MUTATIONS
Change in the amount,content or organization of genetic material is called a mutation.  

TYPES OF MUTATIONS:

a)  POINT MUTATIONS (SINGLE BASE SUBSTITUTION): 

It is a type of small scale GENE MUTATION that causes the replacement of a single base nucleotide with another nucleotide of the genetic material, DNA or RNA.

They maybe:

• Transitions: replacement of a purine base with another purine or replacement of a pyrimidine with another pyrimidine.
• Transversions: replacement of a purine with a pyrimidine or vice versa.

Functional categorization:
1- Missense mutation: which results in a protein in which one amino acid is substituted for another due to base substitution.

2- Nonsense mutation: in which a stop codon replaces an amino acid codon, leading to premature termination of translation.

3- Frameshift mutation: which causes a change in the reading frame due to indels (insertions or deletions) of a number of nucleotides that is not evenly divisible by three from a DNA sequence, leading to introduction of unrelated amino acids into the protein, generally followed by a stop codon.

b)  CHROMOSOMAL MUTATIONS:
Chromosomal abnormalities involve alterations in large segments of DNA.

• Inversions: occurs when a break is rejoined to the correct chromosome but in an incorrect orientation.
• Interstitial deletions: an intra-chromosomal deletion that removes a segment of DNA from a single chromosome, thereby apposing previously distant genes
• Translocations: interchange of genetic parts from nonhomologous chromosomes.
• Insertions: when a segment from one chromosome is inserted into another chromosome.

Functional Categorization:

• Loss-of-function mutations are the result of gene product having less or no function. 
• Gain-of-function mutations change the gene product such that it gains a new and abnormal function
• Dominant negative mutations have an altered gene product that acts antagonistically to the wild-type allele. 

(IF A GENE REQUIRES ONLY ONE PERFECT ALLELE TO WORK AND THAT GENE IS HETEROZYGOUS OR PART MUTANT BUT THE DEFECTIVE PRODUCT OF ONE ALLELE ANTAGONIZES THE PRODUCT OF THE NORMAL THEN ITS CALLED NEGATIVE........)

• Lethal mutations are mutations that lead to the death of the organisms which carry the mutations.
• A back mutation or reversion is a point mutation that restores the original sequence and hence the original phenotype.

• Recessive mutations lead to a loss of function, which is masked if a normal copy of the gene is present. For the mutant phenotype to occur, both alleles must carry the mutation.
• Dominant mutations lead to a mutant phenotype in the presence of a normal copy of the gene. The phenotypes associated with dominant mutations may represent either a loss or a gain of function.)