Explain why disease alleles for cystic fibrosis (CF) are recessive to the normal alleles (CF+), yet the disease alleles responsible for Huntington disease (HD) are dominant to the normal alleles (HD+).

Cystic fibrosis mutation is recessive to normal allele because only one functional or normal allele is enough to produce a functional protein. So, if mutation is present in one allele then also, a normal protein can be made from normal allele. The presence of normal protein prevents the expression of disease.

In addition, mutated allele only results in the loss of function of protein which can be compensated by the expression of normal allele. It does not add any toxic effect to the protein. Consequently, the disease is inherited in autosomal recessive fashion.

In contrast, Huntington mutation not only alters the structure of the functional protein but also adds toxicity to it. The altered protein is enable to interact with 100s of other proteins and inhibit or decrease their function. So, if only one allele is present then also, the mutated protein will be produced and it will result in the phenotype. Consequently, it is inherited as autosomal dominant fashion.