Molecular Biology & Genetics 3- Errors in Meiosis & X inactivation Flashcards
What is aneuploidy?
An abnormal number of a particular chromosome
What is an example of aneuploidy?
Down syndrome which means individuals have 3 of chromosome 21 (trisomy 21)
How does age of the mother affect Down syndrome?
The frequency of cases increases as the age of the mother increases and 40% of cases are in mothers over the age of 45
What is nondisjunction?
The failure of chromosomes to separate properly during meiosis. It may occur during Meiosis I or Meiosis II and effects the gametic products
What are the gametic products if nondisjunction occurs during Meiosis I?
2 of the resulting games have an extra chromosome (n+1) and 2 have one less chromosome (n-1)
What are the gametic products if nondisjunction occurs during Meiosis II?
1 of the resulting gametes has an extra chromosome (n+1), 1 has one chromosome less (n-1) and 2 have the correct number (n)
What is the most common composition of chromosome 21 for Down syndrome babies?
95% of Down syndrome babies have 2 maternal and 1 paternal chromosome 21
What is Klinefelter syndrome XXY?
When individuals have one too many sex chromosome
What is the phenotype of an individual with Klinefelter Syndrome XXY?
Male with a slightly feminine look
What is Turner syndrome XO?
Individuals have one less sex chromosome
What is the phenotype of an individual with Turner syndrome XO?
Phenotypically female but have intellectual impairment and behavioural differences of which the severity depends on whether the X chromosome was passed on from the mother or father
What is polyploidy?
When individuals have a set of chromosomes too many
What is polyploidy common in?
Plants
How does polyploidy occur?
When the gametes produced are unreduced (haven’t gone through meiosis) and so fertilisation could make a tetraploid zygote
What is the fertility of polyploid gametes?
They are likely to be fertile because each chromosome has something to pair up with
What is autopolyploid?
When self-fertilisation occurs
What must a polypoid of two species have?
4n so that each chromosome has something to pair up with
What are the four types of chromosomal aberrations?
Deletion, Duplication, Inversion and translocation
What is deletion?
When a segment of chromosome is removed
What is duplication?
When a segment of chromosome is repeated
How can deletion and duplication occur?
During crossing over if not done correctly
What is inversion?
When a segment of chromosome is reversed
What is translocation?
When a segment is moved from one chromosome to another non-homologous chromosome. It can be reciprocal where segments swap positions
Why may inversion cause problems in meiosis?
Although the genes are all still present, problems may be caused during meiosis because the chromosomes don’t always match correctly
What happens as a result of crossing over of inverted segments?
Insertion and deletion are likely because of “bubbles” where chromosomes don’t fit together correctly in homologous pairs don’t cross over correctly. Gametes may therefore be inviable/infertile
What is the effect of translocation on meiosis?
When parts of chromosomes move to another location it means they can’t be regulated properly (turned on and off)
What are examples of translocation?
Philadelphia Translocation where parts of chromosome 9 and 22 swap and Familial Down syndrome where parts of chromosome 14 and 21 fuse
What happens in Philadelphia Translocation?
95% of patients get chronic myeloid leukaemia but TK over-expression is treatable with Gleevec in 90% of cases
What happens in Familial Down syndrome?
Some parts of the fused chromosomes are also lost. This means there isn’t always a full extra chromosome 21
What are the possible zygotes from someone with a fused chromosome 14 and 21?
Lethal, Downs syndrome, translocation carrier and normal
What is the difference between Down syndrome and familial Down syndrome?
Familial Down syndrome may be passed on to offspring because of the fact that it produces carrier gametes
What is a Barr body?
A black patch found on the edge of the nucleus which consists of the extra X chromosome in females that has been condensed to become inactive
What is added to the inactive X chromosome and why?
Molecular markers to signal to the cell that it should not be used
What is random?
Whether the maternal or paternal X chromosome is inactivated. Therefore traits which are coded for by the X chromosome are a mosaic throughout the individual
What is mapped to the X chromosome?
Many genetic diseases, however, most do not show a mosaic effect in carrier females because the gene products can move around the body
