregor Mendel's pioneering experiments on pea plants (Pisum sativum) conducted between 1856 and 1863, where he observed that certain traits, such as seed color and plant height, appeared consistently in hybrid offspring, leading him to classify them as dominant or recessive. In his seminal 1866 , "Experiments on Plant Hybridization" (Versuche über Pflanzen-Hybriden), Mendel described these patterns quantitatively, noting ratios like 3:1 in the F2 generation for traits where one form masked the other, laying the empirical foundation for understanding dominance as a key feature of particulate inheritance. Mendel's work, presented to the Natural History Society of Brünn, emphasized that traits were inherited as discrete units rather than blending, though it received little attention during his lifetime due to the prevailing views on continuous variation. Mendel's principles remained overlooked for over three decades until their independent rediscovery in 1900 by three botanists: in the , in , and in , who arrived at similar hybridization results while studying plant traits and recognized the alignment with Mendel's earlier findings. This rediscovery sparked renewed interest in dominance and recessiveness, as the scientists confirmed Mendel's ratios through their own experiments on species like evening primroses and , validating the non-blending nature of inheritance. The event marked a turning point, shifting scientific focus from qualitative descriptions of to quantitative genetic analysis. In the early , British played a pivotal role in promoting Mendel's ideas, coining the term "" in 1905 to denote the study of and variation, and actively defending dominance as part of particulate against the blending model favored by biometricians like . , through works like Mendel's Principles of (1909), argued that dominance exemplified discrete genetic units (Anlagen), fueling heated debates between Mendelians, who emphasized discontinuous variation and the masking effects of dominant alleles, and biometricians, who viewed as a gradual, blending process better explained by statistical correlations. These controversies, peaking around 1902–1910, highlighted tensions over how dominance reconciled with evolutionary change, ultimately bolstering the acceptance of Mendelian as a framework for understanding allelic interactions. Post-1920s advancements shifted the view of dominance toward biochemical mechanisms, exemplified by and Edward Tatum's 1941 experiments on the bread mold Neurospora crassa, which proposed the "one gene–one enzyme" hypothesis. By inducing mutations with X-rays and observing how single alterations disrupted specific enzymatic steps in metabolic pathways—leading to auxotrophic mutants unable to synthesize essential compounds—they linked dominance to functional enzyme production, where a wild-type (dominant) suffices to produce the enzyme, masking the recessive mutant's effect. This hypothesis provided a molecular rationale for why dominant traits prevail in heterozygotes, integrating Mendelian observations with emerging insights into action and paving the way for modern .

Core Concepts in Allelic Interactions

Mendelian Inheritance Framework

The foundational framework for understanding dominance in genetics is provided by Gregor Mendel's laws of inheritance, derived from his experiments with pea plants in the mid-19th century. Mendel's Law of Segregation states that during gamete formation, the two alleles for a gene separate, so each gamete receives only one allele, ensuring random distribution to offspring. In a monohybrid cross between individuals homozygous for contrasting traits—such as purple-flowered (PP) and white-flowered (pp) peas—the F1 generation is uniformly heterozygous (Pp) and exhibits the dominant purple phenotype due to complete dominance. Self-pollination of the F1 heterozygotes yields an F2 generation with a genotypic ratio of 1 PP : 2 Pp : 1 pp and a phenotypic ratio of 3 purple : 1 white, as observed in Mendel's pea flower color experiments, where he obtained 705 purple-flowered and 224 white-flowered plants from 929 F2 individuals. Recessiveness, in this context, refers to the lack of phenotypic expression of an when paired with a dominant one, rather than active suppression; the recessive remains present and can be transmitted to . This principle explains why the white flower trait reappears in the F2 despite its absence in the F1. Extending to multiple traits, Mendel's Law of Independent Assortment posits that alleles of different genes assort independently during formation, provided the genes are on different chromosomes. In a , such as between peas differing in seed color (yellow YYRR vs. green yyrr), the F1 dihybrid (YyRr) self-cross produces s in equal proportions (YR, Yr, yR, yr), resulting in an F2 phenotypic ratio of 9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled when dominance applies independently to each trait. This 9:3:3:1 ratio underscores how dominance operates across traits without interference in the Mendelian model.

