LS4 Lect_4

LS4 Lect_4 - 1/13/10 Sex Determination 1. Sex chromosomes...

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1/13/10 1 Sex Determination 1. Sex chromosomes humans; silkworms - presence of Y chromosome Drosophila; nematodes - X to autosome ratio 2. Ploidy (in bees, males are haploid, females diploid) 3. Environment - Bonellia; a sea worm. Larvae are sexually undifferentiated, if it lands on the ocean Foor it develops into a female, if it lands on a female it develops into a male. Human sex chromosomes and sex determination. XY male XXX female XX female XXXY Klinefelter male XO Turner female XXXX female XXY Klinefelter male XXXXY Klinefelter male Turner female - underdeveloped gonads, poorly developed secondary sexual characteristics, poor spatial perception. Klinefelter male - small testes, sterile, mentally below normal. Sex chromosome anueploidy in humans. In birds and moths; the female is ZX and male is ZZ. 1. In humans, sex is determined by Y chromosome. 2. Humans are insensitive to X chromosome aneuploidy. 3. Barr bodies - dark staining bodies in nuclei of cells; Barr bodies = # of X chromosomes - 1. Lyon Hypothesis – explains: 1. how mammals cope with 1X or 2X chromosomes. 2. why X chromosome aneuploidy is tolerated in mammals. 3. what Barr bodies are. Proposed that in mammals only 1 X chromosome is active, the other is inactive in the form of a Barr body. heterochromatin - euchromatin human males and females each have 1 functional X chromosome. (dosage compensation) X inactivation occurs at the few hundred cell stage in the female embryo. In any cell it is random which X chromosome is inactivated. This results in somatic mosaics. Example : coat color in cats.
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1/13/10 2 Mammals: inac=va=on of one of the X chromosome Barr body X inactivation occurs at the few hundred cell stage in the female embryo. In any cell, it is random as of which chromosome is inactivated, which results in somatic mosaics . What is genotype of female cat? Why are there no male calico cats? Calico Cat
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This note was uploaded on 04/15/2010 for the course LS LS 4 taught by Professor Ribaya during the Winter '07 term at UCLA.

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LS4 Lect_4 - 1/13/10 Sex Determination 1. Sex chromosomes...

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