What is Incomplete Dominance?

• Incomplete dominance, a concept in genetics, represents a departure from the classic understanding of Mendelian genetics and was first introduced by the German botanist Carl Correns in the early 1900s. Prior to Correns’ work, the prevailing belief was rooted in Gregor Mendel’s principle of complete dominance, which asserted that the presence of a dominant allele would mask the effects of a recessive allele in a heterozygous individual.
• Mendel, an Austrian monk who laid the groundwork for genetic studies, conducted his groundbreaking experiments using pea plants, wherein he uncovered the phenomenon of complete dominance. In this scenario, when a heterozygous individual possessed both a dominant and a recessive allele for a specific gene, the dominant allele’s phenotype was the one expressed, effectively overshadowing the recessive allele’s phenotype. Mendel’s meticulous crosses between pea plant traits, such as round and wrinkled peas, produced progeny with a dominant round pea phenotype, while the underlying recessive wrinkled pea allele remained concealed. This led to a genotypic ratio of 1:2:1 and a phenotypic ratio of 3:1.
• However, Carl Correns’ work with “Mirabilis jalapa,” also known as the four o’clock flower, unveiled the concept of incomplete dominance. In his investigations, Correns made crosses between plants with red flowers and those with white flowers. Surprisingly, the offspring bore pink flowers, signifying an intermediate heterozygote. Unlike the clear-cut dominance observed in Mendel’s pea plants, the pink-flowered plants exhibited traits that were a blend of their parents’ homozygous alleles. Thus, Correns established the idea that some traits did not follow Mendel’s strict rules of dominance and recessiveness.
• Incomplete dominance, which is also referred to as partial dominance, semi-dominance, or intermediate inheritance, engenders a third phenotypic trait resulting from the mingling of parental alleles. In contrast to complete dominance, heterozygotes under incomplete dominance manifest traits that stand between the characteristics of the two homozygous traits. Neither allele dominates over the other, leading to a unique phenotype that is distinct from the parental phenotypes. This blending of the dominant allele’s effect with the recessive allele’s effect is sometimes described as a dilution of the dominant allele, as the dominance is not fully expressed in the heterozygote.
• The implications of incomplete dominance are profound. It underscores the notion that an organism’s traits can exhibit a range of variations due to the interplay between different alleles.
• Incomplete dominance broadens our understanding of genetic inheritance, challenging the previous notion that traits were merely a combination of parental traits. Instead, this phenomenon introduces the concept of alleles exerting an intermediate influence on phenotype.
• Through Carl Correns’ groundbreaking work, the world of genetics expanded to accommodate the nuanced intricacies of incomplete dominance, enriching our comprehension of hereditary patterns and phenotypic diversity.

Definition of Incomplete Dominance

Incomplete dominance refers to a genetic scenario where a heterozygous individual expresses a unique phenotype that is an intermediate blend of the two homozygous traits, neither allele completely dominating the other.

Incomplete dominance is a genetic phenomenon that is defined by several distinct characteristics:

1. Dilution of Dominant Allele: Incomplete dominance is often described as the dilution of the dominant allele’s effect by the recessive allele. This dilution results in the emergence of a novel heterozygous phenotype. For instance, this phenomenon is evident in the pink coloration of flowers like snapdragons or four o’clock flowers, as well as in traits such as hair shape, hand size, and voice pitch in humans.
2. Intermediate Trait Appearance: Incomplete dominance is characterized by the appearance of an intermediate trait that falls between the phenotypes exhibited by the two homozygous traits. This intermediate trait manifests in the heterozygous organism.
3. Formation of Third Phenotype: The term “incomplete dominance” also refers to the creation of a third phenotype that arises from the combination of alleles inherited from both parents. This phenotype represents a departure from the classic Mendelian view of dominance and recessiveness.
4. Trait Expression: In terms of trait expression, incomplete dominance can be thought of as a situation where neither of the alleles from a pair of alleles dominates the other for a specific trait. Instead, the resulting phenotype is an amalgamation of the effects of both alleles.
5. Assumptions: Different definitions of incomplete dominance exist. Some definitions focus on the blending of traits due to the combination of both dominant and recessive alleles. Others emphasize the concept that heterozygotes display traits that are intermediate between those of the two homozygous traits. Another aspect is that offspring under incomplete dominance might exhibit a specific trait with less intensity than the dominant trait among the paired alleles, resulting in a trait that is neither fully dominant nor fully recessive.
6. Cross of Homozygous Phenotypes: In cases of incomplete dominance, the heterozygous phenotype is notably manifested when a cross is made between two homozygous phenotypes. The initial cross of homozygous alleles in the F1 generation results in the creation of the intermediate trait in the heterozygote. This intermediate trait is then displayed in the F2 generation, with a phenotype ratio of 1:2:1, wherein two phenotypes are intermediate and the other two are either fully dominant or fully recessive traits.
7. Variation in Organism’s Features: Incomplete dominance plays a significant role in the variability of an organism’s characteristics. By contributing to the formation of unique intermediate phenotypes, incomplete dominance enhances the diversity of traits within populations.

