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Barr Body & Lyon Hypothesis MCQ - Practice Questions with Answers
Edited By admin | Updated on Sep 18, 2023 18:34 AM | #NEET
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What is a Barr Body?
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Dosage Compensation, Barr Body & Lyon Hypothesis
- In man it has been found that Y-chromosomes are genetically inert in comparison to the X-chromosomes and other chromosomes and only a few genes are present in the human Y-chromosome.
- As per the chromosomal make-up of human male and female, females contain a higher dose of functional gene containing chromosome than males (Female chromosome numbers = 44 + XX and Male chromosome number = 44 + XY).
- The geneticists have observed that in some cases, female homozygous for the genes in the X-chromosomes do not express a trait more markedly than do hemizygous males.
- So, it must be a mechanism of “dosage compensation”, through which the effective dosage of genes of the two sexes is made equal or nearly so.
- This mechanism of compensating the differential doses of functional sex chromosomes in male and female human is effected by the inactivation of one X-chromosome in the normal female.
- The genetically inactive X- chromosome or condensed X-chromosome is called heterochromatin or sex-chromatin body or Barr body.
- Of the two X-chromosomes in females, which X-chromosome becomes inactive is a matter of chance, but it should be remembered that once an X- chromosome has become inactivated, all cells arising from that cell will keep the same inactive X-chromosome.
- The inactive-X hypothesis or the Lyon’s hypothesis or the Dosage Compensation is widely known from 1961 which states that only one of the two X chromosomes in the homogametic sex is functional while the other condenses and is inactivated.
- The X inactivated in some cells would be that from the father, in other cells it would be that from the mother.
- Hence any tissue in the body of a woman would be a mosaic of cells which would show dominance of all genes having diffusible products.
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