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Chapter 15.2

Chapter 15.2. A Few Outcomes of Gene Controls. AP Biology Fall 2010. X Chromosome Inactivation. In mammalian females, the gene products of only one X chromosome are needed The other is condensed and inactive Called a Barr body. X Chromosome Inactivation.

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Chapter 15.2

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  1. Chapter 15.2 A Few Outcomes of Gene Controls AP Biology Fall 2010

  2. X Chromosome Inactivation • In mammalian females, the gene products of only one X chromosome are needed • The other is condensed and inactive • Called a Barr body

  3. X Chromosome Inactivation • Where does a female get each of her X chromosomes from? • One from: _____________ • One from: _____________

  4. X Chromosome Inactivation • In some cells the parental X chromosome is inactivated • In other cells the maternal X chromosome is inactivated • Each adult female is a mosaic of X-linked traits • Mosaic tissue effect

  5. X Chromosome Inactivation • When alleles on two homologous X chromosomes are not identical, patches of tissues through the body often show variation • This mosaic effect is seen in human females affected by anhidrotic ectodermal dysplasia • In which a mutant gene on one X chromosome results in patches of skin with no sweat glands • When sweat glands are absent, mutant allele is on the active X chromosome

  6. X Chromosome Inactivation • Calico Cat • Colouration is contributed to by X inactivation in select cells • Heterozygous for a certain coat colour allele on their X chromosomes • The Calico females have one X that bears a dominant allele for black fur pigment and another X that bears an allele for orange fur pigment • Various cells have one or the other X chromosome inactivated during embryogenesis, contributing to a random colour pattern in the felines

  7. X Chromosome Inactivation • Dosage compensation: name for the process that shuts down one X of the female so that the overall expression of the remaining X of the female is equivalent to the lone X of the male • Gene control mechanism • Nomal development of females depends on this control

  8. X Chromosome Inactivation • How does one of two X chromosomes get shut down? • Methylation of histones and action of XIST (X linked gene) • XIST product, large RNA molecule, sticks like masking paint to chromosomal DNA • Do not know why, but the XIST gene on only one of the two chromosomes is active • Chromosome and genes get painted with RNA, other one remains paint-free and its genes remain available for transcription • Critical thinking #9: puts twist on generalized picture

  9. Gene Control of Flower Formation • Plants have gene controls too • Studies of plant mutations in Arabidopsis thaliana support an ABC model of floral development and specialization

  10. A = wild B = mutant C = wild D = wild E = mutant F = mutant G = mutant

  11. Gene Control of Flower Formation • Three sets of master genes (A, B, C) guide flower development in a particular pattern • Cell differentiation in the plant depends upon which genes of the ABC group are activated • Support how all the specialized parts of a flower develop in a predictable pattern • Whorls of new tissues become sepals, petals, stamens, and carpels • Page 235

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