Total cholesterol in blood is reported as the number of milligrams (mg) of cholesterol per 100 milliliters (mL) of blood. The normal range is 180–220 mg/100 mL. A gene mutation altering the function of cell-surface cholesterol receptors restricts the ability of cells to collect cholesterol from blood and draw it into cells. This defect results in elevated blood cholesterol levels. Individuals who are heterozygous for a mutant allele and a wild-type allele have levels of 300–600 mg/100 mL, and those who are homozygous for the mutation have levels of 800–1000 mg/100 mL. Identify the genetic term that best describes the inheritance of this form of elevated cholesterol level, and justify your choice.

Key Concepts

Alleles and Genotypes

Alleles are different versions of a gene that can exist at a specific locus on a chromosome. An individual inherits two alleles for each gene, one from each parent, resulting in a genotype that can be homozygous (two identical alleles) or heterozygous (two different alleles). In the context of cholesterol levels, the presence of a mutant allele can significantly affect the phenotype, or observable traits, such as elevated cholesterol levels.

Dominance and Inheritance Patterns

Dominance refers to the relationship between alleles where one allele can mask the expression of another. In this case, the mutant allele affecting cholesterol receptors is likely dominant, as heterozygous individuals exhibit elevated cholesterol levels. Understanding dominance is crucial for predicting how traits are inherited and expressed in offspring, particularly in conditions like familial hypercholesterolemia.

Phenotypic Expression

Phenotypic expression is the observable physical or biochemical characteristics of an organism, determined by both genetic makeup and environmental influences. In this scenario, the phenotypic expression of cholesterol levels is directly influenced by the genotype, with heterozygous individuals showing significantly higher cholesterol levels than those with the wild-type allele. This concept is essential for understanding how genetic variations can lead to health conditions.