First, consider a population where some individuals have cystic fibrosis and others are phenotypically 'normal'. How would natural selection affect the frequency of the cystic fibrosis allele? Address the conditions of natural selection. Second, why hasn't natural selection completely removed the cystic fibrosis allele from the population? Furthermore, explain why the term 'carrier' isn't appropriate for traits that show incomplete dominance or co-dominance. Lastly, why are lethal disorders much more likely to be caused by recessive alleles than dominant alleles?