Which of these illustrates pleiotropy?

The correct option is: Human cystic fibrosis causes many symptoms from respiratory distress to digestive problems

What is Pleiotropy?
Pleiotropy occurs when a single gene influences multiple, seemingly unrelated phenotypic traits. Instead of a gene coding for just one specific characteristic (like eye color), a mutation in a pleiotropic gene will manifest as a variety of symptoms or traits affecting different organ systems or functions in the body.

Analyzing the Correct Option: Cystic Fibrosis
Cystic fibrosis is a classic example of pleiotropy in humans. It is caused by mutations in a single gene, the CFTR gene, which codes for a membrane protein that regulates the flow of chloride ions and water in and out of cells. Because this single protein is crucial for the normal functioning of mucus and sweat glands across various organs, its dysfunction leads to a wide array of diverse symptoms, including:
1. Thick mucus buildup in the lungs, causing respiratory distress and infections.
2. Blocked pancreatic ducts, preventing digestive enzymes from reaching the intestines and leading to severe digestive problems.
3. Excessively salty sweat, poor growth, and male infertility.
Since a mutation in this single gene affects multiple unrelated physiological systems (respiratory, digestive, reproductive), it perfectly illustrates pleiotropy.

Why the Other Options are Incorrect:
1. Matings between earless and long-eared sheep resulting in short-eared offspring: This describes incomplete dominance, where the heterozygous offspring display an intermediate phenotype between the two homozygous parents.
2. Wheat kernel color range governed by multiple genes: This describes polygenic inheritance, where multiple independent genes work together to determine a single physical trait (a continuous gradient of color).
3. Genes located on the same chromosome in fruit flies: This describes gene linkage, where genes physically located close to each other on the same chromosome tend to be inherited together, rather than segregating independently.