Phenotypic Expression in Heterozygotes

In heterozygous individuals, the dominant typically produces a sufficient amount of functional to establish the wild-type , effectively masking the contribution of the recessive , which often results in a null or defective product. This masking occurs because many genes exhibit haplosufficiency, where the output from a single wild-type is adequate to maintain normal cellular or organismal function, rendering loss-of-function mutations in the paired phenotypically silent. For instance, in metabolic pathways, enzymes encoded by haplosufficient genes allow the heterozygous state to sustain typical flux rates despite halved activity from the recessive . Dominance in heterozygotes can also arise from differences in the timing or strength of during development (), where the dominant 's earlier or more robust activation establishes key developmental thresholds before the recessive 's influence becomes relevant. Quantitatively, this often involves threshold effects: a 50% reduction in product level from the single dominant rarely disrupts due to nonlinear responses in biochemical networks, such as those described by metabolic control analysis, where sensitivity coefficients near zero buffer against dosage changes. These thresholds ensure that heterozygotes display the dominant trait unless the gene operates in a highly dosage-sensitive context. A representative example is sickle cell anemia, where the normal hemoglobin allele (HbA) is dominant over the sickle allele (HbS). In heterozygotes (carriers with ), the single HbA allele produces enough normal to support typical oxygen transport and prevent red blood cell sickling under standard physiological conditions, resulting in an asymptomatic phenotype. This illustrates haplosufficiency in action, as the functional product from one allele suffices to override the defective polymerization-prone HbS protein.

Types of Dominance

Complete Dominance

Complete dominance, also known as full dominance, is a pattern of in which the phenotype expressed by a heterozygote is indistinguishable from that of the homozygous dominant individual, as the dominant allele completely masks the expression of the recessive allele. This results in discrete phenotypic categories rather than intermediate forms, aligning with the classical Mendelian model of where traits segregate in predictable ratios, such as 3:1 in the F2 generation of a . In standard genetic notation, alleles are denoted with letters where the dominant form uses an uppercase letter (e.g., R for round seeds) and the recessive form uses the corresponding lowercase letter (e.g., r for wrinkled seeds). Thus, the genotypes RR and Rr both produce the dominant , while only rr expresses the recessive . This notation facilitates the prediction of patterns using tools like Punnett squares. A seminal example of complete dominance comes from Gregor Mendel's experiments with pea plants (Pisum sativum), where seed shape exhibited this pattern: the for round seeds (R) is dominant over the for wrinkled seeds (r). When Mendel crossed true-breeding round-seeded plants (RR) with true-breeding wrinkled-seeded plants (rr), all F1 offspring (Rr) had round seeds, and the F2 generation showed approximately 75% round (RR or Rr) and 25% wrinkled (rr) seeds, confirming the 3:1 phenotypic ratio. In humans, complete dominance is observed in the with respect to the O . The A (I^A) and B (I^B) are each dominant over the O (i), such that genotypes I^A i and I^B i result in type A and type B , respectively, while only i i produces type O with no A or B on red cells. The I^A and I^B code for that add specific sugar to cell surfaces, whereas the i produces a nonfunctional , leading to no expression when homozygous. From an evolutionary perspective, complete dominance facilitates the preservation and spread of advantageous in populations, as heterozygotes express the beneficial dominant and are subject to positive selection, allowing the allele to increase in frequency more rapidly than if it were recessive. This dynamic helps maintain adaptive traits across generations, particularly when the dominant allele confers a survival or reproductive advantage in varying environments.

