WEEK 6 Flashcards
What is the main process that deactivates the X chromosome?
Methylation
What is the Lyon hypothesis?
- Condensed X chromosome is genetically inactive
- Inactivation is mitotically stable
- X activation in humans occurs early in development when embryo consists of about 32 cells (1 to 2 days following fertilisation)
- In early development, one of the X chromosomes is randomly inactivated
- Net effect of this is to equalise phenotypes in males and females for genes that are carried on the X chromosomes
Why are Barr bodies only found in females and not in males?
Barr bodies is X chromosome linked and the rule of Barr bodies is that one must always be activated and males only have one X chromosome
What is Aneuploidy?
An abnormal number of chromosomes such as having a single extra chromosome (47) or a missing chromosome (45)
What is Turner’s syndrome and what are the symptoms?
A condition that affects only females and results when one of the X chromosomes is missing or partially missing
SYMPTOMS:
- Short
- webbed neck
- lymphedema in feet
- menopause before first period
- low chances of fertility (possible with IVF)
What is an example of a disease where Barr bodies cannot occur?
Turner’s syndrome
What is the main cause of Aneuploidy?
Nondisjunction
The failure of chromosomes to disjoin normally during meiosis
Who’s chromosome is usually missing in Turner’s syndrome?
The father’s X chromosome
What is Klinefelter syndrome and what are the symptoms?
Klinefelter syndrome (sometimes called Klinefelter’s, KS or XXY) is where boys and men are born with an extra X chromosome
SYMPTOMS:
- Testicular atrophy (small, firm testes)
- Tall structure with eunuchoid proportions (delayed epiphyseal closure - increased bone length)
- Gynecomastia
- Development delay
- Presence of inactivated X chromosome (Barr body)
- Common cause of hypogonadism seen in infertility workup
- Increased risk of breast cancer
What is ‘Double Y syndrome’ and what are the symptoms?
XYY syndrome is a rare chromosomal disorder that affects males. It is caused by the presence of an extra Y chromosome
SYMPTOMS:
- Less than average IQ
- Tall
- High levels of male hormones
- Large ears
- Sunken chest
- Delayed speech and language skills
Barr body is the ___________________
Inactive X chromosome
Where can Barr bodies be easily demonstrated in female human cells?
Buccal mucosa (cheek cells) or in fibroblasts (undifferentiated connective tissue cells)
What is a Barr body?
a small, densely staining structure in the cell nuclei of females, consisting of a condensed, inactive X chromosome
Define the Lyon hypothesis
Inactivation of X chromosomes occurs randomly in somatic cells at a point early in embryonic development (most likely sometime during the blastocyst stage of development). Once inactivation has occurred, all descendant cells have the same X chromosome inactivated as their initial progenitor cell.
In which stage of the cell cycle can the Barr body be identified?
Interphase
