Mechanisms of Chromosomal Abnormality ✅ Flashcards Preview

TAS Genetics ✅ > Mechanisms of Chromosomal Abnormality ✅ > Flashcards

Flashcards in Mechanisms of Chromosomal Abnormality ✅ Deck (42)
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1
Q

What is mitosis?

A

The process by which most cells replicate

2
Q

What does mitosis result in?

A

The production of 2 identical daughter cells

3
Q

What does each of the daughter cells from mitosis contain?

A

A diploid genome (46 chromosomes)

4
Q

What is meiosis?

A

The process by which gametes are formed

5
Q

What does meiosis involve?

A

2 successive cell divisions

6
Q

What do the cells produced in meiosis contain?

A

Haploid genome (23 chromosomes)

7
Q

Is cell division perfect?

A

No, errors do arise

8
Q

What are the mechanisms of chromosomal abnormality?

A
  • Non-disjunction
  • Anaphase lag
  • Mosaicism
  • Chromosome translocations
  • Reciprocal chromosome translocations
  • Robertsonian translocations
9
Q

Why do numerical chromosome abnormalities occur?

A

Generally because of non-disjunction during meiosis

10
Q

What is non-disjunction?

A

When two paired chromosomes (in meiosis I) or two sister chromatids (in meiosis II) fail to separate and segregate into separate cells

11
Q

What is the result of non-disjunction in meiosis?

A

The gametes will have either too many (24) or too few (22) chromosomes

12
Q

What happens following fertilisation in a gamete with too many or too few chromosomes?

A

The zygote is either trisomic or monosomic for the relevant chromosome

13
Q

Can an embryo be trisomic for every chromosome?

A

Yes

14
Q

What happens if an embryo is trisomic for every chromosome?

A

Most will spontaneously abort

15
Q

Does non-disjunction occur in men or women?

A

It can occur in either

16
Q

What do most cases of trisomy seem to take place because of, in terms of non-disjunction?

A

Non-disjunction at meiosis I in the mother

17
Q

Why do most cases of trisomy take place because of non-disjunction at meiosis I in the mother?

A

Due to this stage being very long in women - starting before birth, and ending at ovulation

18
Q

What happens to the risk of Down syndrome with maternal age?

A

Increases, obviously.

1 in 1500 for 20 year old woman
1 in 30 for 45 year old woman

19
Q

Are all aneuplodies affected by age?

A

Similar age-dependence for all aneuplodies except Turner’s syndrome

20
Q

What is an aneuploidy?

A

The presence of abnormal number of chromosomes in a cell

21
Q

By what mechanism does Turner’s syndrome occur?

A

Anaphase lag

22
Q

What happens in anaphase lag?

A

One of the sex chromosomes (X or Y) moves too slowly to the pole of the daughter cell during cell division. It therefore ends up outside the nucleus, and is broken down

23
Q

When can anaphase lag occur?

A

During meiosis, or during early mitotic divisions of the embryo

24
Q

What does chromosomal mosaicism refer to?

A

The presence of two cell lines, with differing numbers of chromosomes in each, usually one normal and one abnormal

25
Q

Give 2 examples of mosaic chromosomal disorders

A
  • Mosaic Down’s syndrome (46 XX/47 XX+21)

- Mosaic Turner’s syndrome (45,X/46, XX)

26
Q

What does mosaicism usually result in?

A

Milder phenotypes

27
Q

How can mosaicism occur?

A

When the abnormal cell line arises post-zygotically

- When aneuploidy is present from conception, but some cells revert to normal karyocyte

28
Q

Give an example of how aneuploidy can be present from conception, but some cells revert to normal karyotype?

A

Losing a copy of a trisomic chromosome - called trisomy rescue

29
Q

When is a chromosomal translocation described as ‘balanced’?

A

When there is no net loss or gain of genetic material

30
Q

When is a chromosomal translocation described as unbalanced?

A

If there is deletion or duplication of genetic material

31
Q

Why are balanced translocations important to recognise?

A

They can result in transmission of an unbalanced chromosome translocation to the offspring

32
Q

Can balanced translocations cause disease?

A

Occasionally

33
Q

When can balanced translocations cause disease?

A

If a translocation breakpoint disrupts a disease gene

34
Q

When do reciprocal chromosome translocations arise?

A

When any two chromosomes swap non-homologous segments

35
Q

What might happen in a carrier of a reciprocal translocation?

A

They may potentially have offspring with trisomy of one of the translocated segments and monosomy of the other

36
Q

What are Robertsonian translocations?

A

Translocations involving the ‘acrocentric’ chromosomes

37
Q

Which chromosomes are acrocentric?

A
13
14
15
21
22
Y
38
Q

What is an acrocentric chromosome?

A

One that has it’s centromeres close to one end

39
Q

What happens in a Robertsonian translocation?

A

Inappropriate non-homologous recombination means that 2 acrocentric chromosomes join to form a single fusion chromosome with breakpoints on the short arm, just above the centromere

40
Q

Why are balanced Robertsonian translocations important to recognise?

A

Because they may cause transmission of an unbalanced chromosome complement to offspring.

41
Q

What happens if a child inherits an unbalanced Robertsonian translocation?

A

It causes trisomy or monosomy for the affected chromosome

42
Q

What do the most frequently encountered Robertsonian translocations involve?

A

Chromosome 21