Genetic inheritance explains how traits pass from parents to offspring. While some traits follow simple dominant or recessive patterns, others exhibit more complex forms of inheritance. Incomplete dominance is one such pattern, where the heterozygous individual displays a phenotype that is an intermediate blend of the two homozygous phenotypes. Neither allele fully masks the other.
Basic Principles of Genetic Inheritance
Understanding genetic inheritance requires fundamental terms. Genes are segments of DNA that provide instructions for specific traits, located at particular positions on chromosomes. Alleles are different versions of a gene, and an individual inherits two alleles for each gene, one from each parent. The combination of these alleles forms an organism’s genotype. The observable characteristics are known as the phenotype.
In complete dominance, one allele completely masks the effect of the recessive allele. For example, if a plant has one allele for red flowers and one for white, and red is completely dominant, the flowers will appear red. In contrast, incomplete dominance occurs when neither allele is entirely dominant over the other. This leads to a blending of the two traits, creating an intermediate phenotype. A classic illustration in plants is when red and white flowers produce pink offspring.
Familial Hypercholesterolemia: A Key Example
Familial Hypercholesterolemia (FH) provides a clear human example of incomplete dominance, affecting how the body processes cholesterol. This inherited disorder is characterized by elevated levels of low-density lipoprotein (LDL) cholesterol, which can lead to early cardiovascular disease. The gene involved in FH is the LDLR gene, which makes the LDL receptor protein. These receptors remove LDL cholesterol from the bloodstream.
Individuals with two functional copies of the LDLR gene (homozygous dominant) have normal LDL receptor activity and healthy cholesterol levels. They efficiently clear LDL cholesterol. Those who inherit one altered copy of the LDLR gene and one normal copy (heterozygous) exhibit an intermediate phenotype. Their LDL receptor activity is reduced, ranging from 2% to 25% of normal, leading to elevated cholesterol levels, between 350-550 mg/dL. Without treatment, heterozygous individuals may develop heart disease as early as 30 to 40 years old.
In the most severe cases, individuals inherit two non-functional copies of the LDLR gene (homozygous recessive). These individuals have extremely limited LDL receptor activity, less than 2%. This results in high cholesterol levels, exceeding 650-1000 mg/dL, and a severe, early onset of cardiovascular disease manifesting in childhood. The distinct progression of cholesterol levels and disease severity across these three genotypes—normal, elevated (intermediate), and extremely high (severe)—illustrates incomplete dominance, where the heterozygous state is a blend between the two homozygous conditions.
Identifying Incomplete Dominance
Observing the varying degrees of cholesterol elevation in Familial Hypercholesterolemia helps to identify incomplete dominance. Unlike complete dominance, where a single dominant allele results in a fully normal phenotype, heterozygotes for FH show a noticeable, yet less severe, form of the condition. Their cholesterol levels are higher than those with two normal genes but much lower than those with two non-functional genes. This intermediate expression is the hallmark of incomplete dominance.
The continuum of cholesterol levels in FH, from healthy to moderately high in heterozygotes and extremely high in homozygous affected individuals, reflects the partial activity of the LDL receptors. This pattern contrasts with complete dominance, where a heterozygote would appear identical to the homozygous dominant. Recognizing such inheritance patterns helps diagnose and manage genetic conditions, as it informs treatment strategies and provides a clearer understanding of disease progression.