Errors In Meiosis + X-inactivation Flashcards
Down syndrome chromosome
Trisomy 21
PHENOTYPICALLY female and male
How many live births
1/750
Aneuploidy definition
Aneuploidy = abnormal number of a particular chromosome
What age categorie have most cases
Mothers over 45 have 40% of all cases
Nondisjunction definition
Failure of chromosomes to separate properly during meiosis
___ of Down syndrome babies have ___ maternal chromosome _ _
95% of Down syndrome babies have 2 maternal chromosomes at chromosome 21 (from nondisjusction)
Two places nondisjunction can occur
1: nondisjuction at first distinction:
- two chromosomes don’t seperate and end up in one cell - one cell empty - both divide producing n-1, n-1, n + 1, n + 1
2: nondistunction at second division:
- two chromosomes split into two cells at first divison in meiosis 1 - then in meiosis 2, two identical homologous pairs go into one cell, producing - n+1, n-1, n, n
Klinefelter Syndrome
- two X chromosomes and one Y
XXY - feminising features but tall, long limbs and small testes - frontal baldness.
PHENOTYPICALLY MALE
Polyploidy definition
- possession of multiple sets of chromosomes (whole set of chromosomes being multiplied in some way
- offspring with polyploid karyotypes may be viable and self-fertile
What happens in polyploidy - fertile
Karyotype of parent species:
2n = 6
Meiotic error occurs produces 2 sets of unreduced gametes with 6 chromosomes
They self fertilise forming an autopolyploid zygote
4n = 12 (tetraploid)
They are viable and self fertile as they have pairs of chromosomes that can line up and seperate properly during meiosis
What happens in polyploidy - sterile
Gametes each have 9 haploid chromosomes - but the different chromosomes from different parents don’t line up properly
To form a sterile hybrid:
N+n = 9 + 9 = 18 (2n)
- sterile because chromosomes can’t line up properly
To form fertile aphidiploid (aphidiploid = when chromosome is double parental chromosome number):
2n + 2n = 18 + 14 = 36 (4n)
Turner syndrome
1 X chromosome
- will have mental impairment but the severity of the impairment is dependent on whether the X cane from mum or dad.
- phenotypically female
Aneuploidy
Having an abnormal amount of chromosomes in a haploid set
- one too many or one too few
Why can’t bananas reproduce
- they can’t go through meiosis as they are triploids
- on pair doesn’t have anything to line up with so you end up with gametes with many different combos of chromosomes
- triploid pathenogenetic lizard
Chromosomal aberrations
Loss, gain or rearrangement of parts of chromosomes
Deletion
- removes a chromosomal segment
E.G: Lejeune syndrome - cri do chat
- Deletion of tip of short arm of chromosome 5
Inversions
- reverses a segment within a chromosome
- has all the same genes and is phenotypically fine but have fertility issues as they can not line up properly - as a result you get a chromosome where part has been deleted or where a part has been doubles
E.g: cromosome 3
Translocation
- moves a segment from one chromosome to a non homologous chromosome
E.G: Philadelphia translocation t(9;22) - present in 95% of patients with chronic myeloid leukemia
- reciprocal translocation as they both swap parts of the end of their chromosomes
- the break has occurred in the middle of a particular gene which has lead to the over expression of a gene - regulation of the gene has failed so it is on the whole time - the cell divides too much leading to cancer
TK overexpression is treatable with Gleevec in 90% of cases
Familial Down Syndrome
- occurs via translocation not spontaneously from nondisjuction in meiosis
- t(14;21) - individual with this type of Down syndrome’s mother has a fused chromosome 14 and 21 (with bits missing) (concentric fusion) but has a normal phenotype but is a carrier for familial Down syndrome - chromosomes can’t line up properly producing large combination of different gametes. - lethal (when gametes don’t contain 2 of chromosome 21 or 14, or when they’re are 3 copies of 14) could procure a carrier, could produce a normal and when they have a trisome - 3 copies of chromosome 21 they have down syndrome
- normal amount of chromosomes
X - inactivation
- shout down one X in females - not all of it just large parts
- bar bodies in the end be of the nuclear membrane - condensed X chromosome that has been shut down (dark spot)
- one must be deactivated in order for proper development to occur
- none in males
- occurs when a female embryo is 4 days old and consists of 100 cells
(In very rare cases when there are 3 X’s you’ll find 2 X’s shut down)
How the X chromosome is deactivated
- DNA is packed closer together
- modification to dangly tails of histones that signal this inactivation - new structural protiens are added to bind things even closer together - markers added to DNa to signal to it that is isn’t to be read - makes it hard fir molecular machinery to acess thus it is switched off
What does the random inactivation of either paternal or maternal X chromosome result in?
- as cells devide they maintain the active X chromosome they had inside - continues on into adult hood - thus making women stipey
- on cat you can see this as the X chromosome coded for coat colour
Epigenetics
Changes in gene expression due to factories other than changes in the DNA sequence
Absence of sweat glands in humans
- absent in a random mosaic pattern which represents where the gene has been inactivated
Why the fact that many genetic diseases come from the X chromosome doesn’t effect females as much as males
- most genetic diseases do not show a mosaic effect in carrier females because the gene products can be moved around the body
Polyploidy definitions
Abnormal number of chromosomes sets (normal 2, diploid)
In polyploidy if the chromosomes are not homologous:
No meiosis
