Thinking back to the discussion of gain-of-function and loss-of-function mutations in Section 4.1, and putting those concepts together with the discussion of base substitution mutations in this chapter, explain why gain-of-function mutations are often dominant and why loss-of-function mutations are often recessive. Give an example of a type of gain-of-function mutation that is dominant and of a loss-of-function mutation that is recessive.

Key Concepts

Gain-of-Function Mutations

Gain-of-function mutations result in a gene product that has enhanced or new functions. These mutations often lead to a protein that is more active or has a novel activity compared to the wild-type protein. Because the altered protein can exert its effects even in the presence of a normal allele, gain-of-function mutations are typically dominant, meaning that only one copy of the mutated gene is sufficient to manifest the trait.

Loss-of-Function Mutations

Loss-of-function mutations lead to a gene product that is inactive or absent, resulting in a reduced or complete loss of function. These mutations often require both alleles to be mutated (homozygous) for the phenotype to be expressed, making them typically recessive. In heterozygous individuals, the presence of one normal allele can produce enough functional protein to maintain the wild-type phenotype.

Base Substitution Mutations

Base substitution mutations involve the replacement of one nucleotide with another in the DNA sequence. This can lead to changes in the amino acid sequence of a protein, potentially resulting in gain-of-function or loss-of-function effects. Understanding how these mutations affect protein function is crucial for explaining the dominance or recessiveness of the resulting phenotypes.