Most children with cystic fibrosis have frequent lung infections and digestive difficulties. Some people have mild cases, with onset of minor respiratory problems in adulthood. Some men with cystic fibrosis only exhibit the symptom of infertility. Which exception to Mendelian laws of inheritance best explains these symptoms?1.) incomplete dominance2.) pleiotropy3.) epistasis4.) codominance5.) overdominance

The correct answer is 4: "The exception to Mendelian laws of inheritance that best explains the mentioned symptoms is codominance".

Explanation:

In codominance, both alleles can be expressed. In these cases, heterozygote individuals instead of showing an intermediate phenotype, express both of the alleles. Their phenotype is an additive expression of their parents' genes.

In cystic fibrosis, there is a gene responsible for coding for a protein named "cystic fibrosis transmembrane conductance regulator, CFTR".

-Most of the people have two copies of the normal allele and produce the functional CFTR protein form.

-Patients with cystic fibrosis have two copies of the mutated allele and so produce the mutated and dysfunctional form for this protein.

-Heterozygote people possess only one normal CFTR allele and a mutated form for the same allele and produce a normal protein and a mutated protein.

In the last case, both alleles are codominant and they express in heterozygote individuals. Given the fact that the normal allele produces enough functional CFTR protein, these individuals do not have any adverse effect and the mutated allele is recessive at a physiological level.