Numerical Chromosomal Abnormalities

Question 1

What structure is most important in forming the bivalents?

Answer 1

Symptonemal complex

Question 2

What is the kinetochore?

Answer 2

The structure that forms around the centromere which is crucial for correct spindle fibre connection.

Question 3

Describe the normal human karyotype.

Answer 3

Humans have 23 pairs of chromosomes
- 22 pairs autosomal, 1 pair sex chromosomes

Metacentric (eg. 1-3)
- q+p arms even length

Sub metacentric (eg. X chromosome)
- p arm shorter than q

Acrocentric (eg. Y chromosome)
- long q and small p
- p contains no unique DNA

Question 4

What is meant by haploid?

Answer 4

One set of chromosomes as in a normal gamete (n=23)

Question 5

What is meant by diploid?

Answer 5

Cell contains two sets of chromosomes (2n = 46)

Question 6

What is meant by polyploid?

Answer 6

Multiple of haploid number (4n = 92)

Question 7

What is meant by aneuploid?

Answer 7

Chromosomes number which is not an exact multiple of haploid number - due to extra or missing chromosome (2n + 1 = 47)

Question 8

Describe what is meant by meiotic non-disjunction?

Answer 8

An error in the chromosomal or chromatid segregation
Imbalance of chromosomes in the resulting zygote

Aneuploidy

Question 9

Describe what happens during recombination.

Answer 9

Homologous chromosomes align
Form a bivalent structure
Chiasma forms
Exchange genetic material
Recombinant chromosomes form as a result

Question 10

What is chiasma?

Answer 10

The point at which paired chromosomes remain in contact during crossing over.

Question 11

What is independent assortment?

Answer 11

Maternal and paternal chromosomes are separated randomly during meiosis 1 metaphase 1.

This allows for greater variation in the gametes formed

Question 12

What are the most common autosomal aneuploidies?

Answer 12

Trisomy 13 - patau’s
Trisomy 18 - Edward’s
Trisomy 21 - Downs

Others happen but do not carry to term

Question 13

What are the most common sex chromosome aneuploidies?

Answer 13

Turners - (45,X)

Triple X syndrome - (47,XXX)

Klinefelter’s - (47, XXY)

47, XYY

Question 14

What is post zygotes non disjunction?

Answer 14

When non disjunction occurs at mitosis

Only effects small amount of cells are effected by aneuploidy - mosaicism

Question 15

What is mosaicism?

Answer 15

The presence of two or more genetically different cell lines derived from a single zygote.

Occurs as a result of non disjunction in mitosis, or Trisomic Zygots

Question 16

How can mosaicism occur from maternal non disjunction?

Answer 16

During meiosis maternal non disjunction occurs causing the formation of (2m) in the gamete.

After fertilisation a Trisomic zygote forms

This causes some cells to contain both copies of the maternal chromosomes (2m)

Question 17

What is the biological relevance of mosaicism?

Answer 17

Mosaic phenotype is thought to be less severe.

But difficult to assess,
- as the symptoms vary widely
- difficult to predict which tissues and organs are effected
- and the proportion of different cell types

Question 18

How does monopsony arise?

Answer 18

By non disjunction

Question 19

How does partial monosomy/trisomy arise?

Answer 19

Micro-deletion and duplication syndromes

Far more common