In humans, free blood cholesterol levels are influenced in part by an individual's cholesterol receptors, located in liver cells, among other tissues.
One gene determines the expression of a cholesterol receptor (you can represent it with the letter C). When a person has these receptors on the surface of cells, cholesterol in the bloodstream binds to the receptors. The receptors bring the cholesterol into the cell, where it is typically broken down in the smooth endoplasmic reticulum.
When a person is homozygous recessive for the allele that encodes a cholesterol receptor, the cell surface has a normal number of cholesterol receptors on the surface. This person has a cholesterol level in the range of 7.5 to 17.5 mg/ml.
When a person is heterozygous for the allele that encodes the receptor, the person has one recessive allele, which allows for the expression of some cholesterol receptors. The person also has one dominant mutation, which prevents the expression of cholesterol on the surface of liver cells. As a result, these individuals have fewer cholesterol receptors on their cell surfaces. The typical blood cholesterol level for a person who is heterozygous for the receptor is 20 to 25 mg/ml.
In a homozygous dominant individual, no cholesterol receptors are expressed on the surface of cells. The typical blood cholesterol level for such an individual is over 45 mg/ml.
a. Use the letter C to represent the gene that encodes the cholesterol receptor.
b. Using the process you learned earlier this semester, please draw out a Punnett square between two parents, both heterozygous for the C gene.
c. What are the genotypic and phenotypic ratios in F1?
d. What is the name of this phenomenon?
