numerical chromosomal abnormalities

Question 1

What do errors in segregation and what do they lead to?

Answer 1

• both chromatids go to one pole in anaphase (non- disjunction)
• gametes are chromosomal unbalanced so fetus will have too little or too many of a chromosome.
• tripoloid(3N), tetraploid(4N).

Question 2

Define aneuploidy?

Answer 2

missing a chromosome or having an extra chromosme

Question 3

what are 4 examples of autosomal aneuploidy?

Answer 3

1. turners syndrome - only 1 X chromosome.
2. pataeu syndrome - extra 13 (trisomy 13, 2 in 10,000 births)
3. Edward syndrome - trisomy 18 (3 in 10,000)
4. Down’s syndrome, trisomy 21 (15 in 10,000)

Question 4

Outline the process of mitosis

Answer 4

• the cell division process where somatic cells are produced
  1. interphase - chromosomes replicate themselves
  2. mitosis - PMAT
  3. cytokinesis -

Question 5

outline the process of meiosis

Answer 5

the cell division where gametes are produced.

• allows the fertilised cell to have 46 chromosomes bc each egg and sperm have 23 chromosomes which fuse to make 46.
• each somatic cell has 46 chromosomes, 23 pairs.
• 4 daughter cells formed, each with 23 chromosomes.

Question 6

What are different karyotypes?

Answer 6

1. metacentric
   - p&q arms even length
   - 1-3, 16-18
2. submetacentric
   - p arm shorter than q
   - 4-12, 19-20 , X
3. acrocentric
   - long q, short p
   - p contains no unique DNA
   - 13-15, 21-22, Y

Question 7

How many pairs of chromosomes do humans have?

Answer 7

humans have 23 pairs of chromosomes

-22 pair autosomes, 1 pair sex chromosomes XY or XX

Question 8

What are 4 numerical abnormalities definations?

Answer 8

1. haploid : one set of chromosomes(n=23) as in a normal gamete
2. Diploid: cell contain two sets of chromosomes (2n = 46; normal in human)
3. polyploid : multiple of the haploid number (eg. 4n = 92)
4. aneuploid : chromosome number which is not an exact multiple of haploid number - due to extra or missing chromosome(s) (eg. 2n+1 = 47) (trisomy, monosomy)

Question 9

What occurs during meiosis I?

Answer 9

bivalents and recombination

• synaptonemal complex keeps the homologous chromosomes stuck together and enables them to exchange genetic material (recombination/crossing over) forming combinant chromosome.
• important for genetic variations.

Question 10

What happens in the two phases of meiosis?

Answer 10

1. meiosis I

2. meiosis II

Question 11

What are examples of sex chromosome aneuploidies?

Answer 11

1. Turner’s (45,X), 1 in 5000 females
2. triple X syndrome (47, XXX), 1 in 1000 female births
3. Klinefelter’s (47, XXY) (1 in 1000 male births)
4. 47 XYY (1 in 1000 male births)

Question 12

What is mosaic?

Answer 12

• non-disjunction occurs in both meiosis and mitosis
• if non disjunction occurs in the gametes then meiosis it affects zygote and all cells
• in mitosis, occurs after zygote, only proportion of cells affected = mosaic

Question 13

outline the events in meiosis I

Answer 13

1. align as 23 bivalents
2. allows for chiasma formation (ie. recombination)
3. pulls apart homologues from one another
4. daughter cells have 23 chromosomes (each with 2 chromatids)

Question 14

Outline the events in meiosis II

Answer 14

1. align as independent chromosomes
2. sister chromatids pulled apart
3. daughter cells have 23 chromosomes (each of 1 chromatid)

Question 15

Why is it important to introduce natural variation?

Answer 15

• natural variation via independent assortment of chromosomes and recombination
• daughter cells genetically unique
• important to survive disease outbreak.

Question 16

What are clinical relevence?

Answer 16

• mosaic phenotype thought to be less severe

- difficult to assess : what are proportions of different cell types? which tissue/organs are affected?

Question 17

What is partial trisomy and monosomy?

Answer 17

• full monosomy arises by non-disjunction

- partial monosomy/trisomy (microdeletion/duplication syndromes) far more common - mechanism different.