Incomplete Dominance

Incomplete dominance is a pattern of in which neither of a pair is fully dominant over the other, resulting in a heterozygous phenotype that represents an intermediate blend between the phenotypes of the two homozygotes. This occurs due to additive effects of the alleles, where the heterozygote expresses a that is quantitatively midway between the two parental forms, rather than one masking the other completely. In genetic crosses, this pattern deviates from classical Mendelian s, producing a 1:2:1 phenotypic in the F2 generation—1 part homozygous dominant, 2 parts heterozygous intermediate, and 1 part homozygous recessive—contrasting with the 3:1 observed in complete dominance. A classic example of incomplete dominance is observed in the flower color of snapdragons (), where homozygous red-flowered plants (RR) crossed with homozygous white-flowered plants (rr) produce all pink-flowered heterozygous offspring (Rr) in the F1 generation. Self-pollination of these pink heterozygotes then yields the 1:2:1 ratio in the F2: one-quarter red, one-half pink, and one-quarter white. Another well-documented case comes from the four o'clock plant (), first studied by in 1903, where crosses between red- and white-flowered individuals similarly result in pink heterozygotes, demonstrating the same intermediate blending and particulate inheritance in subsequent generations. At the biochemical level, incomplete dominance often arises from the partial activity of enzymes encoded by both alleles, leading to an intermediate level of or metabolic output rather than full suppression of one allele's effect; for instance, in flower pigmentation, the heterozygote may produce half the of the dominant homozygote due to contributions from both alleles (further molecular details are explored in sections on mechanisms). This contrasts with complete dominance, where the heterozygote matches that of the dominant homozygote due to full masking, and differs from codominance by producing a novel blended trait instead of simultaneous distinct expression of both alleles. Importantly, incomplete dominance maintains the particulate nature of Mendelian inheritance, distinguishing it from the outdated concept of blending inheritance, where parental traits would permanently mix without reappearance of original forms in offspring; here, the alleles remain discrete, allowing homozygous parental phenotypes to segregate and reemerge in the F2 generation.

Codominance

Codominance is a form of allelic interaction in which both alleles of a heterozygous genotype are fully and equally expressed in the phenotype, resulting in the simultaneous display of distinct traits from each allele without blending or masking. This contrasts with complete dominance, where one allele masks the other, and with incomplete dominance, where traits blend to form an intermediate phenotype. In codominance, the heterozygous phenotype is distinguishable from both homozygotes, often producing a 1:2:1 genotypic and phenotypic ratio in F2 progeny from a monohybrid cross. A classic example of codominance is the in humans, where the A and B (I^A and I^B) are codominant, while the O (i) is recessive. Individuals with I^A I^B exhibit AB, expressing both A and B antigens on the surface of red blood cells. This results in a where both antigens are detectable simultaneously, allowing for clear identification of the heterozygote. Another prominent example occurs in the coat color of Shorthorn cattle, where the alleles for red (R) and white (W) hair are codominant. Heterozygous (RW) individuals display a roan coat, characterized by an intermingling of red and white hairs, with each hair being fully pigmented in one color or the other rather than a uniform blend. This phenotype arises from the independent expression of both alleles in separate hair follicles, producing the distinctive speckled appearance. At the molecular level, codominance involves the production of distinct functional gene products from each , which contribute separately to the observable . In the ABO system, the I^A encodes an N-acetylgalactosaminyltransferase that adds to the , forming the A , while the I^B encodes a galactosyltransferase that adds to form the B ; in heterozygotes, both enzymes are active, resulting in cells bearing both antigens. For roan cattle, the molecular mechanism centers on alleles affecting pigment production in melanocytes, leading to separate populations of red- and white-pigmented hairs without interference between the products. Codominance in systems like ABO blood groups has significant applications in forensic genetics, particularly in paternity testing and individual identification. The distinct expression of A and B antigens allows exclusion of potential parents based on incompatible blood types—for instance, an AB individual cannot be the biological parent of a type O child, as they cannot transmit two recessive O alleles. Historically, ABO typing provided about 40% exclusion power when combined with other markers, serving as a foundational tool in forensics before the advent of .

