In cystic fibrosis, a single defective gene that produces faulty chloride transport channels leads to vitamin deficiency, respiratory infections, and excessively salty sweat. When a single defective gene is expressed in two or more very different phenotypic effects, the allele is
A dominant
B polygenic
C pleiotropic
D recessive
The correct answer and explanation is:
The correct answer is C. pleiotropic.
In genetics, pleiotropy refers to the phenomenon where a single gene influences multiple, seemingly unrelated phenotypic traits. This can happen because the gene product (often a protein) may have effects on various tissues, organs, or biological processes in the body. In the case of cystic fibrosis, a single defective gene leads to several different symptoms such as respiratory infections, vitamin deficiencies, and excessively salty sweat. These effects are all caused by the malfunctioning chloride transport channels, which are a result of the faulty gene.
The gene responsible for cystic fibrosis is the CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) gene. When this gene is defective, it impairs the movement of chloride ions across cell membranes, affecting various systems in the body, particularly the lungs, pancreas, and sweat glands. The variety of symptoms linked to cystic fibrosis, such as lung infections due to thick mucus, malabsorption of nutrients leading to vitamin deficiencies, and salty sweat, are all consequences of this single genetic mutation. This makes the gene pleiotropic because its effect spans multiple body systems and leads to multiple distinct phenotypic outcomes.
To clarify the other options:
- A. Dominant: A dominant allele only needs one copy to express its effect, but it does not necessarily affect multiple traits.
- B. Polygenic: A polygenic trait is one that is influenced by multiple genes, rather than a single gene causing multiple effects.
- D. Recessive: A recessive allele requires two copies (one from each parent) to express its trait but does not necessarily have pleiotropic effects.
Therefore, the correct answer is pleiotropic, as the CFTR gene mutation in cystic fibrosis causes a variety of different symptoms and affects multiple systems in the body.