Lecture 22 - X Inactivation Flashcards
For an x-linked trait, females have twice the gene product of a male. How does the balance of the products of X-linked genes maintained?
In early embryonic developments, one of the X chromosome is randomly switched off. This inactivated X chromosome is called the Barr Body, and all cell lines derived from this cell will have the same X inactivated.
Explain the concept of mosaicism in female.
If a female is heterozygous for a particular X-linked recessive mutation, the inactivation of random X chromosomes will lead to some cell expressing the normal copy of the gene, while some possessing the unexpressed recessive allele.
Describe the mechanism of X Inactivation.
XIST(X Inactivation Specific Transcript) is a gene locus in the X chromosome which is only transcribed in the inactivated X. RNA produced by the XIST coats the X chromosome and modifies the chromatin, forming a heterochromatin (condensed and methylated), causing the genes to be unexpressed.
Turner syndrome is the genetic abnormality in which a female has only one X chromosome. If X Inactivation does occur, then only one X chromosome would be expressed with the other deactivated. Then, why does Turner Syndrome still displays abnormal phenotype?
The two X chromosomes are active until the 500 cell stage, which may be critical to normal development. Other than that, not all genes are inactivated, so some genes require the presence of two X chromosomes to be expressed normally.
Define atypical lyonization.
Atypical lyonization occurs when heterozygous females may show symptoms of an X-linked recessive trait, if by chance more X chromosomes with the normal allele is turned off.
Define epistasis.
Epistasis: the phenomenon where the presence of one gene determines the expression of the other.
Give examples of sex influenced traits and sex limited traits.
Sex influenced traits:
- Male pattern baldness
- POCD (Polycystic Ovary Disease)
Sex limited traits:
- Precocious puberty in males
- Androgen insensitvity
