Major methods for finding molecular genetic markers
DNA markers are divided into two types: type Ⅰ markers, mainly are some single genes, and used to compare the homologous loci varieties of relative distance and chain and linear correlation; Ⅱ type markers, mainly high polymorphism, information content rich DNA fragments, is one of the most commonly used microsatellite marker. Through Adjuvant Optimization, more and more kinds of molecular markers were introduced, including restricted fragment length polymorphism (RFLP), random amplified polymorphic DNA (RAPD), amplified fragment length polymorphism (AFLP), and Microsatellites. At present, there are 2,505 markers in the world's pig research, including 1,391 microsatellite markers. Ⅰ type 873, type Ⅱ mark 1632.
The main methods to search for molecular genetic markers are candidate gene method and genome scanning method.
1. Candidate gene method as a candidate gene for a trait, it is usually some genes whose biological functions and nucleic acid sequences are known, and they are involved in the growth and development process of the trait. These genes may be structural genes, regulatory genes or genes that affect the expression of traits in biochemical metabolic pathways. Candidate gene method research should follow certain steps, such as candidate gene selection primer design, gene specific fragment amplification, polymorphic locus search and so on. Candidate gene search USES genes that are thought to have a direct physiological function for a trait to find QTLS. In addition, genes found in other species that control some traits can be studied as candidate genes for pigs. For example, the h-fabp gene affects the backfat thickness and intramuscular fat content of pigs (Gerbens et al., 1999). 2000; 2001); The melanocorticoid receptor 4 (MC4R) gene was significantly correlated with the intake, backfat thickness and growth rate of pigs (Kim et al., 1999; 2000).
2. Genome scanning: all genetic information is stored on the 19 pairs of chromosomes of the pig. Reference families were established, such as meishan European and American pig species, wild boar big white pig, and their hybrid offspring were used to find QTL through genetic markers. The most effective design is the genotype analysis of F2 generation isolation population. Figure 4-2 is a simple schematic diagram of single genetic marker and linkage QTL analysis. Alleles of genetic markers and their linkage QTL in F1 generation were heterozygous. In the F2 generation isolation population, the ratio of the three possible genotypes per seat should be 1:2:1, when the average performance of marker genotypes is compared, the existence of linkage QTL can be analyzed.
Anderson (1994), such as reported with a wild boar by the results of the large white building reference group, using the 105 DNA markers in the genetic map, the separation of F2 generation 200 pigs linkage analysis research found that on chromosome 4 seat back fat and control the growth rate, the average genetic effect 24 g/d and 5 mm, respectively, the equivalent of F2 DaiQun total phenotypic variation of 12% and 18%. Daily weight gain can differ by more than 50g between two extreme homozygous genotypes, resulting in a 10kg weight difference at market time in pigs.
(iii) MAS breeding
In pig breeding selection, it is difficult to determine the sex efficiency of low heritability (e.g., reproductive traits), high cost of measurement (e.g., disease resistance), phenotypic values (e.g., lean meat rate) or limited sexual performance (e.g., milk production) early in development. It is estimated that the selection of the marker before the determination of the offspring can increase the selectivity response by 10%~15%. The MAS of compatriots who choose to combine can be increased by about 40%. Combining multiple genetic markers and trait information, the selectivity response can be increased by 50%~200%. Using marker selection in cross breeding can predict and make full use of heterosis. Molecular genetic markers can also be applied to early selection and screening and detection of large populations to select populations with desired genotypes.
For example, there is little progress in the improvement of pig litter, a low genetic trait, by traditional methods. Rothschild et al. found in 1994 that the estrogen receptor (ESR) gene was one of the main genes responsible for the litter size of pigs, which could control the total litter size of 1.5 pigs and the live litter size of 1 pig in the meishan synthetic line of China. In the Chinese two-flower face hybrid population, the agricultural university of China not only confirmed the results of Rothschild et al., but also found another main gene locus controlling the number of piglets - FSH, which can control the total number of piglets and the number of live piglets by 2.0.
Although MAS can improve the effectiveness of selection and the annual amount of genetic improvement, its effectiveness is also affected by many factors. In addition to the heritability of traits, the intensity of selection, and the size of the selected population, the determinants are the linkage between genetic markers and QTLS. Zhang (1992) pointed out that each QTL could be specifically detected by using genetic markers closely linked to QTL, and the final selection of genetic markers would be equivalent to the selection of QTL itself. Therefore, genetic markers closely related to QTL must be obtained in order to improve MAS efficiency. Resources at present, through the establishment of the pig family, has some related to the growth, reproduction and carcass, meat quality of QTL mapping in some microsatellite nearby, such as on chromosome 3 microsatellite Sw2427 - Sw251 area and daily gain of pigs, on chromosome 4 S0101 - S0107 area and back fat belly fat, 7 chromosome S0064 S0066 regional composition and has a strong correlation between birth weight and body. It can be predicted that with the discovery of more genetic markers closely linked to QTL, MAS will be applied more effectively in practical breeding.