Gen.Class+notes_5

Gen.Class+notes_5 - One of the major extensions to...

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One of the major extensions to Mendelian genetics is the inheritance of characters located in the sex chromosomes . Sexual reproduction is the formation of offspring that are genetically distinct from their parents. Each parent contribute genes to their offspring. Most organisms have two sexual phenotypes: female and male. In humans, some obvious differences are the size of the gametes, the size and shape of the sex chromosomes. EGG SPERM Y chromosome X chromosome
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Some definitions : Hetero gametic sex : produce gametes that have two different sex chromosomes. For example XY, XO, ZW. Homo gametic sex : produce gametes that have the same sex chromosomes. For example XX, ZZ. The Sex is determined by the activity of genes that happen to reside in a sex chromosome. It is convenient to follow them if the chromosomes look different.
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There are several mechanisms of sex determination The XX-XO system : This system is used in the grasshopper. The females are homogametic: 2 XX chromosomes. The heterogametic sex is the male with only one sex chromosome (XO). When the males segregate their XO sex chromosomes, ½ gametes are X and the other ½ has no sex chromosome. The ZZ-ZW system : In this system the female is heterogametic (ZW) and the male homogametic (ZZ). This system is used by birds.
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Genic sex determination: These organisms (fungi, plants and protozoan) have no sex chromosomes, there is no difference in the morphology of the chromosomes in the sexes: sex is determined by one or more loci. Environmental sex determination : In many reptiles sex is determined by the temperature during embryonic development. The XX-XY system : (plants, insects, reptiles and all mammals ). The female is homogametic and have two X chromosomes. The male is heterogametic and have one X and one smaller Y chromosome.
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Stevens and Wilson used this system to demonstrate that sex is associated with the inheritance of a particulate chromosome and that genes are in chromosomes. These chromosomes are different in shape size and can be followed. 1) They demonstrated that the X and the Y chromosomes separate into different cells during sperm formation. 2)
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Gen.Class+notes_5 - One of the major extensions to...

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