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Quantitative genetic methods have a variety of applications in understanding the covariance of variations between natural and managed populations. Today, the combination of molecular or genomic selection in combination with the traditional is a very reliable method for faster, more accurate assessment of the breeding value of animals. Important features of body conformation in the meat industry and good growth performance in sheep and goats, including accelerated growth rate are of economic importance. Quantitative genetics methods aim to identify genetic variants associated with quantitative traits in order to achieve genetic improvement in growth performance. Quantitative genetics remains the most reliable tool for estimating the breeding value of domestic animals. The application of practical methods which are used to assess the phenotype, requires excellent theoretical knowledge of the genetic of animals.

IGFBP-3 is responsible for the multiple effects of growth factors in most mammalian species and is considered the major transport factor of growth, used as a marker for different body functions such as growth, metabolism, reproduction, body weight control, immunity, energy balance, and so on. Considered as a candidate gene, used as a marker for the growth and production traits as its essential role in the growth and development of the animals. For the DNA extraction, the blood samples are obtained in the jugular vein using a 10 ml vacutainer containing EDTA as a coagulant in the blood collection of each animal Mis breed of sheep (M), Ile de France (F), and Wurttemberg (W). Isolation of DNA performed using the extraction kit (Quick DNA kit) with primers set the Forward and Reverse. The body weights of lambs from birth to 90 days of age, also been calculated. The results of the agarose gel electrophoresis of PCR amplified IGFBP-3 genes for sheep populations Wurttemberg (W), Mis (M), and Ile de France (F) had 654 bp. In our results showed an absence of polymorphism of the IGFBP-3 gene on the tested sheep populations. The results that there is no polymorphism between the examined sheep breeds, in terms of IGFBP-3 genes, we were interested whether there are differences in the body development of lambs of the mentioned populations because IGFBP-3 is related to the growth of animals. All three breeds have similar weights and growth dynamics, which could link to the growth hormone. Analyzing obtained results, we can suggest that absence of a large difference in the growth of the three breeds of sheep W, M, F does not have to be related to the absence of polymorphism of the IGFBP-3 gene but also other genetic and non-genetic factors can affect this trait. To detect the association between genetic polymorphism in IGFBP-3 genes and body development in lambs, DNA sequencing is required, which will be the subject of our future research.

Success in lamb production depends on their body weight, and in recent years there had been a growing interest in understanding the impact of some factors such as parity on sheep productivity. Knowing the factors that affect the development and growth of lambs can help make changes in breeding plans. This paper was to detect important factors affecting lamb's birth weight which usually are not taken to consideration and have substantial part planning improvement in sheep production. Miss breed sheep were cross or pair with Ile de France rams three times, with the following characteristics observed; Body weights of lambs at birth, type of birth, number of lambs born per sheep per parity in the periods from 2018 to 2020. The result of each lambing per parity of each ewe was the basis of the study. The ANOVA, general linear model, univariate analysis of variance, and descriptive statistics derived to assess the following effects: Effect of parity on lambs’ birth weight; Effect of parity on birth type, and lambs birth weight; Interaction effect of different subject effects on birth weight; The total marginal means of lambing rate per dam in three parities. The result obtained showed a very significant influence of the indicated factors on tested traits (P<0.01). Based on the results of our research, we can conclude that parities have a very significant impact, so they need more attention in the future.

Records of female lambs and their parents of the Mis sheep breed have used. All animals are approximately have weaned at 90 days of age. Descriptive statistics, paired sample test, paired differences, measures of association, correlations and regression of body weights between female lambs and their parents have done. A complementary least body weights at 30 days and weaning between dams and lambs but utmost weight at 30 days, the lambs were higher while at weaning, the dams had higher weight. It can observe that the averages on body weights the rams were the highest, followed by lambs and the lowest the dams’ body weights. The coefficient of determination of R2 varies from low to high, indicating that the lamb's body weight has more influenced by other factors that we have not considered.There were significant correlations between lamb body weight at birth and sire/dam body weight at birth. The results showed highly significant correlations of lamb's body weight at 30 days with dams but with sires, positive and very low. There had positive but no significant correlation between lamb body weight at weaning and sire body weight at weaning. Lamb body weight at weaning and dam body weight at weaning are highly correlated.

Quantitative genetic analysis of variability and relationship of lambs body weight traits in indigenous pirot sheep population are done. The the examined lambs had high variability which very suitable for selection on a larger weight. There is a high and very significant correlation between the body weight of lambs with 30 and 60 days (.969 **) and the weight with 30 and 90 days ( .914 **). There is also a highly significant correlation between the weights of lambs with 60 and 90 days of age (.904 **). From our research, we can see that the first month of the lamb's life is very important for the further development of the body. There is different level of correlation between weight of lambs. This gives us an idea to say that many paragenetic factors are crucial for the growth of lambs from birth to weaning. The coefficient of multiple determination (R2) is 0.845 which means that 8.45% of the lamb's body weight variance at 90 days is determined by the variance of the set of predictor variables (PI-60, 30, 1). Each increase in lamb body weight during the observed periods of age is associated with an increase in the score of depending variable PI90. In particular, any increase in lamb body weight at birth by 1 kg is associated with an increase in lamb body weight from 90 days by 0.238 kg.

Crossbreeding serves as a predictable and cost-effective method to genetically increase lamb body weight by mating two or more breeds of sheep. The crossing over breed comes to a far greater number of combinations of genes and thus is more likely to express favorable allele carriers of economically important traits. The phenomenon of heterosis has used since the beginning of the last century. However, its genetic basis has remained unclear. From the very beginning of the knowledge of heterosis to the present day, there are several theories, but neither theory able to answer all questions that arise regarding the apparent strength of the F1 generation offspring. Not assert anything about the genetic or molecular phenomenon that causes heterosis. It has been increasingly experimentally confirmed that heterosis is the result of highly complex interactions within the genome as well as between the genome and the environment. In Serbia, some activity of domestic researchers regarding crossbreeding of sheep after the Second World War has been recorded up to date. It found out that crosses have a higher body weight than the maternal base in the F1 generation. In other words, they had better fattening capacity than purebreds. However, the results are not always in line with expectations due to the influence of various known and unknown factors.This review paper aims to draw attention to the phenomenon of heterosis through experience in its application in Serbia, in the hope that this biological phenomenon would clarify in the near future by applying modern scientific understandings and technologies.