In essence, incomplete dominance introduces a level of complexity beyond simple dominance and recessiveness, enriching our understanding of how alleles interact to shape the traits exhibited by organisms.

Mechanism of incomplete dominance

The mechanism behind incomplete dominance stems from the fact that neither of the two alleles involved is entirely dominant over the other, leading to a phenotype that represents a combination of both alleles’ effects.

Gregor Mendel, renowned for his experiments with pea plants, explored seven distinct traits displaying contrasting characteristics, all of which followed a consistent pattern of inheritance. Mendel’s findings culminated in the formulation of his fundamental laws of inheritance.

Subsequently, researchers extended Mendel’s work to other plant species, uncovering a surprising deviation from the anticipated pattern of inheritance in the F1 generation. While Mendel’s monohybrid crosses typically resulted in F1 progeny that resembled one of the parent traits, experiments on other plants demonstrated F1 offspring displaying an intermediate phenotype that differed from both parental traits.

The mechanism of incomplete dominance can be illustrated using the example of Snapdragon flowers (Antirrhinum sp). In this case, a monohybrid cross was conducted between red and white colored flowers of the Snapdragon plant. Assuming purebred red flowers have an RR allele pair, and white flowers have an rr allele pair:

1. True-breeding red (RR) and white (rr) Snapdragon flowers were crossed, yielding F1 offspring with pink-colored flowers, characterized by an Rr allele pair.
2. Subsequently, the F1 progeny was subjected to self-pollination, generating F2 offspring with red (RR), pink (Rr), and white (rr) flower phenotypes in a 1:2:1 ratio.

Interestingly, the genotype ratio of the F2 generation in this monohybrid cross, as postulated by Mendel, also resulted in a 1:2:1 ratio. However, there was a significant shift in the phenotype ratio from Mendel’s traditional 3:1 ratio to the observed 1:2:1 ratio. This change in the phenotype ratio is attributed to the incomplete dominance of the allele R over the allele r. This incomplete dominance leads to a blending of colors in the flowers, resulting in the intermediate pink phenotype. This phenomenon underscores how incomplete dominance deviates from the strict dominance-recessive relationships described by Mendel, introducing a more intricate understanding of genetic inheritance patterns.

Concept of Dominance

In genetics, the concept of dominance revolves around the relationship between alleles of a single gene. To truly grasp this concept, it’s essential to delve into the nature of genes themselves.

Genes, as we understand, are fundamental units of heredity found in organisms. They exist in pairs of alleles within diploid organisms, where these pairs can either be similar or dissimilar. In other words, heterozygous genes comprise two different alleles, while homozygous genes possess two identical alleles.

The presence of dissimilar alleles in heterozygous individuals introduces the concept of dominance. When we assert that one trait is dominant over another, there are typically two reasons for this:

1. Non-Functionality: In some cases, the recessive allele might be functionally impaired, unable to carry out its intended role. As a result, the dominant allele effectively masks the recessive allele’s effects, leading to the expression of only the dominant trait.
2. Reduced Activity: Alternatively, the recessive allele might be less active or efficient compared to the dominant allele. This diminished activity can result in a weaker expression of the trait associated with the recessive allele, making the dominant allele’s trait more prominent.

The concept of dominance provides a framework for understanding how certain alleles exert more influence over the phenotype in heterozygous individuals, shaping the observable characteristics of organisms. Through the interplay of dominant and recessive alleles, the genetic landscape gives rise to the diverse array of traits observed in various species.

Incomplete Dominance and Codominance

The foundational principles of inheritance laid out by Mendel, known as Mendelian genetics, introduced the concept of dominance and its effects on allele interactions in diploid organisms. Within the realm of genetics, two significant types of dominance emerged: incomplete dominance and codominance. While Mendel himself didn’t explicitly identify these types, his work indirectly paved the way for their discovery. It’s important to distinguish between these two concepts to understand their distinct characteristics.

1. Incomplete Dominance

• Incomplete dominance, often termed partial dominance, is characterized by an intermediate phenotype that falls between the phenotypic expressions of the homozygous dominant and homozygous recessive alleles. In this scenario, neither allele completely dominates over the other, leading to the emergence of a new phenotype.
• For instance, when red and white flowers are crossed, the resulting offspring might exhibit pink flowers as a manifestation of incomplete dominance. This phenomenon highlights the genetic importance of an intermediate phenotype that arises from the interaction of two distinct alleles.
• Mendel’s Law of Dominance, which postulates that only one allele is dominant and reduces the effect of the recessive allele, aligns with incomplete dominance. Although Mendel’s pea plants did not display incomplete dominance, his proposed 1:2:1 genotypic ratio remains applicable to certain cases, such as the example of four o’clock flowers.

