Molecular Biology & Genetics 3- Errors in Meiosis & X inactivation

Question 1

What is aneuploidy?

Answer 1

An abnormal number of a particular chromosome

Question 2

What is an example of aneuploidy?

Answer 2

Down syndrome which means individuals have 3 of chromosome 21 (trisomy 21)

Question 3

How does age of the mother affect Down syndrome?

Answer 3

The frequency of cases increases as the age of the mother increases and 40% of cases are in mothers over the age of 45

Question 4

What is nondisjunction?

Answer 4

The failure of chromosomes to separate properly during meiosis. It may occur during Meiosis I or Meiosis II and effects the gametic products

Question 5

What are the gametic products if nondisjunction occurs during Meiosis I?

Answer 5

2 of the resulting games have an extra chromosome (n+1) and 2 have one less chromosome (n-1)

Question 6

What are the gametic products if nondisjunction occurs during Meiosis II?

Answer 6

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)

Question 7

What is the most common composition of chromosome 21 for Down syndrome babies?

Answer 7

95% of Down syndrome babies have 2 maternal and 1 paternal chromosome 21

Question 8

What is Klinefelter syndrome XXY?

Answer 8

When individuals have one too many sex chromosome

Question 9

What is the phenotype of an individual with Klinefelter Syndrome XXY?

Answer 9

Male with a slightly feminine look

Question 10

What is Turner syndrome XO?

Answer 10

Individuals have one less sex chromosome

Question 11

What is the phenotype of an individual with Turner syndrome XO?

Answer 11

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

Question 12

What is polyploidy?

Answer 12

When individuals have a set of chromosomes too many

Question 13

What is polyploidy common in?

Answer 13

Plants

Question 14

How does polyploidy occur?

Answer 14

When the gametes produced are unreduced (haven’t gone through meiosis) and so fertilisation could make a tetraploid zygote

Question 15

What is the fertility of polyploid gametes?

Answer 15

They are likely to be fertile because each chromosome has something to pair up with

Question 16

What is autopolyploid?

Answer 16

When self-fertilisation occurs

Question 17

What must a polypoid of two species have?

Answer 17

4n so that each chromosome has something to pair up with

Question 18

What are the four types of chromosomal aberrations?

Answer 18

Deletion, Duplication, Inversion and translocation

Question 19

What is deletion?

Answer 19

When a segment of chromosome is removed

Question 20

What is duplication?

Answer 20

When a segment of chromosome is repeated

Question 21

How can deletion and duplication occur?

Answer 21

During crossing over if not done correctly

Question 22

What is inversion?

Answer 22

When a segment of chromosome is reversed

Question 23

What is translocation?

Answer 23

When a segment is moved from one chromosome to another non-homologous chromosome. It can be reciprocal where segments swap positions

Question 24

Why may inversion cause problems in meiosis?

Answer 24

Although the genes are all still present, problems may be caused during meiosis because the chromosomes don’t always match correctly

Question 25

What happens as a result of crossing over of inverted segments?

Answer 25

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

Question 26

What is the effect of translocation on meiosis?

Answer 26

When parts of chromosomes move to another location it means they can’t be regulated properly (turned on and off)

Question 27

What are examples of translocation?

Answer 27

Philadelphia Translocation where parts of chromosome 9 and 22 swap and Familial Down syndrome where parts of chromosome 14 and 21 fuse

Question 28

What happens in Philadelphia Translocation?

Answer 28

95% of patients get chronic myeloid leukaemia but TK over-expression is treatable with Gleevec in 90% of cases

Question 29

What happens in Familial Down syndrome?

Answer 29

Some parts of the fused chromosomes are also lost. This means there isn’t always a full extra chromosome 21

Question 30

What are the possible zygotes from someone with a fused chromosome 14 and 21?

Answer 30

Lethal, Downs syndrome, translocation carrier and normal

Question 31

What is the difference between Down syndrome and familial Down syndrome?

Answer 31

Familial Down syndrome may be passed on to offspring because of the fact that it produces carrier gametes

Question 32

What is a Barr body?

Answer 32

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

Question 33

What is added to the inactive X chromosome and why?

Answer 33

Molecular markers to signal to the cell that it should not be used

Question 34

What is random?

Answer 34

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

Question 35

What is mapped to the X chromosome?

Answer 35

Many genetic diseases, however, most do not show a mosaic effect in carrier females because the gene products can move around the body