This set of Cytogenetics online quiz focuses on “Dosage Compensation Mechanism for X linked Genes”.
1. Both Drosophila and mammals have XY system of sex determination. Dosage compensation y inactivation of one X chromosome is seen in ___________________
Explanation: Only in case of mammals dosage compensation mechanism is by inactivation of one X chromosome. In Drosophila there is a different mechanism.
2. Barr bodies are found ________________
a) In the cytoplasm of female
b) In the nuclei of female
c) In the cytoplasm of male
d) In the nuclei of male
Explanation: The inactivated X chromosome in female forms a highly condensed structure called the Barr body which is found in the nuclei. As male have only one X chromosome is no inactivation and no Barr body.
3. The X inactivation in female occurs by the process of lyonisation.
Explanation: In female one of the two X chromosomes in somatic cells get condensed and forms the Barr body. This concept was extended by Lyon hypothesis and the mechanism is thus called Lyonisation.
4. If a chromosome loses its XIC due to deletion it will_______________________
a) Always be activated
b) Never be activated
c) Form Barr body
d) Be degraded
Explanation: XIC is X inactivation centre, only one X with XIC stays active in a cell. Now is a chromosome loses is XIC that will not be counted as X chromosome and will stay active.
5. Due to the translocation process, the XIC from one X chromosome in a woman passed to chromosome 3. The individual died. What were the probable reasons of death?
a) The X chromosome was unable to show its effect
b) Both X chromosome being active too much gene product was produced
c) Inactivation of chromosome 3
d) Hyper activation of autosomes
Explanation: As XIC translocated to chromosome 3 now it can be counted as an X chromosome. In some cells randomly this chromosome 3 is condensed and inactivated which prevents the important autosomal gene product from being formed. Thus the individual dies.
6. Lysonization takes place ____________
a) 7th day after fertilization
b) 15th day after fertilization
c) 1st month after fertilization
d) 6th month after fertilization
Explanation: Lysonization process takes place in the 15th day after fertilization when the embryo is 500-1000 cell stage. This inactivated one X chromosome in each cell.
7. _____________________ determines which X chromosome remains active.
Explanation: XCE or X controlling element that is present in the XIC region determines which X chromosome will be inactivated.
8. Which one of the following is the only active gene in the inactivated X chromosome?
c) Histone coding gene
d) Ribosomal gene
Explanation: The X inactive specific transcript (XIST in humans/ Xist in mice) gene, is located in the XIC, it’s expressed exclusively from the inactive X chromosome, producing a 17 kb non-coding RNA that is not translated. Ribosome genes are in nucleolus.
9. X inactivation is an example of epigenetic phenomenon.
Explanation: During x inactivation the X chromosome is converted to heterochromatin by histone remodeling. This is a heritable character that is not present in the DNA sequence but histone code. Thus it is an epigenetic phenomenon.
10. XIST mRNA coats the inactive X and calls in _____________
a) tRNA and ribosome
b) Other mRNA
c) Deacetylase and specific methylase
d) Acetylase and demethylase
Explanation: Histone deacetylases lead to the removal of acetyl group from specific histone residues. This leads to exposure of positive charges on then and causes stronger binding. Specific methylase also plays similar role. Together they help in heterochromatin formation.
11. Tortoise shell or calico cats are always ________________ for O locus that is responsible for orange color development.
Explanation: The O gene is responsible for orange color development. Only dominant O can express it over the mutant B gene that causes black color. O gene is located in X chromosome. Thus in heterozygous condition if O carrying X chromosome is inactivated it leads to black patch as seen here.
12. Calico cats are always______________
Explanation: In calico cats there is development of black and orange and white patches. This is possible only when the dominant O allele is turned off in some X chromosome due to dosage compensation. As males have only one X chromosome they are never turned off. Thus, only females show this trait.
13. The XXY klinefelter survive but trisomy in most autosome is lethal. Why?
a) Autosome carries more genes
b) Autosomes have better dominant recessive relations
c) X chromosomes are inactivated
d) Autosomes are more in number
Explanation: Due to dosage compensation only one X chromosome in XXY says active. So the effect of excess X chromosome is somewhat mitigated. In autosome there is no such inactivation and excess of gene products leads to lethality.
14. Which one of the following is true for Drosophila?
a) They don’t have X chromosome
b) One of the female X chromosome is inactivated
c) The male Y chromosome is inactivated
d) The male X chromosome is hyper active
Explanation: In Drosophila, dosage compensation is achieved in a different way. The male’s X chromosome appears to be hyperactivated, allowing it to be transcribed at twice the rate of either X chromosome in the female.
Sanfoundry Global Education & Learning Series – Cytogenetics.
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