Mutations : 1. Morphological Level (Including Lethal Mutations)

Mutations : 1.  Morphological Level (Including Lethal Mutations)
Brief History
Range of Mutations
Stages of Which Mutations Occur
Types of Mutations
Spontaneous Vs Induced Mutations
Mutation Rates and Frequencies
Induced Mutations 
Detection of Mutations in Drosophila 
Detection of Mutations in Plants
Use of Microbial Systems to Assess Potency of Mutagens
Practical Applications of Mutations
Effect of Genotypes on Induction of Mutations (Mutator Gene and Paramutations)
Adaptive Mutations and Genotrophs
Mutations in a broad sense include all those heritable changes, which alter the phenotype of an individual. Hugo de Vries used the term 'mutation' to describe phenotypic changes which were heritable. He is, therefore, credited to have differentiated between heritable and environmental variations. However, the term mutation is now used in a rather strict sense to cover only those changes which alter the chemical structure of the gene at the molecular level. These are commonly called gene mutations or point mutations. In practice, sometimes it is rather difficult to distinguish between gene mutations and structural changes in chromosomes, because certain structural changes may have same phenotypic effects as gene mutations. Small deficiencies cannot be discovered by cytological observations. Although in Drosophila small deficiencies can also be detected in giant salivary gland chromosomes, in other organisms the only test for a deficiency is that it will not revert back to wild type character. However, gene mutations would be able to give reverse mutations.

The distinction between point mutations and chromosomal aberrations is thus rather superficial. If chromosomes are not studied under the microscope, in certain cases we may not be in a position to say with certainty whether a particular phenotypic character is due to point mutation or due to a structural change. Many mutations, described by de Vries in Oenothera lamarckiana, are now known to be due to certain numerical and structural changes in chromosomes. These are sometimes described as chromosomal mutations. For instance, 'gigas' mutant in O. lamarckiana was later found to be due to polyploidy. The structural and numerical changes in chromosomes have been discussed in Structural Changes in Chromosomes and Numerical Changes in Chromosomes respectively, and this section will be devoted to point (gene) mutations only.