Final answer:
Carriers of cystic fibrosis have some protection against intestinal infections such as cholera, but people with two copies of the cystic fibrosis allele develop a life-threatening disease. The mutant allele is common in people of northern European descent due to a heterozygote advantage.
Step-by-step explanation:
Carriers of cystic fibrosis have some protection against intestinal infections such as cholera, but people with two copies of the cystic fibrosis allele develop a life-threatening disease. The mutant allele is common in people of northern European descent due to a heterozygote advantage. It has been hypothesized that carriers of the cystic fibrosis mutation may have greater-than-normal resistance to certain infectious diseases.