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This study aimed to evaluate the effect of thermal manipulation during different periods of embryonic development on the amount of DNA methylation during the first and second generations of chicks and its effect on some productive traits, and to study the possibility of transferring tolerance to high temperatures to the second generation. 150 eggs were divided into three equal groups. The eggs of the first group (control group) were incubated under standard conditions for chick egg incubation, and the eggs of the other groups were incubated at 41°C for three hours on three consecutive days of different periods of embryonic development (early and late period). Blood samples were collected when the birds reached sexual maturity to study the required indicators. The eggs were collected from the birds and incubated under standard conditions and the birds were raised until maturity, then blood samples were collected again from the second generation birds. DNA was extracted from the blood samples of the first and second generations, and an ELISA test was performed to estimate the amount of DNA methylation. No significant decrease was recorded in hatching indicators (hatching percentage - hatching percentage) for the early and late heat treatment groups for eggs that produced first generation birds compared to the control group. As for the weight of chicks on the first day, both the early heat treatment groups outperformed the control group by 0.35 grams and the late heat treatment groups by 0.95 grams. The weight of chicks on the first day in the late heat treatment group was heavier than the control group. As for the second generation, no differences were recorded in all hatching indicators between the three groups. The second generation birds showed a decrease in the amount of methylation compared to the first generation birds, and this decrease was not significant (p<0.05) except in the early heat treatment group. It can be said that the heat treatment programs during embryonic development led to an improvement in some productive traits of the birds and their ability to resist heat stress, and this ability appeared in the second generation birds, especially for the early and late heat treatment groups.
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