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Homonally Controlled Sex Determinig Mechanisms |
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Hormonally Controlled Sex Determining Mechanism
In higher animals, early differentiation, including that of the sex organs, is influenced by hormones. In salamanders, for example E. Witschi found that a peculiar type of hormonal balance is involved in the differentiation of the gonads; antagonistic substances are formed by the cortex and medulla of the embryonic gonad. The male hormone is produced by medulla where as the female hormone comes from the cortex. When male and female salamanders were experimentally grafted together, the male substance suppressed the development of the ovary in the female member. When the male graft was considerably smaller than the female, however, the female hormone was dominant and genetically determined males developed ovaries. The quantity of the hormone evidently was a factor in this transition.
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Partial sex reversals sometimes occur in adults of higher animal species, indicating that at least the secondary sex characteristics of the opposite sex are potentially present throughout the life. For example, in birds only one gonad of a normal female develops into functional ovary. The other gonad remains rudimentary. If the functional ovary of a hen is destroyed, the rudimentary gonad develops into a testis and other masculine secondary sexual characters such as crowing ability, cock feathering and wattles develop. Thus, the female sex is reversed into male sex due to the phenomenon called sex reversal. Such a sex reversed hen fathered chicks which showed a sex ratio of two females to one male, since in birds the female is the heterogametic sex. The case of sex reversal of hen can be interpreted as follows:
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During embryonic development, the XY (ZW) genotype stimulates the pituitary gland to produce female hormones that the gonad of the hen to develop into an, ovary. After the development of ovary, the pituitary ceases to produce female hormones, due to inhibition of the pituitary by hormones produced by the ovary, thus, acting as a developmental feed-back system. The high level of female hormones secreted sequentially by the pituitary and the ovary is sufficient to suppress the action of male hormone producing cells of the body such as the steroid producing cells of the adrenal glands.
When the ovary of a hen is removed, the steroid cells of adrenal become active and provoked the development of rudimentary gonad into testis, which itself is an endocrine gland. Both endocrine glands (e.g., adrenal and testis) produce large amount of male hormones which sufficiently suppress the action of the female hormone producing cells of pituitary.
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Removal of the gonads of either sex in mammals is followed by the development of secondary sex characteristics, of the other sex. For example, sexual differentiation in man is influenced by hormones. When the testes of the male are removed before puberty, female characteristics of body form, voice, and hair pattern develop in the adult. Tumors of the adrenals ill women are associated with the development of masculine characteristics such as lower pitched voice and increased growth of hair.
Another classical example of hormonal control of sex determination has been found in cattles. In cattle, when twin calves of different sexes occur, the female member is usually a sterile intersex, called a freemartin. The freemartin has external female genetalia but internal sex organs are more or less like those of male.
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The male twin is usually normal. F. R. Lillie. (1917) has suggested that the formation of freemartin was due to a fusion of the foetal membranes of the twin calves, while they were in uterus of the mother. The fusion of the foetal membranes permitted the blood of each twin to circulate in the blood vessel of the other. The male hormones produced by the male, twin are presumed to suppress the differentiation of the female internal sex organs of the co-twin. The hormonal influence occurs only in one direction. This also indicated that in cattle the female hormone is produced later in development than the male hormone. Recently, R. D. Owen and C. Stormont have supported the Lillie's hypothesis by performing serological tests in these cattle twins
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ns. Instead, those lateral roots that escape infection by Frankia develop into nodule roots serving as antennae for oxygen diffusion.
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