Genetics

Question 1

Discuss trisomy 21
-Incidence
-Risk factors
-Genetic causes (2)
-Features (6)

Answer 1

1. Incidence - 1:400
2. Risk factors - maternal age
3. Genetic causes
   -Non-disfunction of chromosome 21 during meiosis (95%)
   -Balanced Robertsonian translocation (1-3%)
4. Features
   -Growth failure
   -Mental impairment
   -Broad face, slanting eyes, epicanthic eye fold
   -Big tounge
   -Congential heart disease
   -Poor muscle tone

Question 2

Discuss trisomy 18
-Incidence
-Prognosis (2)
-Features (6)

Answer 2

1. Incidence - 1:5000
2. Prognosis
   -60% die in first 2 months
   -95% die by first 12 months
3. Features
   -Prominant occiput
   -Small mouth and jaw
   -Clenched hand
   -Rocker bottom feet
   -Cardiac anomilies
   -GIT anomilies

Question 3

Discuss trisomy 13
-Incidence
-Prognosis
-Features

Answer 3

1. Incidence - 1:5000
2. Prognosis
   -97% fetal loss.
   -If born alive most die within 4 months
3. Features:
   -Defects in all the major organ systems

Question 4

Discuss translocations
-Definition
-Balanced translocations defintion
-Unbalanced translocations defintion
-Implication of balanced transloactions
-Implications of unbalanced translocations

Answer 4

1. Defintion
   -abnormal arrangement of chromosomes
2. Balanced translocation
   -Even exchange of of genetic material between chromosomes with no genetic material lost
3. Unbalanced translocation
   -Uneven exchange of genetic material resulting in missing or extra genes
4. Implications of balanced translocations
   -Nothing for the individual
   -Increased risk of miscarriage
   -Risk of unbalanced translocation in offspring
5. Implications of unbalanced translocation
   -Increased risk of aneuploidy in offspring 1-3% T21
   -Increased risk of miscarriage

Question 5

Discuss fragile X syndrome
-Inheritance type
-Incidence in males and females
-Pathophysiology
-Features

Answer 5

1. Inheritence type - thought to be dominant with variable penetrance but behaves link X-linked ressessive
2. Incidence
   -Males - 1:4000
   -Females - 1:8000
3. Pathophysiology
   -Due to increase in CGG repeats on the X chromosome
   -Normal to have 6-50 CGG repeats
   -50-200 CGG repeats = pre-mutation
   ->200 CGG repeats = full mutation
4. Features
   -Most common cause of mental impairment: Autism
   -Prominent forehead and jaw
   -Large ears and long face
   -Murmur
   -Ovarian dysfunction in fragile X pre-mutation