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Definition of Incomplete Dominance

Incomplete dominance is a fascinating genetic phenomenon that plays a crucial role in shaping the diversity of life forms on our planet. This article explores the definition of incomplete dominance, its mechanisms, and provides various examples of its occurrence in different species.

What is Incomplete Dominance?

Incomplete dominance, also known as partial dominance or intermediate inheritance, is a genetic phenomenon that occurs when two true-breeding parents, each with a specific trait, produce an offspring that displays an intermediate or blended phenotype. Unlike the traditional dominant and recessive alleles, in incomplete dominance, neither allele exerts complete dominance over the other. Instead, the dominant allele is expressed in a reduced ratio, resulting in a unique phenotype.

To better understand incomplete dominance, it's essential to clarify some key terms:

• Allele: Alleles are different forms or versions of a gene.
• Organism: An organism inherits two alleles for each gene, one from each parent.
• Dominant Allele: The dominant allele masks or suppresses the effects of other alleles and is prominently expressed in the offspring.
• Recessive Allele: The recessive allele's effects are suppressed by the dominant allele and do not appear in the offspring.
• Homozygous: An organism with two identical alleles for a specific gene, either both dominant or both recessive.
• Heterozygous: An organism with two different alleles for a specific gene.
• Genotype: The set of genes inherited by an organism, determining its observable physical features.
• Phenotype: The organism's appearance, characteristics, behavior, and physically observable features.
• Genotypic Ratio: The ratio of trait appearances in offspring after gene or allele crossing, often understood through a Punnett square.

It's important to note that while we commonly refer to alleles as dominant or recessive, dominance is a property of the phenotype, not the allele itself. Incomplete dominance is one of several forms of dominance, which also include complete dominance and codominance.

Defining Incomplete Dominance

Incomplete dominance can be defined in various ways:

• Dilution of Dominant Allele: In incomplete dominance, the dominant allele is not fully expressed in the heterozygous genotype, resulting in a phenotype that appears diluted compared to the homozygous dominant phenotype. For example, the pink color of snapdragon flowers is a result of incomplete dominance.
• Intermediate Trait Appearance: Incomplete dominance leads to an intermediate phenotype in heterozygous individuals, which falls between the phenotypes of the two homozygous genotypes. This intermediate appearance is a hallmark of incomplete dominance.
• Combination of Parent Alleles: It can also be described as a phenomenon where the offspring's phenotype is a result of the combination of alleles from both parents. Neither the dominant nor the recessive allele completely masks the other's effects.
• Neither Dominant nor Recessive: In incomplete dominance, the heterozygous genotype does not exhibit the dominant trait as seen in complete dominance, nor does it display the recessive trait. Instead, it showcases an entirely distinct phenotype.
• Role in Phenotype Variation: Incomplete dominance plays a crucial role in generating diversity in an organism's traits or characteristics. It introduces an additional layer of complexity in the expression of genetic traits.

In practical terms, when two homozygous individuals with different traits are crossed, the resulting heterozygous offspring displays an intermediate phenotype. In the F2 generation, this intermediate phenotype appears alongside the dominant and recessive phenotypes in a ratio of 1:2:1, respectively. This ratio exemplifies the blending or intermediate nature of incomplete dominance.

Mechanisms of Incomplete Dominance

Incomplete dominance operates through the interaction of alleles, and Punnett squares are used to predict the outcomes of breeding experiments involving these alleles.
Here's a breakdown of how incomplete dominance works:

• Breeding Experiment: Incomplete dominance is demonstrated through a breeding experiment where two individuals with different homozygous traits are crossed. For example, one parent has a homozygous dominant trait (red flowers), and the other has a homozygous recessive trait (white flowers).
• Punnett Square: A Punnett square is a tool used to visualize the possible genotypes and phenotypes of the offspring resulting from this cross. The alleles from each parent are represented on the top and left sides of the square.
• Genotype Prediction: The Punnett square shows the combinations of alleles that can result from the cross. In this case, the alleles for red (R) and white (W) flower colors are crossed. The square reveals that the offspring will inherit one R allele from the red parent and one W allele from the white parent, resulting in a heterozygous genotype (RW).
• Phenotype Observation: The key observation is the phenotype that corresponds to the genotype RW. In the case of incomplete dominance, neither allele is dominant over the other. Instead, the expression of both alleles occurs simultaneously, leading to a blended or intermediate phenotype—in this example, pink-colored flowers.
• Intermediate Trait: The intermediate phenotype of pink demonstrates that incomplete dominance doesn't involve one allele dominating over the other. Instead, both alleles contribute to the phenotype, creating a trait that falls between the two homozygous parental traits.
• F2 Generation: When these heterozygous offspring (RW) are crossed in the F2 generation, the phenotypic ratio resembles Mendel's classic 1:2:1 ratio, with 25% displaying the dominant trait (red), 25% displaying the recessive trait (white), and 50% showing the intermediate phenotype (pink).

