Obstetric Genetics

Question 1

Common X-linked conditions

Answer 1

Becker/Duchenne Muscular Dystrophy

Haemophilia

Red-Green Colour Blindness

Question 2

Common Autosomal Recessive Disorders

Answer 2

Cystic Fibrosis

Haemochromatosis

Beta Thalessaemia

Question 3

Frequency of Cystic Fibrosis carriers and affected

Answer 3

Carriers - 1 in 25

Affected - 1 in 2000

Question 4

Point of Hardy-Weinberg Principle

Answer 4

Allows calculation of carrier rates once condition incidence is known (as long as gene frequency is in equilibrium)

Question 5

Hardy-Weinberg Principle Formula

Answer 5

If Allele A has frequency P; b has q

p²+2pq+q²=1

Derived from p+q=1

Or punnett square

Question 6

Example of Hardy-Weinberg Principle::

Calculate carrier rate of PKU - Incidence 1 in 10,000

Answer 6

q²=1/10000

q=1/100

p=99/100

2pq=2*99/10000=188/10000 ≈ 1/50

Question 7

Factors that disturb Hardy-Weinberg Principle

Answer 7

Assortative Mating - Tendency to choose similar mates (e.g. IQ)

Consanguinity - Incest/relationships between family relatives leads to increased carrier risk

High New Mutation Rate like duchenne

Mutations/Genetic Conditions with a benefit like sickle cell anaemia (malaria benefit)

Question 8

Dating Scan (prenatal)

Answer 8

As early as 8 weeks but usually 12

Shows how many foetuses, general feasibility and can also date the pregnancy

Question 9

Nuchal Translucency Scan

Answer 9

Looks at back of foetus’ neck and can indicate likelihood of chromosomal abnormalities like down syndrome

Offered to all pregnancies in the UK

Question 10

AFP Testing

Answer 10

AFP elevates when issues of foetal formation (particularly spina bifida - neural tube defect)

Question 11

Full Foetal Scan Date

Answer 11

Around 18-20 weeks

Question 12

Invasive targeted tests for foetuses

Answer 12

Chorionic Villus Sampling (CVS)

Amniocentesis

Cordocentesis (foetal blood sampling)

Question 13

CVS

Answer 13

Usually around 11-13 weeks

Putting needle into womb and taking out placental cells

Question 14

Amniocentesis

Answer 14

Usually 15-16 weeks

Putting needle and getting sample of amniotic fluid

Question 15

PGD (Pre-implantation Genetic Diagnosis)

Answer 15

Process of testing embryos produced by IVF to avoid having a child with a genetic defect

Question 16

Universal Newborn Screening

Answer 16

Blood spot screening offered to all 7 day old babies for phenylketonuria (PKU) and congenital hypothyroidism

CF, Sick Cell Anaemia and MCADD as well

Question 17

Discuss Gender Effect in polygenic inheritance

Answer 17

Take a condition like autism is more common in males;

if someone’s female relative has autism, they are more likely to have it as well as the expression of related genes must be higher