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Gene expression at the phenotypic level varies due to a number of influences from the environment in which the animals are reared. Ignoring this reality or due to insufficient knowledge, farmers are often disappointed when choosing a population of sheep when they do not get the production results that the breed achieves where they bought it. The investigation of the reproductive and production characteristics of parent herds of Sjenica sheep was conducted on four farms. In the research, it included a total of 921 lambs and 474 sheep. The influence of sheep body weight on the weight of lambs at birth within a farm was analyzed. Based on the research conducted on the sheep population and after the obtained and processed data, we can state the following: certain differences were found in the fertility of sheep depending on the farm and body weight. We also perceived that certain differences in the body weight of the lambs are evident depending on the weight group of the sheep and the farm where the sheep were raised. All of the above leads us to the general conclusion that management is extremely important in sheep farming. If the selection of the breeding population is carried out correctly and adequate measures of keeping, nutrition and reproduction are applied, success is guaranteed.

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.

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.

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Copyright of Biotechnology in Animal Husbandry is the property of Institute for Animal Husbandry and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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.