The first research investigated the polymorphism of heat resistance-related genes across several sheep breeds from diverse climatic conditions. A study involving 51 SNPs across 30 genes associated with heat stress was conducted on 720 sheep from 17 distinct breeds acclimated to various temperatures in Hungary, Bosnia and Herzegovina, Morocco, and Romania, utilizing KASP-PCR technology. A total of 32 SNPs (62.74%) from the initial list of 51 SNPs were successfully genotyped in this study; 17 of these were identified as polymorphic (33.33%). All breeds were successful in genotyping for four SNPs: rs161504783-HSP12A, rs588145625-HSPA8, rs588498137-STAT3, and rs602521720-HCRT. Genotype frequency tendency was detected only for the HSPA12A and HSPA8 genes. The Botosani Karakul breed exhibits the highest number of SNPs (4 SNPs) deviating from Hardy-Weinberg equilibrium and has decreased heterozygosity noted in 10 SNPs. PCA failed to distinctly discriminate the breeds, however plot concentration exhibited slight variation among the three groups, with the loading values of HSP90AA1 and IL33 SNPs strongly influencing PC1 (11.21%) and PC2 (9.98%). We validated prior studies indicating that the SNPs of HSPA12A, HSPA8, HSP90AA1, and IL33 are possible candidate markers for thermotolerance adaption in sheep. The second research is a gene expression analysis of seasonal variations in heat-related genes (HSP70, IL10, TLR2, TLR4, and TLR8) across various sheep breeds in Hungary — the indigenous Tsigai, Hungarian Merino, and White Dorper — using qRT-PCR during the peak summer, fall, winter, and spring seasons. The THI on all sampling days remained within the thermoneutral zone, except during the summer season (THI 78.99). Among all genes of interest in this study, the lowest expression relative to the spring season was observed in the autumn. This study observed that in the Hungarian Merino, a cold-tolerant breed highly susceptible to heat stress, the relative gene expression of HSP70 was elevated throughout summer, demonstrating heat-stress-induced HSP70 expression. An overexpression for IL10 has been observed over the summer season for Hungarian Tsigai, but for Hungarian Merino and White Dorper was in winter season. Spring and autumn are seasons when TLR2 and TLR8 gene expression is lower, whereas winter and summer are seasons when relative gene expression is higher. All breeds exhibited overexpression of both genes during the summer. The peak expression of TLR4 was recorded in the summer for Hungarian Merino and White Dorper, but Hungarian indigenous Tsigai exhibited its maximum expression in the winter The indigenous Hungarian Tsigai was clearly the hardiest breed, having developed remarkable thermotolerance and immunity to the harsh Hungarian climate. The third research carried a transcriptome analysis of the skin of Hortobágyi Racka sheep with black (n=4) and white (n=5) coat colors, used high-throughput RNA sequencing, and linked it with thermoregulatory capacity. A total of 5,525,285 reads were sequenced, which were effectively annotated to 21,328 genes from the Ovis aries genome (ARS-UI_Ramb v30). Based on the criteria of FC>1.5 and p-value<0.05, a total of 108 genes demonstrated differential expression between black-coated and white-coated sheep. Of these, 25 genes were identified as downregulated, whilst 83 genes were classified as upregulated. The gene exhibiting the greatest downregulation was KRTAP6-1 (adj.P.Val= 0.192) which a keratin related genes whereas the most significantly elevated genes (adj.P.Val<0.05) included TYRP1, PMEL, TRPM1, MLANA, SLC24A5, SLC24A4, SLC45A2, TYR, IRF4, DCT, OCA2, and DDC, which are genes linked to pigmentation and the biological mechanisms of melanogenesis. The GO and KEGG pathway analysis of DEGs indicated that the majority were linked to oxidative phosphorylation, tyrosine metabolism, metabolic pathways, thermogenesis, and melanogenesis. The protein-protein interactions shows the interaction network associated with pigmentation and melanin synthesis (IRF4, PAX3, SLC24A4, DCT, KIT, TRPM1, MLANA, OCA2, SLC24A5, SLC45A2, TYRP1, and TYR) as well as immune response and inflammation (PECAM1 and GPNMB) ribosomal proteins. The findings indicated that the roles of these pigmentation-related genes in heat stress adaptation occur via pleiotropic effects. This dissertation research has contributed to the understanding of the genetic architecture and molecular basis of adaptive traits in sheep, mediated by complicated mechanisms involving large gene networks in both direct and indirect (pleiotropic) ways.