الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
This study was conducted at the Nucleic Acids Research Department, Genetic Engineering and Biotechnology Research Institute (GEBRI), City of Scientific Research & Technological Applications, Alexandria, Egypt. This study was divided into two basic parts: Part I (Phylogenetic relationship between the three species of tilapia) the study were done on three pure tilapia species (Oreochromis niloticus, Oreochromis aureus, and Tilapia zillii) DNA was extracted from fine tissue (muscle), followed by DNA Pooling of each species RAPD technique was used to characterize the studied fish species at molecular level and to reveal if any genetic diversity exists between these species. Results showed that the genetic dissimilarities among these three species were as follow; 54% (O. aureus x O. nilotica), 60% (O. aureus x T. zillii) and 59% (O. nilotica x T. zillii). However, this closer proximity or the highest genetic similarity was observed between O. aureus and O. nilotica (46%), while the lowest was observed between O. aureus and T. zillii (40%). Part II was designed to study the association between body weight and Polymorphism in growth hormone gene in tilapia nilotica. Fifty tilapia nilotica fish were selected from three grads of fish sizes (large, medium and small). DNA was extracted from the collected 50 muscle samples. PCR was done for amplification of the 2nd exon, 2nd intron, 3rd exon, 3rd intron and 4th exon of the growth hormone gene (GH). Restriction Fragment Length Polymorphism (RFLP) The amplified DNA fragment of the growth hormone gene was digested with restriction enzyme (HinfI). Results indicated that genotype GG showed large size fish, while, GH mostly common in small size fish. DNA sequencing: sequencing was performed to 19 PCR products of the target genes in 9 of large size fish and ten of small size fish. The results indicat that there was single nuclutid polymorphism (SNPs) which can be used for improvement of body weight trait in tilapia niloticus fish using marker-assisted selection (MAS).