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A Few Outcomes of Gene Controls

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1 A Few Outcomes of Gene Controls
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: _____________

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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