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.

Verified step by step guidance

1. Step 1: Begin by identifying the key genetic terms in the problem. The problem describes individuals with different cholesterol levels based on their genotype: wild-type homozygous individuals have normal cholesterol levels, heterozygous individuals have intermediate levels (300–600 mg/100 mL), and homozygous mutant individuals have the highest levels (800–1000 mg/100 mL).
2. Step 2: Recognize that the heterozygous individuals exhibit a phenotype that is intermediate between the two homozygous phenotypes. This suggests that the inheritance pattern is not strictly dominant or recessive.
3. Step 3: Recall the genetic term 'incomplete dominance,' which describes a situation where the heterozygous phenotype is intermediate between the two homozygous phenotypes. This matches the pattern described in the problem.
4. Step 4: Justify the choice of incomplete dominance by explaining that in this case, the mutant allele does not completely dominate the wild-type allele, nor is it completely recessive. Instead, the heterozygous individuals show a blend of the effects of both alleles.
5. Step 5: Conclude that the inheritance of elevated cholesterol levels in this scenario is best described as incomplete dominance, based on the intermediate phenotype observed in heterozygous individuals.

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 elevated cholesterol levels varies significantly between heterozygous and homozygous individuals, highlighting the impact of genetic variations on health outcomes. This concept is essential for understanding how genetic mutations can lead to specific health conditions.