Final answer:
The cystic fibrosis (CF) gene frequency in populations would be expected to increase if it conferred protection against diseases like cholera during the Middle Ages due to the survival advantage of the carriers.
Step-by-step explanation:
Cystic Fibrosis and Population Genetics
If the cystic fibrosis (CF) gene provided protection against cholera during the cholera epidemics of the Middle Ages, the gene frequency in those populations would be expected to increase. Carriers of the recessive CF gene, having just one copy, could have a survival advantage over those without the gene because carriers may have a greater resistance to certain diseases like cholera without suffering from the full effects of CF. This is known as heterozygote advantage.
Since carriers of the CF gene were more likely to survive and reproduce during cholera outbreaks, the allele frequency of the CF gene in the population would rise. This is consistent with natural selection, where alleles that confer a survival advantage increase in frequency within a population over time. This principle suggests that the frequency of the CF gene increased because carriers had an advantage during times when cholera was prevalent.