2. Codominance

• In codominance, both alleles of a gene, typically from homozygous parents, are simultaneously expressed in the offspring’s phenotype.
• This dominance pattern doesn’t entail a single allele suppressing the other; rather, both alleles coexist, resulting in a mixture of their phenotypic effects. Codominance can sometimes be referred to as “no dominance” due to the equal expression of both alleles.
• For example, in certain plants, white color (recessive allele) and red color (dominant allele) result in pink and white-spotted flowers when crossed. Similarly, humans display codominance in cases like blood types.
• Codominance is distinguishable by using uppercase letters with superscripts to denote the alleles’ equal expressions in writing. In the pea plant context, Mendel’s focus was limited to a few traits, which didn’t encompass codominance. However, subsequent research expanded our understanding of this type of dominance.

In summary, both incomplete dominance and codominance reflect deviations from the classic Mendelian dominance pattern. In incomplete dominance, the heterozygous phenotype is an intermediate blend, while in codominance, both alleles’ expressions are distinct but simultaneously present in the phenotype. Despite Mendel’s work not directly exploring these dominances, his foundational genetic laws paved the way for their recognition, allowing scientists to appreciate the diverse ways alleles interact to shape the phenotypic diversity observed in various organisms.

How does incomplete dominance work?

The mechanism of incomplete dominance is best understood through the utilization of a Punnett square, a tool that assists in predicting the outcomes of breeding experiments. By considering a cross between plants producing red and white flowers, the nature of incomplete dominance becomes evident.

1. Punnett Square Application: Botanists employ Punnett squares to anticipate the results of genetic crosses. In this scenario, one parent plant with red flowers (RR) and another with white flowers (rr) are mated.
2. Heterozygous Offspring with Intermediate Trait: The Punnett square analysis reveals that the F1 generation comprises heterozygous offspring with an intermediate trait—pink color. This outcome signifies that neither allele dominates over the other. Unlike complete dominance where one allele masks the other’s effect, here, both alleles contribute to the phenotype, resulting in a blending of the two traits.
3. Blend of Parental Traits: In the case of incomplete dominance, the phenotype of the heterozygote is a combination of both parental traits rather than an absolute blend. The heterozygote possesses both the red and white color traits, which are not fully obscured or blended together.
4. F2 Generation Phenotype Ratio: Following Mendel’s principles, the F2 generation’s phenotype ratio remains 1:2:1. The phenotypes observed are 25% red flowers, 25% white flowers, and 50% pink flowers. This underscores the idea that incomplete dominance does not entail complete blending, as the red and white traits remain distinct and are still expressed in the F2 generation.
5. Blended Phenotype: In incomplete dominance, the heterozygous genotype (Rr) does not produce enough pigment for fully red flowers. Despite carrying the dominant allele for red pigment, the quantity of pigment produced under the influence of a single R allele is inadequate for full coloration. As a result, the flowers appear pink—a blended phenotype that falls between the fully red and fully white extremes.
6. Illustrative Example: Consider a flower color scenario, where R symbolizes the dominant allele for red pigment and r represents the recessive allele for no pigment. In cases of incomplete dominance, a plant with a heterozygous genotype (Rr) will display an intermediate pink color. This is due to the limited amount of pigment produced by the single R allele, insufficient for full color saturation.

In conclusion, incomplete dominance operates by creating an intermediate phenotype in the heterozygous state, where both alleles contribute to the trait, but neither dominates the other completely. The Punnett square elucidates how this phenomenon results in a range of phenotypic outcomes that deviate from the strict dominance-recessive relationships proposed by Mendel, introducing a more complex understanding of allele interactions and trait expression.

Examples of Incomplete Dominance

Examples of incomplete dominance can be found across various organisms, including plants, animals, and even humans, illustrating the nuanced interplay of alleles in genetics.

Incomplete Dominance in Plants:

1. Carnation Plants: German scientist Kolreuter’s work with carnation plants led to the discovery of incomplete dominance. Crossing true-breeding red and white carnations resulted in offspring with pink flowers, a blending of the parental red and white traits.
2. Four O’Clock Plants: Cross-breeding red and white four o’clock plants produces pink-flowered progeny, showcasing an intermediate phenotype where neither allele is fully dominant.
3. Snapdragon Flowers: When red and white snapdragons are cross-pollinated, the resulting pink-colored snapdragons exemplify incomplete dominance.
4. Eggplants: The light violet color of some eggplants is a result of incomplete dominance, occurring when deep purple eggplants are bred with white eggplants.

