Similarly, cretinism results when the tyrosine to thyroxine pathway is mutated. Therefore, in a strict genetic sense, if appropriate experiments are performed, a morphological mutation can be explained at the biochemical level. For some mutations to be expressed, the individual needs to be placed in a specific environment. This is called the restrictive condition . These are called conditional mutations.
Lethal mutations are also possible.
As the term implies, the mutations lead to the death of the individual. Death does not have to occur immediately, it may take several months or even years. But if the expected longevity of an individual is significantly reduced, the mutation is considered a lethal mutation. Wild type alleles typically encode a product necessary for a specific biological function. If a mutation occurs in that allele, the function for which it encodes is also lost.