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
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