Errors In Meiosis + X-inactivation Flashcards
(27 cards)
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
