Obstetric genetics Flashcards
What are the different types of inheritance
- autosomal dominant
- Autosomal recessive
- x-linked recessive
Whats autosomal dominant inheritance
Heterozygous with one copy of abnormal gene are affected
Whats autosomal recessive inheritance
`homozygous with two copies of the abnormal gene are affected
Whats x-linked recessive inheritance
Males which one copy of the abnormal gene on X-chromosome are affected
Who is not affected by x-linked inheritance
Carrier women
Who can’t x-linked inherited genes be passed on from-to
Male to male
Who are affected by x-linked recessive diseases
All men who inherit the mutation
Examples of x-linked diseases
- Haemophilia
- Duchene muscular dystrophy
common examples of recessive inherited diseases
- CF
- Haemochromatis
- Beta thaloesemia
What’s the chance of being a carrier when you are not affected, but your sibling is
2/3
Formula for risk
Risk in A x Risk in B x Probability that child will inherit 2 copies of the mutation
What formula do you use when you don’t know the carrier rate
Hardy Weinburg (as long as the gene frequency is in equilibrium)
When is chronic villus sampling done
11-13 weeks
How is chronic villus sampling done
needle stuck through stomach and chromosomes are tested
risks of chronic villus sampling
1% risk of miscarriage
When is amniocentesis done
15-16 weeks
Where are amniocytes derived from
Fetal tissue and amnion
How long does chromosome analysis in amniocentesis take
2-3 weeks
Negatives of amniocentesis
Low yield of fetal DNA, 1% miscarriage
What happens in PGD
1) stimulation ovulation
2) oocyte retrieval
3) fertilization and culture
4) blastocyst biopsy on day 5
When does transfer of embryos occur in PGD
embryos are frozen when testing takes place and the transfer of embryo occurs 2 months later
What is the live birth rate of PGD
35%
What gives a persons liability to be affected?
Sum of environmental and genetic predisposition
What is recurrence risk
Empiric figures from population studies
Rules for recurrence relation in multifactorial inheritance
- people who have relatives with the condition have curve shifted to the right so more individuals cross the threshold
- risks bigger for close relatives
- Risks greater if relative is more severely affected
- risk higher if more than one relative is affected
- Someone with a high genetic liability will not be affected if the environment is favorable
- risk higher if a child of the lower risk gender is affected in the family. (i.e. autism more prevalent in boys so if female in family has it, then you are more likely to get it)
What are the screening tests offered in a routine pregnancy
- scans
- trisomy screening
When is an ultrasound done
8+ weeks
When is an NT scan done and what does it detect?
-what does an increased NT caused by
Measures width of back of babies neck
increased NT caused by:
- downs syndrome
- major congenital heart disease
When is biochemical serum screening
10-14 weeks detects beta HCG and PAPP-A
second trimester detects afp, estriol and beta HCG
Abnormal MSAFP levels and what do they indicate
<0.5 = down’s
>2.5+ neural tube defect, missed or threatened miscarriage, multiple pregnancy
When is the second ultrasound done
18-20 weeks (fetal anomaly scans)
Whats cardocentesis
Checking the fetal blood (invasive)
What does universal newborn screening check for
- CF
- PKU
- Sickle cell
- MCADD
What is polygenic inheritance
Effect of many genes
What affects polygenic inheritance
Genes at many loci
Example of polygenic inheritance
Height
What has an impact on all polygenic inheritance
environment
What are the rules for the effect on environmental polygenic iheritance
- relatives of an affected person have got a greater genetic tendency
- Risks are greatest for close relatives
- Risks greater for relatives when the person is more seriously affected
- Risk higher if more people are affected
- If there are conditions which affect one gender more (i.e. boy), and a girl in family is affected you are more likely to get it
How to reduce recurrence risk of neural tube defects
folic acid
Congenital example of multifactorial inheritance
- cleft lip
- congenital heart defect
- Talipes
Adult onset disorders of multifactorial inheritance
- Diabetes mellitus
- EPilepsy
- Glaucoma
