Incomplete Dominance Example In Humans

Incomplete Dominance: When Neither Allele Dominates in Humans

Incomplete dominance, also known as partial dominance, is a type of inheritance where neither allele is completely dominant over the other. This results in a heterozygous phenotype that is a blend or intermediate between the two homozygous phenotypes. Unlike complete dominance where one allele masks the other completely, incomplete dominance reveals a unique expression in the offspring. Understanding incomplete dominance helps us interpret a range of human traits, although it's less common than complete dominance. This article delves into the intricacies of incomplete dominance, using examples from human genetics to illustrate this important concept.

Understanding the Basics of Incomplete Dominance

In Mendelian genetics, we typically encounter complete dominance, where one allele (the dominant allele) completely masks the expression of the other allele (the recessive allele). For instance, if 'B' represents the allele for brown eyes and 'b' represents the allele for blue eyes, an individual with genotype 'Bb' will have brown eyes because 'B' is dominant over 'b'.

However, in incomplete dominance, the heterozygote ('Bb' in our example) displays a phenotype that is a mixture of the two homozygous phenotypes ('BB' for brown eyes and 'bb' for blue eyes). This means the heterozygote doesn't express either parental trait fully; instead, it shows a blend or intermediate expression. This is significantly different from codominance, where both alleles are fully expressed simultaneously.

Examples of Incomplete Dominance in Humans

While fewer human traits exhibit classic incomplete dominance compared to complete dominance or codominance, several examples illustrate this inheritance pattern. It’s important to note that the expression of these traits can be influenced by other genetic and environmental factors, leading to variability within populations.

1. Familial Hypercholesterolemia (FH)

FH is a genetic disorder affecting cholesterol metabolism. Individuals with FH have elevated levels of low-density lipoprotein (LDL) cholesterol, significantly increasing their risk of cardiovascular disease. The inheritance pattern of FH demonstrates incomplete dominance.

• Homozygous dominant (FF): Individuals with two copies of the dominant allele (FF) exhibit severe hypercholesterolemia, with extremely high LDL cholesterol levels. They often experience premature cardiovascular disease.

• Heterozygous (Ff): Individuals with one dominant and one recessive allele (Ff) have moderately elevated LDL cholesterol levels. They experience a less severe form of the disease compared to homozygous dominant individuals.

• Homozygous recessive (ff): Individuals with two copies of the recessive allele (ff) have normal LDL cholesterol levels.

The intermediate phenotype in heterozygotes reflects the incomplete dominance of the FH allele. The severity of the disease correlates directly with the number of dominant alleles present.

2. Tay-Sachs Disease

Tay-Sachs disease is a devastating neurodegenerative disorder caused by a deficiency in the enzyme hexosaminidase A. This deficiency leads to the accumulation of harmful substances in the brain and nervous system, resulting in progressive neurological deterioration.

While often described as recessive, Tay-Sachs exhibits aspects of incomplete dominance in its severity.

• Homozygous recessive (tt): Individuals with two copies of the recessive allele (tt) have a complete deficiency of hexosaminidase A and experience the full-blown, severe form of Tay-Sachs disease.

• Heterozygous (Tt): Carriers (Tt) have approximately 50% of the normal enzyme activity. They usually don't exhibit symptoms of the disease, but they have an increased likelihood of passing on the faulty allele to their offspring. The reduced enzyme activity compared to homozygous dominant individuals hints at incomplete dominance.

• Homozygous dominant (TT): Individuals with two normal alleles (TT) have normal enzyme activity and don't have the disease.

The incomplete dominance here lies in the enzyme activity level. Heterozygotes possess partial enzyme function, a phenotype intermediate between the severely affected homozygotes and unaffected homozygotes.

3. Sickle Cell Anemia

Sickle cell anemia is another example where the inheritance pattern demonstrates a complex interplay between incomplete dominance and codominance. It’s caused by a mutation in the gene that codes for hemoglobin, the protein responsible for carrying oxygen in red blood cells.

• Homozygous dominant (HbAHbA): Individuals with two normal hemoglobin alleles (HbAHbA) have normal red blood cells and don't have sickle cell anemia.

• Heterozygous (HbAHbS): Individuals with one normal hemoglobin allele (HbA) and one sickle cell allele (HbS) exhibit sickle cell trait. They usually don't show severe symptoms, but they may experience mild anemia or other complications under certain conditions (like low oxygen levels). This is an example of incomplete dominance – the trait is less severe than homozygous recessive condition, but not entirely normal. It's also codominance because both HbA and HbS are expressed.

