mutant alleles are recessive to the wildtype! - true / untrue?? (Jun/20/2007 )
was reading something on evolution and came across this..
'mutant alleles are recessive to the wildtype'.
What the statement was referring to was lost of function mutants. Ie mutations that cause the gene to stop working. If a mutant allele stopped working, then the normal allele would be dominant, as only the normal form is encoding protein production.
However in reality, the mutant allele may still encode a functional protein. (especially if it is a single codon change, and not a frame shift mutation) Although with slightly different kinetics/binding affinity/stability. In this situation, the mutant allele and normal allele are codominant. Both contribute to the final phenotype of the organism. Such examples are alcohol dehydrogenase. There are several forms in humans, all are functional but with different activity.
Alternatively the mutant allele may encode a protein which has gained a function. Ie the mutant protein now does something the normal protein doesn’t do. In such a situation, the mutant allele is dominant. Again there are examples. The most common used example is Huntington’s disease.
The mutant allele need not be subservient to the normal allele.
The mutant alleles are not always recessive to the normal alleles. Like perneseblue said, there are examples of dominant or codominant mutant alleles. The thing is, just by itself, the mutant allele could be either dominant or recessive. It is quite random. By itself, actually nothing will dictate it has to be either this or that. However, you should also note that for any mutant to be able to exist in a gene pool for certain time, it needs to be able to pass through generations. Which means that if it encodes anything that affect severely the individual's chance to survive and produce progeny, it will be quickly removed from the gene pool due to its own nature. So if a mutant cause the individual to die early (stillborn or when it is too young for breeding), or make it unable to produce progeny, or simply affect its chance of survive (e.g. a mutant that affects the colour of a butterfly, make it unable to hide and thus easily get eaten by birds) will have no, or at least less chance to stay in the gene pool for long. This is the force that selects the recessive and dominant mutant alleles. Obviously, the recessive ones get more chances to hide in the gene pool in heterozygous genotype. The dominant ones that is harmful for the survival rate of the individual will be removed, along with that unfortunate individual. That is one reason which leads to the statement: 'mutant alleles are recessive'.