Question

in dogs and other animals, a transmembrane protein called mdr1 helps pump chemicals out of brain tissue and into the bloodstream. the mdr1 protein is encoded by the mdr1 gene.
a dog with a parasitic infection is often treated with the drug ivermectin. ivermectin kills parasites throughout the dog’s body, but it can also cause damage to the dog’s brain. normally, the brain is protected when the mdr1 protein transports ivermectin out of brain tissue. however, in some dogs, the drug accumulates in the brain and causes negative neurological effects. these dogs are said to have ivermectin sensitivity.
ivermectin sensitivity is associated with a four base - pair deletion in the mdr1 gene that introduces an early stop codon near the start of the gene’s sequence. dogs that are heterozygous for this mutation are not typically sensitive to ivermectin, while dogs that are homozygous typically are.
which of the following best explains why dogs that are homozygous for the mdr1 gene mutation are sensitive to ivermectin, while dogs that are heterozygous for the mutation are not?
choose 1 answer:
a homozygous dogs make only mdr1 proteins that cannot transport ivermectin due to an amino acid substitution, while heterozygous dogs make some wild type mdr1.
b homozygous dogs have more functional mdr1 transporters in their cells, so more ivermectin is pumped into the brain in homozygous dogs.
c homozygous dogs do not make functional mdr1 transporters, so more ivermectin stays in their brains.
d homozygous dogs make only mdr1 transporters that pump ivermectin out of the brain at a higher rate, while heterozygous dogs make some wild type mdr1.

Explanation:

1. Analyze Option A: The mutation is a base - pair deletion causing an early stop codon, not an amino acid substitution. So A is incorrect.
2. Analyze Option B: Homozygous dogs have a mutation that should lead to non - functional MDR1, not more functional transporters. And MDR1 pumps ivermectin out of the brain, not into. So B is incorrect.
3. Analyze Option C: Homozygous dogs have two copies of the mutated MDR1 gene. The mutation (four base - pair deletion with early stop codon) means they don't make functional MDR1 transporters. So ivermectin can't be pumped out of the brain and accumulates, causing sensitivity. Heterozygous dogs have one wild - type gene, so they make some functional MDR1 to pump out ivermectin, so they aren't sensitive. This matches the situation.
4. Analyze Option D: The mutated MDR1 in homozygous dogs is non - functional, so it can't pump ivermectin out at a higher rate. So D is incorrect.

Answer:

C. Homozygous dogs do not make functional MDR1 transporters, so more ivermectin stays in their brains.