Genetics 1 : Chromosomes, genetic counselling and ethics 1

Question 1

What are chromosomes ?

Answer 1

Vessels of genetic inheritance and allow cellular DNa to be stabalised.

Question 2

what happens in chromosomes during mitosis?

Answer 2

DNA replication
chromosome segregation
stable maintenance of genetic material through cell division

Question 3

what happens in the chromosomes in meiosis?

Answer 3

two divisions – meiosis I and meiosis II
reduction of diploid to haploid
chromosome complement

Question 4

what are key things that cause genetic variation in meiosis?

Answer 4

genetic recombination/synapsis:
faithful segregation
exchange of genetic material

Question 5

describe the chromosome structure

Answer 5

short (p) arm, long (q) arm
telomere: protect ends
centromere:
holds sister chromatids together 
chromosome segregation

Question 6

define metacentric,

sub-metcentric and acrocentric

Answer 6

Metacentric - centromere placed in middle
Sub-metacentric - closer to one telomere
Acrocentric - centromere near telomere

Question 7

what is the human karyotype?

Answer 7

2 haploid sets of 23 chromosomes
44 autosomes, 2 sex chromosomes
kayotyping: G-banded, classified according to size

Question 8

what are the two type of chromosomal abnormalities?

Answer 8

Aneuploidy – too many or too few chromosomes

Structural – chromosome rearrangements

Question 9

what are the two types of rearrangements?

Answer 9

Balanced rearrangements – translocations, inversions

Unbalanced rearrangements – deletions, duplications

Question 10

what is the most common cause of aneuploidy?

Answer 10

non-disjunction during maternal meiosis I is the most common cause of aneuploidy

Question 11

Describe what Trisomy 21 is and what the features are?

Answer 11

most common autosomal abnormality
- 1:700-1:1000 live births
maternal age effect: 1:35 at 43
learning difficulties, dysmorphic features, heart defects

Question 12

what are the symptoms of Down’s ?

Answer 12

learning disabilities: mild to moderate
Dysmorphic features
Cardiac defects (40%)
Alzheimer’s 20-30 years earlier

Question 13

Describe Trisomy 18 and some of the features?

Answer 13

severe mental disability
dysmorphic features: Micrognathia, prominent occiput
rocker-bottom feet, clenched fists with overlapping fingers
congenital heart defects
many die within weeks, 90% within 1 year
incidence: 1:3000
maternal age effect: 1:500 at age 43
primary trisomy 18: 47,XX,+18 or 47,XY,+18

Question 14

Describe trisomy 13 and some of the features?

Answer 14

Severe mental disability
Dysmorphic features (cleft lip and palate, polydactyly, holoprosencephaly)
renal abnormalities, cardiac defects
many die within weeks, 95% within 1 year
incidence: 1:5000 live births ; maternal age effect: 1:1100 at 43
primary trisomy 13: 47, XY, +13 (or 47, XX, +13)

Question 15

what is the biggest risk factor for trisomy?

Answer 15

main risk factor for trisomy is maternal age

Question 16

what is the genetic abnormality in Klinefelter’s ?

Answer 16

XXY

Question 17

what are the signs and sx of Klinefelter’s ?

Answer 17

tall stature, long limbs
gynaecomastia (30-50%)
small testes, sometimes Crypthochirdism
infertility, azoospermia
Delayed or incomplete puberty
incidence: 1:500 to 1:1000 newborn males – often undiagnosed

Question 18

what is the chromosomal abnormality in Turner Syndrome?

Answer 18

45,X monosomy 40-60%

Question 19

what are the signs and sx of turner’s?

Answer 19

short stature
infertility
1 in every 2,000 live female births
maternal age is not a risk factor

Question 20

what are the sx of XYY syndrome?

Answer 20

often asymptomatic
symptoms may include learning disabilities, speech delay, low muscle tone (hypotonia), and being taller than expected
usually normal sexual development, and normal fertility, sometimes develop testicular failure

Question 21

what are the sx of triple X (XXX) syndrome?

Answer 21

largely asymptomatic
in 10% seizures or kidney abnormalities
may be taller but usually does not cause unusual physical features, nor infertility
increased risk of learning disabilities and delayed development of speech and language skills
incidence: 1:1000 newborn females

Question 22

what is the mechanism of X-inactivation aka lysonization?

Answer 22

begins X inactivation centre (XIC)
X-inactive specific transcript RNA (Xist) expressed on inactive X
Xist transcripts coat X chromosome leading to inactivation

Question 23

what is a robertsonian translocation?

Answer 23

fusion of two acrocentric chromosomes
most common: der(13;14), der(14;21)
phenotype normal in balanced carriers but reproductive risks

Question 24

what is translocation Down’s?

Answer 24

translocation Down’s : fusion of long arms of chr 14 and chr 21

Question 25

Describe reciprocal translocation

Answer 25

two-way exchange of material between any two non-homologous chromosomes

Question 26

what is chromosomal inversion?

Answer 26

chromosome segment reversed and re-inserted
may be balanced or unbalanced
may involve centromere or arm

Question 27

what is an isochromosome?

Answer 27

centromere fission

Question 28

what is a ring chromosome?

Answer 28

telomeric region lost, arms fuse

Question 29

what is duplication?

Answer 29

More than one copy of chrosome segment

Question 30

what is deletion ?

Answer 30

chromosome segment is lost

can be large or small

Question 31

what type of syndrome is cri du chat?

Answer 31

deletion syndrome

Question 32

what are the signs and sx of cri du chat?

Answer 32

Cri-du-chat named for high-pitched cry
incidence  1:20,000 to 50,000 newborns
most de novo, 10% inherited
slow growth, microcephaly
moderate to severe mental disability