The central dogma of sexual differentiation states that only the secretion of gonadal hormones causes all somatic sex differences that arise in tissues and organs. Nonetheless some traits in mice and bird models have been shown to differentiate independently of the animal's gonadal sex. Studies sought to investigate whether levels of sex chromosome gene expression might contribute to the development of these sex distinctions in somatic cells. Animal models such as the four core genotype mice and the gynandomorphic zebra finch allow the effects of gene expression to be assessed independently of hormonal influence. Behavioral, organ morphological, and other somatic cell differences were observed to be caused by differential gene expression in the sex chromosome. These findings have major implications in trying to understand the origins of differences between males and females in terms of behavior, physiology and susceptibility to disease. This better understanding will allow for the development of more efficient treatments for sex biased diseases.
[...] Influences of Genes in the Sexual Differentiation Process of Rodents : Similar to results found in zebra finches, some sexually dimorphic traits seen in mice seem to develop independently of the sex chromosome complement or Y). Studies done on the aggressive behavior of XX,XY- females and XXSry , XY-Sry males showed that both types of males were equally aggressive, but showed greater aggression than the XY and XX females{{69 Canastar,A. 2008}}. Interestingly, no significant difference in the degree of aggression was observed between XX and XY females. [...]
[...] (Table This animal model is of extreme importance to the field of sexual differentiation because it allows studies to determine whether a particular phenotype is caused by differential gene expression in the sex chromosome or by gonadal hormones. Comparison between mice of the same gonadal sex allows for the observation of the effects of sex chromosomes in the process of sexual differentiation of somatic tissue. On the other hand when gonadal males and females (XXSry vs. XY-Sry and XY- vs. [...]
[...] The analysis of brain tissue that belonged to a rare gynandomorphic zebra finch provided an insight as to whether genes located in the sex chromosomes could act as possible mediators in the process of sexual dimorphism. As previously mentioned since both sides of this bird's brain were exposed during development to the same gonadal environment, they should be morphologically identical; however, the right side of the gynandomorphic zebra finch's brain was more masculine than the left side. Agate,R.J. 2003}} These differences were associated with the differential gene expression observed on both sides of the brain. [...]
[...] Studies done to investigate the origins of sex specificity of autoimmune disease have shown that XX mice demonstrated greater susceptibility to both autoimmune encephalomyelitis (EAE) and lupus. Further analysis of these mice showed that XY mice had greater Th2 cytokine production which has been shown to be protective against EAE and lupus. The process of sexual differentiation of somatic tissues in the rodent model is, similarly to the zebra finch model - a multifaceted process involving both genetic and hormonal components. [...]
[...] Only with further improvements in the animal models and gene manipulation techniques used will we be able to assess the relevance of gene expression in sexual differentiation. Conclusion Genes in the sex chromosomes influence the development of sexually dimorphic organs both by controlling the secretion of gonadal hormones and by acting directly on the somatic cells in these organs. Because of their ease of manipulation, gonadal hormones and their effects have been more extensively studied; thus, the effects of hormones in the process of sexual differentiation are better understood than the direct action of sex chromosome genes. [...]
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