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In genetics, transgressive segregation is the formation of extreme phenotypes, or transgressive phenotypes, observed in segregated hybrid populations compared to phenotypes observed in the parental lines.〔Nolte, Arne W.; H David Sheets (2005-06-29). "Shape based assignment tests suggest transgressive phenotypes in natural sculpin hybrids (Teleostei, Scorpaeniformes, Cottidae)". Frontiers in Zoology〕 The appearance of these trangressive (extreme) phenotypes can be either positive or negative in terms of fitness. If both parents' favorable alleles come together, it will result in a hybrid having a higher fitness than the two parents. The hybrid species will show more genetic variation and variation in gene expression than their parents. As a result, the hybrid species will have some traits that are transgressive (extreme) in nature. Transgressive segregation can allow a hybrid species to populate different environments/niches in which the parent species do not reside, or compete in the existing environment with the parental species. ==Causes== Genetic There are many causes for transgressive segregation in hybrids. One cause can be due to recombination of additive alleles. Recombination results in new pairs of alleles at two or more loci. These different pairs of alleles can give rise to new phenotypes since gene expression has been changed at these loci. Another cause can be elevated mutation rate. When mutation rates are high, it is more probable that a mutation will occur and cause an extreme phenotypic change. Reduced (developmental stability ) is another cause for transgressive segregation. Developmental stability refers to the capability of a genotype to go through a constant development of a phenotype in a certain environmental setting. If there is a disturbance due to genetic or environmental factors, the genotype will be more sensitive to phenotypic changes. Another cause arises from the interaction between two alleles of two different genes, also known as the epistatic effect. Epistasis is the event when one allele at a locus prevents an allele at another locus to express its product as if it is masking its effect. Therefore, epistasis can be related to gene over dominance caused by heterozygosity at specific loci.() What this means is that the heterozygote (hybrid) when compared to the homozygote (parent) is better adapted and therefore shows more transgressive, extreme phenotypes. All of these causes lead to the appearance of these extreme phenotypes and creates a hybrid species that will deviate away from the parent species niche and eventually create an individual "hybrid" species. Environmental Other than the genetic factors solely causing transgressive segregation, environmental factors can cause genetic factors to take place. Environmental factors that cause transgressive segregation can be influenced by human activity and climate change. Both human activity and climate change have the capability to force species of a specific genome to interact with other species with different genomes. For example, if a bridge is built that connects two isolated areas to one another, a gene flow door would open. This open door will increase the interactions between different species with different genomes can create hybrid species that can potentially show transgressive phenotypes. Human activity can open the gene flow door by pursuing harmful actions such as cutting down forests and pollution. Climate change on the other hand can open the gene flow door by breaking climate and environmental barriers that were present before. This convergence between species can give rise to a hybrid species that will have more phenotypic variation when compared to the parent species. This increase in phenotypic variation has the potential for transgressive segregation to occur.〔Hegarty, M. J. "Invasion of the hybrids." Molecular Ecology 21.19 (2012): 4669–4671.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Transgressive segregation」の詳細全文を読む スポンサード リンク
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