Problem 11
What is a Barr body? How is its structure different from that of other chromosomes in the cell? How does the structure of a Barr body affect the level of \(\mathrm{X}\)-linked gene expression?
Problem 13
Describe when X-chromosome inactivation occurs and how this leads to phenotypic results at the organism level. In your answer, you should explain why XCI causes results such as variegated coat patterns in mammals. Why do two different calico cats have their patches of orange and black fur in different places? Explain whether or not a variegated coat pattern due to \(\mathrm{XCI}\) could occur in marsupials.
Problem 14
Describe the molecular process of X-chromosome inactivation. This description should include the three phases of inactivation and the role of the Xic. Explain what happens to the X chromosomes during embryogenesis, in adult somatic cells, and during oogenesis.
Problem 15
On rare occasions, a human male is born who is somewhat feminized compared with other males. Microscopic examination of the cells of one such individual revealed that he has a single Barr body in each cell. What is the chromosomal composition of this individual?
Problem 16
How many Barr bodies would you expect to find in humans with the following abnormal compositions of sex chromosomes? A. \(\mathrm{XXY}\) B. \(\mathrm{XYY}\) C. XXX D. X0 (a person with just a single \(\mathrm{X}\) chromosome)
Problem 17
Certain forms of human color blindness are inherited as X-linked recessive traits. Hemizygous males are color-blind, but heterozygous females are not. However, heterozygous females sometimes have partial color blindness. A. Discuss why heterozygous females may have partial color blindness. B. Doctors identified an unusual case in which a heterozygous female was color-blind in her right eye but had normal color vision in her left eye. Explain how this might have occurred.
Problem 18
A black female cat \(\left(\mathrm{X}^{B} \mathrm{X}^{B}\right)\) and an orange male cat \(\left(\mathrm{X}^{0} \mathrm{Y}\right)\) were mated to each other and produced a male cat that was calico. Which sex chromosomes did this male offspring inherit from its mother and father? Remember that the presence of the \(Y\) chromosome determines maleness in mammals.
Problem 19
What is the spreading phase of \(\mathrm{X}\)-chromosome inactivation? Why do you think it is called a spreading phase?
Problem 20
When does the erasure and reestablishment phase of genomic imprinting occur? Explain why it is necessary to erase an imprint and then reestablish it in order to always maintain imprinting from the same sex of parent.
Problem 21
In what types of cells would you expect de novo methylation to occur? In what cell types would it not occur?
