There are a number of ways of classifying the different types of genetic mutations, such as the effect on fitness, and the effect on the DNA sequence - i.e. a point mutation is a single base substitution.
The classification of mutations based on their behaviour was created by Hermann J Muller (1890-1967). His classification scheme for genetic mutations consists of the following terms; amorph, antimorph, hypermorph, hypomorph, and neomorph.
This is the most common type of mutation and it causes a loss of function of the protein product. It is also known as a ’null mutation.’ Genetic mutations in an amorph allele (an alternative form of a gene at a specific locus) can cause any of the following;
- No gene - it is not present in the chromosome
- No transcription
- No translation
- Non-functional protein product
These are rare types of genetic mutations;
Changes to the DNA base pair sequence of an antimorphic allele does result in a gene being present, but its protein product works against the normal gene’s protein product. Antimorph mutations are generally dominant, and they are also known as ‘dominant-negative’ mutations.
Hypomorph is a common type of genetic mutation and results in the loss of function of a gene product function. It is also sometimes referred to as a ’leaky’ mutation.
The DNA sequence changes in a hypomorph allele can cause;
- Reduced translation
- Reduced transcription
- Reduced function of the protein product
This is a rare type of genetic mutation that results in a gain of function by increasing the expression of the normal gene. They tend to be dominant mutations.
Changes in the DNA sequence may result in gene duplication, so there could be an extra copy present. Or if there is no gene duplication a hypermorph mutation may involve;
- Increased transcription
- Increased translation
- Increased function of the protein product
This is a rare type of genetic mutation that can cause a gain of function or a new function from the mutant gene/protein product.
Changes in the sequence of promotor regions or regulatory elements can lead to new transcription and/or new functions from the altered protein structure.