Molecular and Biochemical Basis

Gene Expression Mechanisms

Dominance in arises in part from differential at the transcriptional level, where dominant often possess stronger enhancers or promoters that bind more effectively, leading to higher expression compared to recessive counterparts. For instance, in regulatory genetic interactions, a dominant at a cis-regulatory site can enhance binding to the promoter, outcompeting the recessive and driving sufficient for the wild-type . This mechanism is exemplified in models where heterozygous states result in asymmetric expression due to variant-specific enhancer strengths. At the protein level, dominance is frequently determined by the functional consequences of allelic variants during and protein activity. Gain-of-function , which confer novel or enhanced protein activity, typically exhibit dominant effects because the altered protein interferes with or overrides the normal allele's function, as seen in certain signaling pathways where hyperactive variants disrupt . In contrast, loss-of-function are often recessive, as the remaining wild-type produces enough functional protein to maintain normal activity; however, in cases of , a single wild-type copy is inadequate, resulting in dominant phenotypes. Classic examples of include in the gene causing autosomal dominant bone marrow failure syndromes, where reduced levels impair hematopoiesis, and deletions in the gene leading to supravalvular in , where insufficient production affects vascular development. Dosage compensation underlies many instances of dominance, particularly for , where the product of one wild-type suffices to achieve physiological requirements due to the kinetics of metabolic pathways. In enzyme-catalyzed reactions, the steady-state through a pathway is often insensitive to reductions in enzyme concentration down to 50% of normal levels, as the maximum (Vmax) is not rate-limiting under typical substrate conditions; thus, heterozygotes for loss-of-function alleles exhibit the wild-type . This principle, derived from flux control analysis, explains why most recessive mutations do not manifest in heterozygotes, as the control coefficient of individual enzymes is low in branched networks. Modern genome editing techniques, such as , have enabled direct interrogation of dominance by precisely altering alleles and observing phenotypic reversals. Post-2012 studies using have demonstrated dominance reversal by targeting dominant mutant alleles for disruption while sparing the wild-type copy, restoring normal function in heterozygous models. For example, allele-specific editing of the dominant KRT14 mutation in cells eliminated the pathogenic keratin aggregates, shifting the toward the recessive wild-type state and highlighting how editing can modulate dominance hierarchies. These approaches underscore the plasticity of dominance at the molecular level and inform therapeutic strategies for dominant disorders.

Regulatory Factors Influencing Dominance

Environmental factors can significantly modulate the expression of dominance by altering the stability or activity of products, thereby influencing phenotypic outcomes in heterozygotes beyond classical allelic interactions. serves as a prominent example, where function is sensitive to thermal conditions, leading to spatially variable dominance. In Siamese cats, the Himalayan allele (c^h) of the , resulting from a (p.Gly302Arg), produces a temperature-sensitive that is less active at warmer body temperatures (around 38.5°C) but functional at cooler peripheral temperatures (below 35°C). In homozygotes for this recessive , this results in darker pigmentation in cooler areas like the ears, paws, and tail, illustrating how environmental gradients can modulate the expression of a recessive trait. Dosage effects, often stemming from gene duplications or copy number variations, can buffer heterozygous phenotypes and shift dominance relationships by compensating for reduced expression from one . In cases like nucleolar dominance in allopolyploids, unequal rRNA dosages between parental genomes lead to the selective silencing of one set, mediated by dosage-dependent processes that establish a dominant ribosomal . modifiers, such as small RNAs and transcription factors, further enhance or suppress dominance by regulating allele-specific expression across distant genomic loci. For instance, in self-incompatibility systems, small RNAs produced by the dominant SCR repress the recessive counterpart, enforcing a clear that depends on the modifier's dosage and activity. These elements often operate within broader regulatory networks, where their concentration influences the competitive balance between alleles. Recent studies using single-cell sequencing (scRNA-seq) have illuminated the dynamic variability of dominance within gene regulatory networks, revealing how extrinsic regulatory factors contribute to cell-to-cell heterogeneity in allelic expression. In systems, scRNA-seq analyses show that allelic imbalances—indicative of dominance—fluctuate across cell states due to influences from regulators like transcription factors and epigenetic modifiers, challenging the view of dominance as a fixed trait. Tools such as ASPEN enable robust detection of these dynamics, demonstrating that dominance can switch between alleles in response to network perturbations, with implications for understanding regulatory plasticity in development and disease. For example, in mammalian tissues, eQTL hotspots enriched for dominance effects coordinate and expression biases across multiple , underscoring the role of network-level modifiers in variable dominance. Such regulatory influences extend to practical contexts like antibiotic resistance, where dominance of resistance alleles can conditionally shift based on drug concentration, affecting bacterial fitness and evolution. In exposed to beta-lactam antibiotics, a dominant resistance (e.g., conferring AmpR) masks fitness costs at high drug levels near the , establishing clear phenotypic dominance. However, at subinhibitory concentrations, negative interactions with compensatory mutations reduce the effective dominance of the resistance , promoting the co-selection of multi- combinations that restore fitness and alter the dominance landscape. This concentration-dependent modulation highlights how environmental drug levels act as extrinsic regulators, influencing the dominance of resistance phenotypes in populations.

