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| Sex Determination |
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| It is the phenomenon where in the sex of an individual organisms becomes fixed soon after the formation of a zygote, due to the specific composition of sex chromosomes. |
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| The process of sex determination by allosomes is called genetic or chromosomal sex determination. It is brought about by heterogametes, which is the formation of two kinds of gametes in one of the two sexes. There are three mechanisms of chromosomal sex determination and they are as follows. |
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| This type of sex determination occurs in insects and mammals including human beings. Here, the females have two identical homologous sex chromosomes designated as XX and the males have two dissimilar sex chromosomes designated as X and Y. Thus, females are homogametic, producing only one type of ovum, and males are heterogametic, producing two types of gametes: half carrying the X chromosome and the other half carrying the Y chromosome. In Drosophila the Y chromosome has a small hook like projection. In some plants such as Melandrium alba the Y chromosome is distinctly longer than the X chromosome. In humans, the Y chromosome is shorter in length than the X chrmosome. |
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| Hence, in all examples of XX-XY type of allosomes, the females are homogametic and the males are heterogametic. The male gametes will be of two types and this condition is called digamety. The X-carrying sperms are called gynosperms and the Y carrying sperms are called androsperms. |
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| In all such cases, sex gets determined at the time of fertilization and depends on the type of sperm that fuses with the ovum. If the X-carrying ovum is fertilised by a X-carrying sperm the resulting zygote will have a sex chromosome composition of XX. Such a zygote develops into a female. If the X carrying ovum is fertilised by a Y-carrying sperm. The resulting zygote will have a sex chromosome composition of XY. Such a zygote develops into a male. |
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| fig. 19.6 XX-XY Type |
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| In this type, the females have two homomorphic sex chromosomes XX and hence they are homogametic. The males have one sex chromosome and are represented as XO. If the X-carrying ovum is fertilised by a X-carrying sperm, the resulting zygote XX will develop into a female. If the sperm carrying no sex chrmosomes unites with the ovum, the zygote XO will develop into a male. |
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| fig. 19.7 XX-XO Type |
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| The XX-XO type of sex determination is seen in some insects such as bugs, cockroaches and grasshoppers. |
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| In this type, the females carry two different types of sex chromosomes (ZW) and are heterogametic while males carry identical sex chromosomes (ZZ) and are homogametic. The ova will be of two types: Zcarrying and Wcarrying while all sperms will be only Z-carrying. The ovum will have a sex chromosome composition of ZZ and it develops into a male. A zygote with ZW chromosomes, formed by a fusion of Z-carrying sperm with a W-carrying ovum, results in the formation of a female offspring. |
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| fig. 19.8 ZW-ZZ Type |
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| This type of sex determination occurs in some insects like butterflies and moths. It is also known to occur in several examples of fishes, reptiles and birds. |
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| In this type, the females have only one sex chromosome and hence represented as ZO. Females are heterogametic. Males carry two identical sex chromosomes designated ZZ. Males are homogametic. On fertilization by a Z-carrying sperm the Z-carrying ovum would develop into a male (ZZ) and on fertilization, the Z-lacking ovum would develop into a female. |
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| fig. 19.9 ZO-ZZ Type |
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| This type of sex determination is seen in some moths and butterflies. |
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