sex_topics - Sex-Related Topics A grab bag of subjects,...

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Sex-Related Topics A grab bag of subjects, vaguely related to the typical eukaryotic condition of having 2 sexes
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Sex Determination Many groups use sex chromosomes to determine sex. Mammals have the X and Y chromosomes XX = female, XY = male All other chromosomes are called “autosomes” Thus, humans have 46 chromosomes, 44 autosomes plus 2 sex chromosomes.
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X and Y Chromosomes The X has many genes on it, just like the autosomes. Most of the genes on the X have nothing to do with sex. The Y has very few genes on it. It consists of mostly inactive DNA. One gene on the Y is very important: SRY. The SRY gene is the primary determinant of sex. If SRY is present, testes develop in the early embryo. The testes secrete the hormone testosterone, which causes development as a male. If SRY is absent (no Y chromosome), ovaries develop instead of testes, and the embryo develops into a female. The X and Y chromosomes share a common region at their tips, the pseudoautosomal region. Crossing over in meiosis occurs in this region.
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Sex Determination in Birds Birds use a system of sex chromosomes very similar to mammals. The bird sex chromosomes are called Z and W. Big difference from mammals: in birds, a ZZ individual is male, and a ZW individual is female. We can define some terms: “homogametic” means having both sex chromosomes the same, like female (XX) mammals and male (ZZ) birds. “Heterogametic” means having different sex chromosomes, like male (XY) mammals and female (ZW) birds.
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Sex Determination in Drosophila Drosophila also have X and Y chromosomes, with XX female and XY male. However, Drosophila don’t use the SRY gene to determine sex. Instead, they use the ratio of X’s to sets of autosomes. 1 X plus 2 sets of autosomes is a normal diploid male. 2 X’s plus 2 sets of autosomes is a diploid female. The difference between sex determination mechanisms comes in the odd cases: --an XXY individual has a Y, so is a male mammal. However, 2 X’s plus 2 sets of autosomes makes it a female Drosophila. ---an XO individual (i.e. only 1 X, no other sex chromosomes, but otherwise diploid) is a female mammal (no Y) but a male Drosophila (1 X plus 2 sets of autosomes).
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Other Mechanisms Hymenopterans (wasps, bees, ants) are mostly female. Females are diploid, and males are haploid. Thus, a virgin female can lay unfertilized eggs that will hatch into males that can then fertilize her to produce more females. Nematodes (roundworms) have a single sex
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sex_topics - Sex-Related Topics A grab bag of subjects,...

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