The marker used for selection is associated at high frequency with the gene of interest due to genetic linkage. There are selectable markers which eliminate certain genotypes from the population and screenable markers which cause certain genotypes to be readily identifiable, at which point the ex- perimenter must score or evaluate the population and act to retain the preferred genotypes. Depending on the trait of interest, marker-assisted selection may be cheaper and faster than conventional phenotypic assays. Multiple markers can be evaluated using the same DNA sample, and once DNA has been extracted and purified, it may be used for multiple markers for the same or different traits, thus reducing the time and cost per marker.
Identifying genetic markers Genetic markers can be identified by a range of molecular techniques, such as microsatellites or single nucleotide poly- morphism detection. Linked markers are molecular markers located very close to major genes of interest. There are sever- al known phenotypic and genetic markers for gastrointestinal parasite resistance in naturally infected animals that could potentially assist response to selection. The Major Histocom- patibility Complex (MHC) involves a series of highly polymor- phic genes which are responsible for the initiation of the im- mune response when an animal is challenged by pathogens
or parasites. The MHC is divided into three regions: class 1, class 2 and class 3. Faecal Egg Count (FEC) is used as an indicator trait to deter- mine resistance to gastrointestinal parasites. The heritability of the FEC varies considerably depending on both the para- site species and animal breed. Immune response evaluation is another indicator trait that can be used.
Selective breeding Selective breeding to take advantage of within-breed var- iation in disease resistance is an important disease control strategy. Selection for parasite resistance alone can result in negative traits, such as lower live-weight gains. Therefore, it is essential to apply appropriate selection policies and to understand the genetic architecture of underlying resistance to predict genetic risk or selective breeding. The application of selective breeding for parasite resistance in combination with other integrated control methods is considered an alternative means of parasite control in the long run. However, disease resistance varies between species, between breeds and between individuals within breeds, and identification of the phenotype for disease re- sistance is challenging, because in a population containing both healthy and sick animals, not all healthy animals may be disease resistant.
▶ POULTRY WORLD | No. 7, 2022
The application of selective breeding for parasite resist- ance in com- bination with other integrated control methods is considered an alternative means of par- asite control in the long run.
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