X-inactivation Flashcards
Monosomic
Missing one chromosome (2n-1)
Trisomic
Extra chromosome present (2n+1)
Down syndrome
Trisomy 21, incidence increases with maternal age
Klinefelter syndrome
XXY, extra X chromosome
Turner syndrome
XO, single X chromosome
Deletion
Loss of chromosome segments; can lead to severe effects
Duplication
Extra segment; often harmful effects
Inversion
Segment of chromosome inverted; may disrupt gene order but not quantity
Translocation
Segment attaches to a non-homologous chromosome; can affect gene expression
Meiosis I
Homologous chromosomes fail to separate
Meiosis II
Sister chromatids fail to separate
nondisjunction
Failure of chromosomes or chromatids to separate properly during cell division
aneuploidy
A condition where a zygote has the wrong number of chromosomes
Polyploidy
when a cell or organism has more than two complete sets of chromosomes
What is the Lyon Hypothesis?
It proposes that one X chromosome in females is randomly inactivated early in development to balance gene dosage between sexes.
What is a Barr body?
A condensed, inactivated X chromosome found in the nuclei of female cells.
Where are Barr bodies found?
Against the inner nuclear envelope in female somatic cells.
What does X-inactivation lead to in terms of gene expression?
It causes mosaic gene expression in females because different cells may express different active X chromosomes.
What is the purpose of X-inactivation?
To balance the expression of X-linked genes between males (XY) and females (XX).
What is the evidence for the Lyon Hypothesis?
In sex chromosome aneuploidies (e.g. XXX, XXY), only one X is active, and the rest form Barr bodies.
What kind of inheritance pattern is associated with X-inactivation?
X-linked inheritance — particularly visible in heterozygous females due to mosaic expression.
Why do most X-linked diseases not show mosaic effects?
Because the gene products often circulate systemically in the body
Give an example of X-inactivation mosaicism in humans.
Anhidrotic ectodermal dysplasia – heterozygous females have patches of skin without sweat glands.
What is the sex and expected Barr body presence in someone with XXY?
Male with one Barr body in each nucleus — associated with Klinefelter syndrome.
