Risk Calculations Flashcards
What probabilities need to be considered when calculating the risk of a carrier female and normal male having a child affected with an X-linked recessive disorder?
The child will only be at risk of being affected if it is male AND inherits the affected X-chromosome from the mother. The chance of it being male is 1/2 and the chance of it inheriting the faulty X-chromosome is 1/2 - therefore the overall risk of such a couple having an affected child is 1/4.
An individual is at a population risk of carrying CF or 1 in 25. They are screened using a screen which excludes 90% of known mutations. What is this individuals new risk?
9 in 10 mutations are excluded. 1 in 10 mutations are not excluded.
This individuals new risk has to take into account the probability of being a carrier (1/25) and the probability of testing negative for CF mutations on our screen even though they are actually a carrier (1/10).
The probability of him being a carrier AND testing negative therefore is 1/25 x 1/10 = 1/250
If we know an offspring is not affected by an autosomal recessive disorder, but both of their parents are known carriers, what is their risk of being a carrier?
From a punnet square there are 3 out of 4 possible outcomes that could lead to the offspring not being affected. 2 out of these 3 outcomes would result in the individual being a carrier, 1 out of 3 would be normal and a non-carrier. The risk of an unaffected individual with 2 carrier parents being a carrier is 2/3.
In Bayes what is the initial probability of an event known as?
The prior probability.
What does Bayes allow us to do?
Bayes allow us to take into account new information to modify risks.
It essentially allows us to work out the probability of an outcome occurring based on the initial probability of the event and taking into account new information that is likely to modify the outcome.
Watch Risk Calculation Lectures again. Flash cards not good for these.
Watch.
An individual is at risk of being heterozygous for an HD mutation. The individual is 40y/o and has not yet shown any symptoms. About 80% of people carrying the mutation would be expected to be symptomless at this age. What would be the conditional probabilities included in the Bayesian analysis table?
1) . The conditional probability of the individual being asymptomatic but still being het for the mut is 8/10 as we know this would be the case in 80% of people with the mutations.
2) . The conditional probability of the individual not having the mut and being asymptomatic is 1 (10/10) as if he did not have the mutation we would never expect him to show any symptoms.
Can we perform risk estimation for complex disorders?
Yes, but where the modes of inheritance are unknown we need to rely on empirical tables of recurrence risks to estimate risk of a second affected child.
e.g. Risk of a second child affected by spina bifida to a couple with one affected child = 4%.
If a child of 2 CF carrier parents is not symptomatic for CF what is the child’s prior risk of being a CF carrier before being tested for CFTR mutations?
The child’s prior risk is 2/3. If we draw a punnet square we see that there is a 1/4 chance of the child being affected, a 1/4 chance of the child being totally free of the CFTR mutation, and a 2/4 chance of the child being a carrier. However, as the child is not symptomatic we can exclude this risk from our calculations. We are left with 3 options. There is now a 1/3 chance that the child is totally free of CFTR mutations and a 2/3 chance of the child being a carrier.
Watch lectures again.
See ibooks.
