numerical chromosomal abnormalities

Question 1

describe mitosis

Answer 1

normal form of cell division
sister chromatids identical 
each daughter cell receives 1 chromatid of each chromosome 
each chromosome behaves independently 
daughter cells genetically independent to parent 
homologues don't interact 
align as 46 separate chromosomes 
all somatic cells

Question 2

bivalents and recombination

Answer 2

homologous chromosomes align
form bivalent structure
exchange genetic material

Question 3

describe crossing over

Answer 3

reciprocal breaking and rejoining of homologous chromosomes in meiosis

results in exchange of chromosomal segments and new allele combinations

Question 4

describe meiosis

Answer 4

two phases: 1 and 2 
meiosis 1 :
- align as 23 bivalents 
- allows for chiasma formation
- pulls apart homologues from one another 
- daughter cells have 23 chromosomes 

meiosis 2

• align as independent chromosomes
• sister chromatids pulled apart
• daughter cells have 23 chromosomes
• very important to introduce natural variation via independent assortment of chromosomes and recombination
• daughter cells genetically unique

Question 5

describe karyotype

Answer 5

humans have 23 pairs of chromosomes
- 22 pairs autosomes, 1 pair sex chromosomes XX or XY

metacentric

• p and q arms even length
• 1-3, 16-18

Submetacentric
p arm shorter than q
4-12, 19-20, X
Acrocentric
Long q, small p 
p contains no unique DNA
13-15, 21-22, Y

Question 6

chromosomal changes detection

Answer 6

numerical = can detect through traditional karyotyping, FISH, QF-PCR and NGS

structural = can detect through traditional karyotyping, FISH and arrayCGH

Question 7

numerical abnormalities - definitions

Answer 7

HAPLOID:
one set of chromosomes (n=23) as in a normal gamete.
DIPLOID:
cell contains two sets of chromosomes (2n=46; normal in human)
POLYPLOID:
multiple of the haploid number (e.g. 4n=92)
ANEUPLOID:
chromosome number which is not an exact multiple of haploid number - due to extra or missing chromosome(s) (e.g. 2n+1=47) (trisomy, monosomy)

Question 8

autosomal aneuploidies

Answer 8

Trisomy 13 (Patau’s, 2 in 10,000 births)
Trisomy 18 (Edward’s, 3 in 10,000)
Trisomy 21 (Down’s, 15 in 10,000) 

Others happen but do not carry to term

Question 9

sex chromosome aneuploidies

Answer 9

Turner’s (45,X) (1 in 5000 female births)
Triple X syndrome (47,XXX) (1 in 1000 female births)
Klinefelter’s (47,XXY) (1 in 1000 male births)
47,XYY (1 in 1000 male births)

Question 10

describe mosaicism

Answer 10

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

Question 11

clinical relevance

Answer 11

mosaic phenotype thought to be less severe

difficult to assess

Question 12

partial trisomy and monosomy

Answer 12

Full monosomy arises by non disjunction

Partial monosomy/trisomy (microdeletion/duplication syndromes) far more common – mechanism different