This resistance is inferred by a particular gene that can be followed using MAS (Marker Assisted Selection) and QTL (Quantitative Trait Loci). QTL's identify particular variants within phenotypes or traits and typically identify where the GOI (Gene of interest) is located. Once the cross has been made, sampling of offspring may be taken and evaluated to determine which offspring inherited the traits and which offspring did not. This type of selection is becoming more beneficial to breeders and farmers because it is reducing the amount of pesticides, fungicides and insecticides. Another way to insert a GOI is through mechanical or bacterial transmission. This is more difficult but may save time and money. It has 5 applications in fisheries and aquaculture:  Biochemical markers are generally the protein marker. These are based on the change in the sequence of amino acids in a protein molecule. The most important protein marker is alloenzyme. alloenzymes are variant forms of an enzyme that are coded by different alleles at the same locus and this alloenzymes differs from species to species. So for detecting the variation alloenzymes are used. These markers are type-i markers. Advantages: Disadvantages: Applications: