Solved) USMLE Genetics: Updated A + Guide Solution: Updated

What are the three things a USMLE question about genetics will ask you to do using a pedigree? (Ans- Recognize a mode of inheritance Predict the recurrence risk of a disease, say in an unborn child Identify an OBLIGATE carrier for disease How do you determine the probability of an unborn child being affected by an AR trait when you know the father is a carrier and the mother has probability X of being a carrier? (Ans- Multiply the odds of the baby being affected IF the mother was a carrier (1/4) by the chances of her BEING a carrier (X, or say 1/10)--- 1/40 How do you determine the probability of an unborn child being affected by an AR trait when you only know the Father has chance X of being and carrier and the Mother has chance Y of being a carrier (Ans- X time Y multiplied by the chances of being affected IF bother are carriers, which is 1/4 so X*Y*1/4 What is a manifesting heterozygote? (Ans- Normally X inactivation occurs in all cells of the female so one X works and the other doesn't. This is normally about 50/50 so even if half the cells dont make an enzyme, such as HGPRT, this is still enuf to get by ...IF in a rare case and due to bad luck all the good X-chromosomes get turned off...They will manifest the symptoms of HGPRT even tho they are a heterozygote What does Variable Expression mean? (Ans- The nature and severity of a phenotype varies widely between individuals (often explained by the type of mutation to blame, missense v nonsense.) ALL people affected have symptoms