Topic 14.2: Chromosomal abnormalities

Question 1

what are the two types of chromosomal abnormalities

Answer 1

numerical and structural

Question 2

what are the two types of chromosomal abnormalities considering their consequences

Answer 2

balanced and unbalanced

Question 3

what is balanced abnormalities

Answer 3

no genetic information lost and/or no phenotypic changes

Question 4

what is unbalanced abnormalities

Answer 4

anything that is not balanced

Question 5

what are some numerical abnormalities

Answer 5

POLYPLOIDY
gain of extra haploid set of chromosomes (23)
eg: triploidy - 3n=69
most commonly caused by polyspermy - two sperm fertilize one egg
occurs in pregnancies and causes around 15% of miscarriages
ANEUPLOIDY
loss or gain of whole chromosome
this is trisomies and monosomies
caused by meiotic non disjunction
MOSAICISM
presence of two or more cell lines in an individual
throughout the body or tissue specific
caused by mitotic non disjunction

Question 6

what are examples of triosomies that are viable for life

Answer 6

downs syndrome 47(+21)
patau syndrome 47 (+13)
edwards syndrome 47 (+18)
klinefelter syndrome 47 (XXY)` (males)

Question 7

what are examples of monosomies that are viable for life

Answer 7

turner syndrome 45, X (females)

Question 8

what are the types of structural abnormalities

Answer 8

deletion or microdeletions- of a whole part of a chromosome
duplication - of a whole part of a chromosome
inversion - part of chromosome swapped. either paracentric - not involve centromere or pericentric - over the centromere
substitution/insertion - one part of one chromosome inserted into another
translocation - two chromosomes swap information, either reciprocal or robertsonian
isochromosomes - one chromosome has two p arms and the other of the same pair has two q arms
ring chromosomes - can fuse to form a ring
marker chromosome - extra bit of DNA on top of 46 chromosome, genes on them can cause disease

Question 9

what are the two types of translocation

Answer 9

reciprocal or roberstonian

Question 10

what is reciprocal translocation

Answer 10

where a bit of chromosome swaps from one chromosome to another
- can be balanced (all info is there) or unbalanced (causes disease)
instead of forming two bivalents during meiosis, forms quadrivalent (sequence similarities combine), which when segregated will have 3 possible ways, 6 gametes (alternate, homologous, non homologous) and will result in a 3:1 non disjunction (unbalanced:balanced). balanced = alternate, contains all the correct amount of information

Question 11

what is the karyotype of a normal male

Answer 11

46, XY

Question 12

what is the karyotype of a normal female

Answer 12

46, XX

Question 13

what is robertsonian translocation

Answer 13

2 acrocentric chromosomes fuse (small p arm)
q arms fuse and become one chromosome and p arms lost
results in 45 chromosomes in balanced carriers
females higher risk than males
problem during meiosis not mitosis
trivalent will form during meiosis, segregation with 6 possible outcomes
when mixed with normal chromosomes from other gamete (sex cell) = aneuploidy risk

Question 14

what can robertsonian translocation cause

Answer 14

downs syndrome as happens on chromosome 21

Question 15

what does balanced mean

Answer 15

lost DNA but not lost genetic information as p arms contain very little information so phenotype unaffected

Question 16

how can robertsonian translocation cause downs syndrome

Answer 16

3 copies of chromosome 21 in one of the gametes as two q arms fuse plus a copy from a normal chromosome

Question 17

what is cytogenetic testing

Answer 17

analysis = karyotyping
flourescent in situ hybridisation
microarray hybridisation
DNA sequencing
and others

Question 18

why do we perform cytogenetic testing

Answer 18

• diagnosis and prognosis of clinical problems - syndrome, account for pregnancy loss
• better clinical management - hormone treatment
• access future reproductive risks - downs
• prenatal diagnosis

Question 19

why would someone want cytogenetic testing

Answer 19

prenatal diagnosis
birth defects
abnormal sexual development
infertility
recurrent fetal loss
leukemia
solid tumors
prognostic information

Question 20

what does chromsomal inversion cause

Answer 20

Inversion does not cause the loss of genetic information. It simply rearranges the gene order or the gene sequence. There are two types of inversions namely paracentric and pericentric inversions. A paracentric inversion occurs in one arm of the chromosome without involving the centromere. Both arms of the chromosome break including the centromere during the pericentric inversion.

Question 21

Non invase prenatal testing tests for

Answer 21

downs
fetal sex
single gene disroders

Question 22

what does wild type allele mean

Answer 22

normal

Question 23

how are mRNA missense detected

Answer 23

NMD

Question 24

what would happen if premature stop codon in mRNA

Answer 24

protein degraded rather than shorter