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BIO 325 September 27th Lecture Notes

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University of Texas at Austin
BIO 325
Sanghamitra Mohanty

9/27 Lecture Friday, September 27, 2013 12:39 PM Sex determination is done by looking at the sex chromosomes --- but the sex determining genes aren't always present on the sex chromosomes --- in mice, some sex determining genes are present on the autosomes Sexual dimorphism - either male or female Primary sexual differentiation - involves only the gonads where gametes are produced Secondary sexual differentiation - the overall appearance of the organism There are situations where primary and secondary sex are different! Unisexual (dioecious or gonochoric) - individuals that contain only male or female reproductive organs Bisexual (monoecious or hermaphroditic) - individuals that contain both male and female reproductive organs (can produce both male and female gametes) Chlamydomonas (algae) -- produces asexually and spends most of their life cycle in haploid phase  Unfavorable nutrient conditions --> certain daughter cells function as gametes o The two gametes fuse together during mating that aren't usually morphologically distinguishable  Isogametes  Two mating types: mt and mt + o Subtle chemical differences; behave differently o mt cells can mate only with mt cells  Example: mt can't mate with mt , same with mt - Zea mays ---- maize  The diploid sporophyte stage predominates and both male and female structures are present on the adult plant o This indicates that sex determination must occur differently in different tissues of the same plant C.elegans (nematode worm) ---  Only has 2 sexual phenotypes: o Males, which only have testes o Hermaphrodites - both testes and ovaries  Self-fertilization  Produces hermaphrodite offspring with less than 1% male offspring o THERE'S NO FEMALE OR Y CHROMOSOME For a long time we didn't know sex chromosomes were in charge of sex determination ---- after long research there were 3 types of sex determinations discovered  XX/XO mode of sex determination o First demonstrated in butterflies o The O means nothing (just one X)  Males have one X  Females have two XX  XX/XY mode of sex determination o First seen in a bug, and later seen in humans o Females have XX chromosome [homogametous] o Males have either X and o Y chromosome [heterogametous]  ZZ/ZW sex determination o Seen in fish and bugs o Females = heterogametic (ZW) set o Males = homogametic (ZZ) set If you got a Y, then it's a guy. Whenever there's an abnormal number of chromosomes, i
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