Interactions with Broader Genetic Phenomena

Multiple Alleles and Dominance Hierarchies

In , multiple alleles refer to the presence of more than two alternative forms () of within at a single locus, extending beyond the simple biallelic systems described in classical . These alleles can interact through dominance hierarchies, where one allele masks the expression of others in a ranked order rather than a binary dominant-recessive relationship, leading to a series of distinct phenotypes in heterozygotes depending on the specific allelic combination. For instance, in the human ABO blood group system, the alleles IAI^A, IBI^B, and ii exhibit a hierarchy where IAI^A and IBI^B are codominant to each other—resulting in the AB phenotype when both are present—but both are dominant over the recessive ii allele, which produces no antigen and yields type O blood only in homozygotes. A classic example of a dominance hierarchy is observed in the coat color of rabbits, controlled by multiple s at the C locus. The full-color (C) is dominant over all others, producing or self-colored ; the chinchilla (cchc^{ch}) is recessive to C but dominant over lower alleles, yielding light gray with white undercolor; the Himalayan (chc^h) is recessive to both C and cchc^{ch} but dominant over the albino (c), resulting in white with dark extremities; and c is recessive to all, producing full . This series, first systematically described in early 20th-century breeding experiments, illustrates how allelic interactions can generate a graded phenotypic spectrum in heterozygotes, such as CchC c^h yielding full color despite the presence of the Himalayan . Dominance hierarchies are not always complete, and incomplete dominance can occur within the series, particularly among subordinate alleles. In , the white locus (ww) governing features multiple s forming an incomplete , where the wild-type (w+w^+) dominates all others; however, combinations among recessive alleles like (waw^a), (whnw^{hn}), and white (ww) produce intermediate shades—such as pale apricot in wawhnw^a w^{hn} heterozygotes—rather than full masking, reflecting partial expressivity and blending effects. In , the presence of multiple at a locus enhances heterozygote diversity by increasing the number of possible diploid genotypes, which expands the genotypic and phenotypic variation within a beyond what biallelic systems allow. This diversity is quantified by expected heterozygosity (He=1−∑pi2H_e = 1 - \sum p_i^2, where pip_i are allele frequencies), which rises with more alleles even at moderate frequencies, promoting evolutionary flexibility through greater adaptive potential and resistance to .

