High ambient temperatures in tropical and subtropical regions adversely affect the growth and egg productive performance of both broiler and laying chickens. Relative humidity and ventilation rate influences bird's performance, particularly when they are reared in open sided poultry houses. Genetic approaches that aid in reducing or altering the extent of plumage cover has, as a result, been adopted in many hot regions. Fortunately, there are some major and marker genes can reduce the deleterious effects of heat stress, naked neck (Na) and frizzle (F) genes being the most notable ones. The bulk of relevant literature has shown that their favourable effects on growth and laying performance are well established. Therefore, the naked neck and frizzle genes can be a viable alternative for broiler and layer production particularly, in the tropical countries where summer temperatures are very high.

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The present study aimed to outline the population differentiation of Sinai and Norfa chicken, native to Egypt, with microsatellite markers. Twenty microsatellite loci recommended by the Food and Agriculture Organization (FAO) were used. Fifty eight birds were sampled (29 for each strain: 12 males + 17 females). Data were collected and genetic diversity indicators were assessed utilizing the approaches implemented in FSTAT, Cervus 3.0.7 and GenAlEx 6.5 software programmes. A total number of 182 alleles were detected with an average value of 9.1 allele per locus. The expected heterozygosity was 6.625 and 6.343 in Norfa and Sinai chickens, respectively. Norfa chickens produced 15 private alleles, while there were 9 unique alleles detected in Sinai chickens (13.18% private alleles as a percentage of the total observed number of alleles). Fixation indices' (F ST , F IS , and F IT) values were 0.060, 0.410 and 0.438, respectively, across all 20 loci investigated. Results indicated that the studied populations were genetically differentiated. Consequently, they have high breeding potential. Efforts should be made to incorporate the other local chicken strains as unique genetic resources into conservation programmes. This should begin with proper management of these flocks to ensure the maintenance of their genetic diversity over time by avoiding inbreeding. Such information is likely to have a profound effect on the success of genetic improvement and completes information from phenotypes and biometric measurements of the domestic chickens in Egypt.

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Major marker genes are believed to confer not only adaptability to the tropical climate, but also resistance to diseases. During the last two decades, many investigations have observed that naked neck and frizzle genes have favourable effects on immunocompetence in chickens raised under high ambient temperatures. As a result, several tropical countries have introduced these genes in breeding programs to improve both the productive performance and survival rate of chickens.

Major genes could be introgressed into laying hens to attenuate heat stress. However, under cold and/or moderate ambient temperature, these genes might possess different behaviour. The main objective of this study was to evaluate laying performance, immune response, and antioxidant status of native laying hens segregating for naked neck (Na) and frizzle (F) genes under low ambient temperature. Five genotypes were studied: homozygous naked neck (NaNaff), heterozygous naked neck (Nanaff), homozygous frizzle (nanaFF), heterozygous frizzle (nanaFf), and normally feathered (nanaff). The hens were raised under temperature range 22.2–16.7°C. No adverse effect due to ambient temperature was detected in laying performance for naked neck genotypes. Significant decrease in egg weight was recorded in nanaFF genotype compared to the other genotypes leading to significant decrease in egg mass. Significant improvement in shell thickness was associated with Na and F genes. NaNaff genotype had a significantly higher eggshell strength compared to nanaff. Furthermore, Na and F genes improved cellular mediated immune responsiveness, whereas this improvement did not extend to humoral immunity. Birds carrying F gene in homozygous state had a higher total antioxidant activity compared to the remaining genotypes. It could be concluded that the presence of Na and F genes in laying hens raised under low ambient temperature significantly increased shell thickness and, in turn, improved shell strength. Moreover, they greatly enhanced cellular immunity, particularly in heterozygous naked neck status.

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