and translocations. Similarly, short wave UV light works in the same way as ionizing radiation which can also induce mutations generating chromosomal rearrangements. When DNA absorbs short wave UV light, dimerizing and oxidative mutations can occur which can cause severe damage to the DNA sequence of an organism. Insertional mutagenesis Mutations can also be generated by insertional mutagenesis. Most often, insertional mutagenesis involves the use of transposons, which introduces dramatic changes in the genome of an organism. Transposon movements can create random mutations in the DNA sequence by changing its position within a genome, therefore modifying gene function, and altering the organism’s genetic information. For example, transposable elements containing a marker are mobilized into the genome at random. These transposons are often modified to transpose only once, and once inserted into the genome a selectable marker can be used to identify the mutagenized individuals. Since a known fragment of DNA was inserted this can make mapping and cloning the gene much easier. Post mutagenesis Once mutagenized and screened, typically a complementation test is done to ensure that mutant phenotypes arise from the same genes if the mutations are recessive. If the progeny after a cross between two recessive mutants have a wild-type phenotype, then it can be inferred that the phenotype is determined by more than one gene. Typically, the allele exhibiting the strongest phenotype is further analyzed. A genetic map can then be created using linkage and genetic markers, and then the gene of interest can be cloned and sequenced. If many alleles of the same genes are found, the screen is said to be saturated and it is likely that all of the genes involved producing the phenotype were found. Human diseases Human diseases and disorders can be the result of mutations. Forward genetics methods are employed in studying heritable diseases to determine the genes that are accountable. With single-gene or mendelian disorders a missense mutation can be significant; single nucleotide polymorphisms (SNPs) can be analyzed to identify gene mutations that are associated with the disorder phenotype. Before 1980 very few human genes had been identified as disease loci until advances in DNA technology gave rise to positional cloning and reverse genetics. In the 1980s and 1990s, positional cloning consisted of genetic mapping, physical mapping, and discerning the gene mutation. Discovering disease loci using old forward genetic techniques was a very long and difficult process and much of