Lecture 21 Flashcards
What is a karyotype? How is one collected?
A karyotype is an ordered, visual representation of the chromosomes in a cell, it is gotten by taking a blood sample, treating the cells with mitogen, then cholchicine and then staining, (then organising the chromosomes).
What are the two major seperations of the chromosome types? What are the subdivisions based on centromere location?
The two major groups are the autosomes (1-22) and the sex chromosomes (X and Y). The chromosomes can be further arranged based on centromere location:
Metacentric: centromere roughly in the middle
submetacentric: arm lengths are unequal
acrocentric: short arm is so small that its difficult to see, but still present
telocentric:centromere located at the erminal end of the chromosome.
subtelocentric: centromere located closer to end then centre.
holocentric: entire length of chromosome acts as centromere.
Note that humans only have metacentric, acrocentric and submetacentric.
What is downs syndrome? What symptoms does it have and what is the biggest risk factor?
Downs syndrome is genetic disorder caused by three copies of chromosome 21, it occurs in 1/750 live births and has phenotypic differences like: short fingers, strong crease on hand lifeline, tends to lead to some form of mental retardation which can vary in severity, slanted eyes, shortened nose, increased risk of heart disease. Risk increases heavily with the age of the mother and can be found in other animals.
What two fetal testing options are there for downs syndrome?
Aminocentesis: done between 16-20 weeks, involves collection of fetal cells by extracting amniotic fluid and subsequent biotic testing via generation of a karyotype after centrifuging. Has a 0.1% chance of miscarriage.
Chorionic villus sampling: can be done as early as 10-13 weeks, fetal cells are collected from chorionic villus to generate a karyotype, has a 1% chance of miscarriage.
What is downs syndrome typically caused by?
Non disjunction of chromosome 21 at either first or second meiotic division (95% of downs syndrome have two maternal copies of chromosome 21).
What is klinefelter syndrome? What symptoms does it have?
Klinefelter syndrome is caused by possession of two X chromosomes and one Y chromosome, individuals with this will have: tall stature, slightly feminized physique, mildly impaired IQ, tendency to lose chest hairs, female type pubic hair, testicular atrophy, osteoporosis, breast development, poor beard growth and frontal baldness absence on the forehead.
What is turner syndrome? What are some symptoms?
Turner syndrome is caused by possession of only one X chromosome as their sex chromosomes (XO). Symptoms include: short stature, low hair line, characteristic facial features, heart issues, low breast development, no menstruation, weirdly arranged hands and brown spots on the skin known as nevi.
What are some common human aneuploids?
Down syndrome (trisomy 21), Patau syndrome (trisomy 12), Edward syndrome (trisomy 18), klinefelter syndrome (XXY), Turner syndrome (XO), Supernumary Y (XYY), supernumary X (XXX).
What do these terms mean: aneuploidy, monosomy, trisomy?
aneuploidy: an abnormal number of chromosomes.
Monosomy: only one chromosome from a chromosome pair.
trisomy: one more chromosome from a chromosome pair (3).
What are barr bodies? Why do they exist and at roughly which stage do they form?
One X chromosome is perfectly functional for providing necessary proteins as males are fine with just one. Females have two and as such this dosage compensation is achieved by inactivation of one X chromosome in each cell (known as lyonisation). This inactivates X chromosome condenses heavily, leading to dark spots in the nucleus known as barr bodys based on how many X chromosomes are extra, one for each X chromosome above one. X inactivation occurs at roughly the 500-1000 cell stage in embryonic development and is random for each cell, all cells after this point will have the same inactivated X chromosome as the parent cell.
How does X inactivation occur?
Methyl groups are added to the DNA of the inactivated X chromosome and it is also densely packed while the activated X chromosome is more widely spread out.
Why are women safer from recessive X linked traits?
Recessive genes typically code for a lack of a protein, if a man has a copy of this gene he will not produce the protein as he only has one X chromosome, however if a woman has a copy of this recessive X linked genes then half the cells will have this copy active while he other cells won’t, these other cells will still be able to produce the necessary amount of the protein and it will circulate around the body, hence no affect is noticable.