Incomplete Dominance in Animals:

1. Andalusian Chickens: Breeding a white-feathered male chicken with a black-feathered female chicken results in blue-tinged feathered offspring. This outcome is due to the dilution of genes that reduce the intensity of melanin’s effect on feather color.
2. Rabbits: Crossing long-furred and short-furred rabbits produces medium-length fur in the offspring, demonstrating incomplete dominance.
3. Dog Tail Length: Breeding a long-tailed dog with a short-tailed dog yields offspring with medium-length tails, showcasing incomplete dominance.
4. Spots in Animals: Breeding a spotted animal with a non-spotted animal leads to offspring with few spots. This phenomenon is visible in animals like dogs, cats, and horses.

Incomplete Dominance in Humans:

1. Hair Texture: Crossing parents with straight and curly hair can result in children with semi-curly or wavy hair, displaying incomplete dominance.
2. Human Height: In families with varied parental heights, children often fall between their parents’ height ranges due to incomplete dominance in height patterns.
3. Tay Sachs Disease: Tay Sachs disease, an autosomal recessive neurological condition, is an example of partial dominance in humans. Carriers of Tay Sachs have incompletely dominant genes causing an enzymatic imbalance, producing only half of the required enzyme for a normal life.
4. Familial Hypercholesterolemia (FH): FH is a condition illustrating incomplete dominance. One allele leads to liver cells lacking cholesterol receptors, while another allele produces cells with normal receptors. The incomplete dominance results in cells lacking sufficient receptors to effectively remove harmful cholesterol from the bloodstream.

These examples emphasize how incomplete dominance generates unique phenotypes that blend parental traits, contributing to the diversity of traits observed in different organisms.

Incomplete dominance vs. Codominance

Incomplete dominance and codominance are two distinct genetic phenomena that illustrate different ways alleles interact in heterozygous organisms. Here’s a comparative overview of these two concepts:

Incomplete Dominance:

1. Definition: In incomplete dominance, the dominant allele does not completely mask the recessive allele’s effect. Instead, an intermediate phenotype arises in the heterozygote.
2. Phenotype: The offspring’s phenotype is a blend or mix of the parents’ homozygous traits. This leads to an intermediate appearance that is different from either parental trait.
3. Expression of Alleles: The expression of alleles in incomplete dominance is evident, with neither allele dominating over the other. Both alleles contribute to the phenotype.
4. Parental Phenotype: The offspring do not resemble either parental phenotype directly.
5. Dominance Relationship: Incomplete dominance does not exhibit a typical dominant-recessive relationship; the dominant allele is not entirely dominant over the recessive allele.
6. Examples: Pink flowers of four o’clock flowers (Mirabilis jalapa) and human physical traits like hair color, hand size, and height.

Codominance:

1. Definition: In codominance, neither allele is dominant, and both alleles from homozygous parents are expressed simultaneously in the phenotype of heterozygotes.
2. Phenotype: The phenotypic expression of both homozygous alleles is independent, resulting in the simultaneous appearance of both traits in the offspring.
3. Expression of Alleles: Codominance features uniformly conspicuous allele expression, where both alleles have an equal chance of expressing their effects.
4. Parental Phenotype: The offspring show both parental phenotypes distinctly.
5. Dominance Relationship: Codominance lacks the traditional dominant-recessive relationship entirely.
6. Examples: Human blood types (A, B, AB, O) and certain animal traits like spots on feathers or hairs, such as those found in livestock.

Key Takeaways:

• In incomplete dominance, an intermediate phenotype emerges due to a blending of allele effects, while in codominance, both alleles express themselves simultaneously without one suppressing the other.
• Incomplete dominance does not exhibit a clear dominant-recessive relationship, while codominance lacks any dominance relationship altogether.
• Incomplete dominance leads to an intermediate phenotype that does not resemble either parent, while codominance displays both parental phenotypes distinctly.
• Examples of incomplete dominance include pink flowers and various human physical traits, while codominance is observed in human blood types and certain animal traits.

Understanding these distinctions enhances our grasp of how alleles interact and contribute to the vast diversity of traits in living organisms.

Incomplete dominance vs. Codominance Chart

Aspect	Incomplete Dominance	Codominance
Definition	Dominant allele incompletely dominates recessive	Both alleles from homozygous parents are equally expressed
Phenotype	Intermediate blend of parental traits	Both parental traits expressed independently
Expression of Alleles	Both alleles contribute to phenotype	Both alleles have equal chance of expression
Parental Phenotype	Offspring phenotype distinct from parents	Offspring phenotype displays both parents’ traits
Dominance Relationship	No clear dominant-recessive relationship	No dominance relationship
Examples	Pink flowers (four o’clock), human traits (hair color, hand size, height)	Human blood types (A, B, AB, O), animal traits (spotted feathers, hairs)

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References

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