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1. DOSAGE COMPENSATION IN MAMMALS
Presented by :-
Name- Ipsita Priyadarshini panda
Branch- M.Sc. In Zoology
School- School of Applied Sciences
Regd. No.- 220705180046
Subject- Genetics and Epigenetics
Code- CUTM1454

2. CONTENTS :-
• Introduction
• Methods of Dosage Compensation
• Dosage Compensation in humans
• Lyon Hypothesis
• Formation of Barr bodies
• Discussion
• Conclusion
• References
• Acknowledgment

3. Introduction
• Dosage compensation is the mechanism to create balance between the sex chromosomes
of organisms that have different set of sex chromosomes.
• It is established that humans, most mammals and some other species follow the XY system
of sex determination.
• Human females have 22 pairs of autosomes and 1 pair of X chromosomes (XX); where as
males differ in having only 1 X and 1 Y sex chromosome(XY).

4. Methods of Dosage Compensation
• It can be done in three ways:-
1. Two fold up-regulation in the expression of the X-linked genes in male.
- When transcribed twice it will compensate for that extra X-chromosome that is
present in the female(XX).
2. Two fold down-regulation in the expression of X-linked genes in females.
- If the XX chromosome half their transcription ,it can compensate for the XY
chromosome in males.
3. Complete inactivation of one of the two X-chromosomes present in the females
and formation of Barr bodies.
- If one of the X-gene is completely silenced, it will compensate for only one X-
chromosome in the males.

5. Dosage compensation in humans
• As is evident, females have two copies of the X-chromosome where as male possess
only one. Moreover, X-chromosomes are larger than Y-chromosomes.
• Also, X-chromosomes have more genes on them than Y-chromosome, so there is
possibility of the X-linked genes to be transcribed twice as compared to males as males
have only one X-gene.
• The nature takes care of this inequality and the effect of unequal distribution of X-
chromosome is fulfilled by dosage compensation.

6. Fig.1- X-chromosome inactivation in mammals

7. Lyon Hypothesis
• The process of dosage compensation or inactivation of a X-chromosome in females (XX)
begins in the somatic cells in early embryonic developmental stages (i.e- the blastocyst
stage of development).
• The process of X- Chromosome inactivation is random (i.e- the blastocyst stage of
development).
• The process of X-chromosome inactivation is random (i.e- X-chromosome from any of the
parents can be inactivated).
• The inactivated X-chromosome is called Barr body or 'sex-chromatin body’.

8. Formation of Barr bodies
• Barr bodies was first observed by Murray L. Barr and Ewart G. Bertram.
• Susumu ohno was the first to predict that Barr bodies results due the inactivated
X-chromosomes in females.
• This prediction was affirmed when no Barr bodies were seen in the somatic cells of turner
(45, X) females ; one in seen in Klinefelter’s (47, XXY) syndrome; two in (47, XXX)
females, and so on.
• The number of Barr bodies follows as (N-1) rule, where N is the total number of X-
chromosomes present.
• If one of the X-chromosome in females is inactive it can compensate the expression of
genes in males and females making it equal.

9. Fig. 2 – formation of Barr bodies in humans

10. Discussion
• The presence of different numbers of X chromosomes in males and females presents a
special problem in development.
• Because females have two copies of every X-linked gene and males possess one copy, the
amount of gene product (protein) from X-linked genes would normally differ in the two
sexes: females would produce twice as much gene product as males.
• This difference could be highly detrimental because protein concentration plays a critical
role in development.
• Animals overcome this potential problem through dosage compensation, which equalizes
the amount of protein produced by X-linked genes in two sexes.

11. Conclusion
• The process of dosage compensation or equalization of the expression of genes
between different biological sexes in an organism is facilitated through numerous
pathways.

12. References
• Muller, HJ (1932). "Further studies on the nature and causes of gene mutations".
• The Epigenetics Revolution: How Modern Biology is Rewriting Our Understanding
of Genetics, Disease and Inheritance Book by Nessa Carey.
• Epigenetics: How Environment Shapes our Genes Richard C. Francis, 2011.
• The Developing Genome: An Introduction to Behavioral Epigenetics by. David S.
Moore.

13. Acknowledgement
• Special thanks to My Subject teacher, Dr. Gagan Kumar Panigrahi, School of Applied
Sciences , CUTM, Bhubaneswar.
• All the faculty members of Department of Zoology, School of Applied Science, CUTM,
Bhubaneswar.
• Family and Friends.