Explain why disease alleles for cystic fibrosis (C F) are...

Explain why disease alleles for cystic fibrosis $(C F)$ are recessive to the normal alleles $\left(C F^{+}\right),$ yet the disease alleles responsible for Huntington disease $(H D)$ are dominant to the normal alleles $\left(H D^{+}\right)$.

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Key Concepts

Loss-of-Function Mutations

This concept describes mutations that result in a reduced or abolished function of the gene product. In diseases like cystic fibrosis, the mutation typically leads to the loss of function of a crucial protein. However, because one normal allele produces enough functional protein (a phenomenon known as haplosufficiency), the disease manifests only when both alleles are mutated.

Gain-of-Function Mutations

This concept refers to mutations that result in a new or enhanced function of the gene product, often producing a toxic product. In the case of Huntington disease, the mutated allele leads to the production of an abnormal protein that interferes with normal cellular functions, and its dominant nature means that the presence of one mutated allele is sufficient to cause disease.

Allelic Dominance in Genetic Traits

This concept involves the relationship between alleles, where one allele can mask or override the effect of another. For cystic fibrosis, the normal allele is dominant because it compensates for the defective one, while for Huntington disease, the mutated allele is dominant as its deleterious effects overshadow the presence of the normal allele.

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