Q: A couple seek a genetic counsellor's advice about their chances of having a child with the autosomal recessive condition of Tay Sachs disease. Both the female and the male have a sibling with Tay Sachs but no other family members have the disease. The advice would be that the probability of their child having Tay Sachs would be?
The answer says 1/9, but I'm a bit confused as to how they ended up at that answer.....
These questions are hard to do before you know how but fairly simple after. You have to work out the possible genotypes of the man/woman and then the probability they'll have a kid with the condition. It's complicated because you can't actually determine what their genotype is, only what possibilities it could be.
TSD is autosomal recessive, so the parents of each member of the couple must both be carriers (Rr).
Therefore, there is a 2/3 chance that the male partner is a carrier (rR or Rr) and a 66% chance the female partner is a carrier. Since they don't have TSD, there are only three possible genotypes, which is why it's out of 3 (rr is impossible). The probability of them being carriers is independent so 2/3 * 2/3 = 4/9
If they're both carriers, there is a 1/4 chance that they have an affected child. Therefore 4/9 * 1/4 = 4/36 = 1/9
You should try not to give extra info. If any of that info is wrong, you'll be marked down. If it's correct, you won't be. It's best to resist showing off though.
VCAA don't mark you down if you give extra, wrong information unless something has changed. They're usually instructed to only mark the first X things written on the paper though.