Unit 3 -Chromosomal Abnormalities

Question 1

describe a sub-metacentric chromoome and give examples

Answer 1

short (p) arm
long (q) arm
- examples: chromosomes 6-12, X

Question 2

what is a metacentric chromosome?

Answer 2

centromere is in middle - e.g. chromosome 1

Question 3

what is an acrocentric chromosome?

Answer 3

very short (p) arm which can be deleted without harm - examples: 14, 21, Y

Question 4

when genes are being labelled, in what order are the autosomes?

Answer 4

of decreasing size

Question 5

what are the chromosomes stained with?

Answer 5

giemsa

Question 6

how are genes labelled?

Answer 6

chromsome number, arm (either p or q), position on arm (main band and sub types)

Question 7

what is the difference between a karyogram and karyotype?

Answer 7

karyogram - depiction (image) of chromosome set

karyotype - description of chromosome set

Question 8

how is a karyotype recorded?

Answer 8

number of chromosomes, identity of sex chromosomes and anomalies

Question 9

what are the karyotypes of normal male and female?

Answer 9

Male: 46, XY
Female: 46, XX

Question 10

what is aneuploidy? how does it arise?

Answer 10

presence of extra individual chromosomes or absence of chromosomes
arises from non-disjunction during meiosis - leads to disomic gametes (two copies) and nullisomic gametes (no copy)
- when gametes fertilise, it will have three copies= trisomy or 1 copy = monosomy

Question 11

what is the karyotype for a female with down’s syndrome?

Answer 11

47, XX, +21

Question 12

what is the karyotype of a triploid male?

Answer 12

69, XXY

male has polyploidy - extra set of chromosomes

Question 13

what is the karyotype of a female with turner’s syndrome? state the consequences of this sex chromosome aneuploidy

Answer 13

45, X

short stature,infertility - incomplete ovarian development

Question 14

what is the karyotype of a male with Klinefelter’s syndrome?

Answer 14

47, XXY - extra X chromosome - infertility, small testicles

Question 15

why are the effect of sex chromosomes aneuploidies so mild?

Answer 15

X chromosome activation ensures balance in number of X-chromosome between males and females so only 1 X chromosome is active

Question 16

what type of structural abnormality is Cri du Chat syndrome? give karyotope of male wih this condition

Answer 16

deletion of short arm of chromsome 5 - development delay, cognitive impairment
46, XY, del(5p)

Question 17

what is a reciprocal translocation?

Answer 17

exchange of material between non-homologous chromosomes

Question 18

what is a robertsonian translocation? - give consequences

Answer 18

fusion between long arms of acrocentric chromosomes with loss of short arms

• risk loss/ gain of materials in gametes
• if occurs between two copies of chromosome 21 -all offspring will have down’s syndrome

Question 19

what are the two forms of prenatal diagnosis of common aneuploidies?

Answer 19

non-invasive - biochemical markers in circulation + ultrasound - produces false positives (1:9)
invasive - amniocentesis (small amount of amniotic fluid retrieved) and chromosomal analysis - definitive result

Question 20

what is the “philadelphia” chromosome?

Answer 20

reciprocal translocation between chromosome 9 and 22 = common in myeloid leukaemia (bone cancer)
- produces fusion gene = BCR-ABL - codes for tyrosine kinase- abnormal cell proliferation