Codominance and Incomplete Dominance

• Mendelian genetics, proposed by Gregor Mendel, laid the foundation for understanding dominance factors and allele effects on phenotypes in diploid organisms. However, two other types of inheritance, known as incomplete dominance and codominance, were not directly identified by Mendel. Instead, these concepts were later discovered by botanists building upon Mendel's work. While these two types of dominance are often confused, it is important to understand their distinguishing factors.
• Incomplete dominance refers to partial dominance, where the phenotype lies between the dominant and recessive alleles. For example, in a cross between red and white flowers, the resulting offspring may have a pink color trait due to incomplete dominance. Unlike in complete dominance, the dominant allele does not completely mask the recessive allele, resulting in a unique phenotype. Mendel did not study incomplete dominance as it was not observed in pea plants. However, the genotypic ratio of 1:2:1 proposed by Mendel for complete dominance is often applicable to incomplete dominance, as seen in the example of four o'clock flowers.
• Codominance, on the other hand, refers to a type of dominance where both alleles are expressed together in the offspring's phenotype. Unlike in complete dominance, there is no dominant or recessive allele in codominance. Instead, both alleles are present and contribute to the phenotype, resulting in a mixture of the two traits. Codominance can be easily observed in plants and animals, such as in flower colors or blood types in humans. Mendel did not consider codominance in his studies due to the limited traits of pea plants.
• In summary, incomplete dominance involves an intermediate phenotype, while codominance involves the expression of both alleles in the offspring's phenotype. In incomplete dominance, the parent alleles remain in the heterozygote, whereas in codominance, no allele dominates over the other. The offspring's phenotype in incomplete dominance is a combination of the parents' traits, while in codominance, it shows both parental phenotypes. 

Incomplete dominance vs. Codominance

Examples of Incomplete Dominance

Incomplete dominance is indeed a fascinating genetic phenomenon that leads to the expression of intermediate traits when two different alleles for a particular gene are present. Your examples illustrate how incomplete dominance occurs in plants, humans, and other animals:

• In Plants:
  • Carnation Flowers: Incomplete dominance is observed in carnation plants, where a cross between true-breeding white and true-breeding red-flowering plants results in offspring with pink flowers.
  • Four O'Clock Flowers: Red and white flowering four o'clock plants breed to produce offspring with pink flowers, showcasing incomplete dominance.
  • Snapdragons: Cross-pollination between red and white snapdragons leads to pink flowers due to incomplete dominance.
  • Corn Crops: Incomplete dominance is utilized in corn crops to create high-yielding and healthier varieties.
• In Humans:
  • Hair Color: Some individuals exhibit semi-curly or wavy hair due to incomplete dominance when parents with straight and curly hair alleles have offspring.
  • Eye Color: Human eye color is a classic example of incomplete dominance. Brown and blue eye alleles can produce intermediate eye colors, such as green or hazel.
  • Height: Offspring of parents with different heights tend to have a height that falls between their parents' heights rather than resembling one parent more closely.
  • Skin Color: Incomplete dominance is also seen in human skin color. Genes responsible for melanin production do not exhibit dominance over each other, resulting in offspring with intermediate skin tones.
  • Sound Pitch: High-pitched sounds in males and low-pitched sounds in some homozygotes result in intermediate voice pitches in heterozygous individuals.
  • Hand Size: Similar to other traits, hand sizes can show incomplete dominance, leading to offspring with medium-sized hands when parents have different hand sizes.
  • Genetic Diseases: Genetic diseases like Tay-Sachs disease and familial hypercholesterolemia (FH) can exhibit incomplete dominance in their inheritance patterns.
• In Other Animals:
  • Andalusian Chickens: These chickens exhibit incomplete dominance in feather color. The offspring of a white-feathered male and a black-feathered female have blue and tinged feathers.
  • Rabbits: Breeding long-furred and short-furred rabbits can produce offspring with medium-length fur due to incomplete dominance.
  • Dog Tails: The length of a dog's tail can show incomplete dominance when a long-tailed dog is bred with a short-tailed dog, resulting in offspring with medium-sized tails.
  • Labradoodles: Labradoodles have wavy hair due to incomplete dominance resulting from breeding straight and curly-haired parent dogs.
  • Spots in Animals: Spots on the bodies of animals like cats, dogs, and horses can vary when more spotted and less spotted animals are bred together, leading to offspring with varying degrees of spotting.

Summary

Incomplete dominance is a captivating genetic phenomenon that enhances the diversity of life forms through the blending of dominant and recessive alleles. It leads to the creation of intermediate phenotypes, allowing for a wide range of trait variations. Understanding incomplete dominance can shed light on the complexity of genetics and the formation of unique characteristics in different species.