• Homozygous recessive (HbSHbS): Individuals with two sickle cell alleles (HbSHbS) have sickle cell anemia, a severe condition causing chronic pain, organ damage, and reduced life expectancy.

In sickle cell anemia, the heterozygous state shows an intermediate phenotype, but the expression of both alleles (codominance) also plays a significant role. The incomplete dominance is evidenced by the less severe condition in heterozygotes compared to homozygotes for the sickle cell allele.

4. Hair Color and Skin Pigmentation

While complex polygenic inheritance significantly affects hair color and skin pigmentation, the concept of incomplete dominance can be observed in simplified models. Consider a simplified model with alleles for dark and light pigmentation.

• Homozygous dominant (DD): Might represent very dark hair or skin.

• Heterozygous (Dd): Would display an intermediate shade, possibly brown hair or medium skin tone, showcasing an incomplete expression of both alleles.

• Homozygous recessive (dd): Represents very light hair or skin.

The actual inheritance of these traits involves multiple genes and environmental factors, creating a wide spectrum of phenotypes. However, the principle of incomplete dominance provides a basic framework for understanding the blending of characteristics.

Distinguishing Incomplete Dominance from Other Inheritance Patterns

It's crucial to differentiate incomplete dominance from other inheritance patterns, especially complete dominance and codominance.

• Complete Dominance: One allele completely masks the expression of the other. The heterozygote displays the phenotype of the dominant allele.

• Incomplete Dominance: Neither allele is completely dominant. The heterozygote displays an intermediate phenotype, a blend of the two homozygous phenotypes.

• Codominance: Both alleles are fully expressed in the heterozygote. There is no blending; both traits are equally visible.

For example, in complete dominance of flower color, a red flower (RR) crossed with a white flower (rr) will produce all red flowers (Rr). In incomplete dominance, the offspring (Rr) would be pink. In codominance, the offspring (Rr) might have red and white patches.

The Scientific Basis of Incomplete Dominance

At a molecular level, incomplete dominance reflects the relationship between the genotype and the amount of gene product produced. In complete dominance, a single copy of the dominant allele is sufficient to produce enough gene product to fully express the dominant phenotype.

In incomplete dominance, however, the heterozygote produces less gene product than the homozygous dominant individual. This reduced gene product level results in an intermediate phenotype. This reduced expression can be due to several factors:

• Reduced enzyme activity: As seen in Tay-Sachs disease, a single copy of the functional allele may not produce enough enzyme to fully compensate for the deficiency.

• Reduced protein production: A single copy of a functional gene might not produce enough protein to fully express the dominant phenotype.

• Gene regulation: The regulation of gene expression might differ between homozygous and heterozygous states, leading to a reduced amount of gene product in the heterozygote.

Challenges in Studying Incomplete Dominance in Humans

Studying incomplete dominance in humans presents several challenges:

• Environmental influences: Many human traits are influenced by both genetics and the environment. This can make it difficult to isolate the effects of incomplete dominance.

• Complex interactions: Many human traits are polygenic, meaning they are controlled by multiple genes, making it harder to identify the role of incomplete dominance in individual genes.

• Ethical considerations: Studying human genetics requires careful consideration of ethical issues related to informed consent, privacy, and potential discrimination based on genetic information.

Frequently Asked Questions (FAQ)

Q: Are all human traits determined by incomplete dominance?

A: No, many human traits are determined by complete dominance or codominance, or are polygenic and influenced by environmental factors. Incomplete dominance is relatively less common.

Q: Can incomplete dominance be used to predict offspring phenotypes?

A: Yes, Punnett squares can be used to predict the probabilities of different genotypes and phenotypes in offspring when incomplete dominance is involved, although the calculations are identical to those used for complete dominance.

Q: Is incomplete dominance the same as blending inheritance?

A: While incomplete dominance may appear like blending inheritance, the key difference lies in the reappearance of parental phenotypes in subsequent generations. Unlike true blending inheritance, where the blended phenotype is permanent, the original phenotypes can reappear through specific crosses demonstrating the underlying discrete nature of alleles.

Conclusion

Incomplete dominance represents a significant departure from the classic Mendelian model of inheritance, highlighting the complexity and nuances of gene expression. While less frequent in humans than complete dominance, examples like familial hypercholesterolemia and Tay-Sachs disease illustrate its crucial role in determining the severity and expression of certain genetic disorders. Understanding incomplete dominance is fundamental to comprehending human genetic diversity and the relationship between genotype and phenotype. Continued research into human genetics will undoubtedly reveal further examples of this intriguing inheritance pattern and shed more light on its underlying molecular mechanisms. Further research will also help us better understand the interplay between incomplete dominance, environmental factors, and the phenotypic expression of various human traits.