Epistasis and Dominance Modulation

refers to the interaction between at different loci where the phenotypic expression of one masks or modifies the effect of another, thereby altering the apparent dominance relationships observed at the affected locus. In this context, the epistatic influences the pathway or product necessary for the hypostatic 's expression, leading to a deviation from expected Mendelian ratios in dihybrid crosses. This masking can make a dominant appear recessive or nullify dominance hierarchies, highlighting how dominance is not solely an intrinsic property of alleles but can be modulated by intergenic interactions. Recessive epistasis occurs when a recessive at the epistatic locus masks the phenotypic expression of alleles at another locus, typically resulting in a modified 9:3:4 phenotypic in F2 progeny from a . A classic example is coat color in mice, involving the agouti (A/a) locus, which determines pigment pattern ( dominant to non-agouti), and the color (C/c) locus, which controls deposition. The homozygous recessive cc prevents melanin production, yielding albino mice regardless of the A locus , thus the cc alleles are epistatic and recessive to C. This interaction modifies the expected 9:3:3:1 to 9 : 3 non-agouti : 4 albino, demonstrating how the C locus masks dominance at the A locus. In contrast, dominant epistasis arises when a dominant at one locus suppresses the expression of alleles at a second locus, often producing a 12:3:1 . color in () illustrates this, with the W/w locus where W (dominant) inhibits color development, resulting in white fruit, and the Y/y locus where Y produces yellow and y green. The dominant W is epistatic, masking Y/y effects to yield white fruit in 12/16 , yellow in 3/16 (wwY_), and green in 1/16 (wwyy), thereby overriding dominance at the Y locus. The Bombay phenotype in humans provides another example of recessive epistasis affecting dominance in the . Individuals homozygous for the recessive h allele at the H locus (FUT1 gene) fail to produce the , a precursor required for A and B synthesis, resulting in type O-like red blood cells despite genotypes that would otherwise express A or B. Thus, the hh is epistatic to the ABO locus, nullifying ABO dominance and appearing phenotypically as O, which complicates blood typing and transfusion compatibility. This was first described in and exemplifies how can obscure allelic dominance across loci. In modern genetics, particularly post-2000 research, synthetic lethality represents an extreme form of epistasis relevant to dominance modulation in complex traits like cancer. Synthetic lethality occurs when mutations in two genes are individually viable but jointly lethal, often involving pathways where one mutation's dominance is contextually altered by the other. For instance, in BRCA1/2-mutant cancers, inhibition of PARP (via drugs like ) exploits synthetic lethality, as the dominant wild-type PARP compensates for BRCA loss, but its inhibition reveals the recessive-like lethality of BRCA defects. This approach has led to targeted therapies, with clinical trials showing response rates up to 50% in BRCA-mutant ovarian cancers, underscoring epistasis's role in modulating dominance for therapeutic gain.

Pleiotropy and Polygenic Traits

Pleiotropy refers to the phenomenon where a single influences multiple phenotypic traits, and the dominance relationship of its s can vary across those traits. In such cases, the dominant or recessive nature of an may manifest differently depending on the specific trait affected, complicating the prediction of inheritance patterns. A classic example is , caused by mutations in the FBN1 gene encoding fibrillin-1, which exhibits by affecting the skeletal, ocular, and cardiovascular systems. This disorder follows an autosomal dominant inheritance pattern, where a single mutated leads to abnormalities across multiple traits, such as tall stature, lens dislocation, and aortic dilation. While typically dominant, rare recessive forms have been reported, highlighting variability in dominance expression. In polygenic traits, controlled by multiple genes at different loci, dominance effects contribute additively to the overall rather than dominating in a simple Mendelian fashion. These traits, such as , result from the cumulative action of numerous genetic variants, where dominance variance (V_D) forms part of the total genetic variance (V_G) alongside additive (V_A) and interaction (V_I) components. (h²) is estimated as the ratio of genetic variance to total phenotypic variance (V_P), expressed as: h2=VGVPh^2 = \frac{V_G}{V_P} where V_G = V_A + V_D + V_I, allowing dominance to influence the proportion of trait variation attributable to . For , this model explains up to 80% , with dominance effects playing a minor but detectable role in quantitative genetic analyses. Genome-wide association studies (GWAS), prominent since the , have illuminated the role of dominance in complex diseases by identifying variants with non-additive effects across polygenic architectures. In traits like and , dominance deviations from additivity are often small but contribute to missing , with studies estimating that dominance variance accounts for less than 10% of genetic effects in most cases. This underscores the need for models incorporating dominance to refine risk prediction in polygenic disease contexts. Human skin color exemplifies a polygenic trait with partial dominance, influenced by at least 10-20 genes regulating production, where alleles exhibit incomplete or additive dominance leading to a continuous of pigmentation rather than discrete categories. Variants in genes like MC1R and SLC24A5 show partial dominance effects, contributing to intermediate tones in admixed populations.

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