Pedigree attached
Answer:
1) 2/3
2) 1/2
3) 1/12
Explanation:
1) Individual III.1 is affected. He inherited one r allele from his carrier mother, but must also have inherited another allele from his father, who's genotype we are not told. His father is unaffected, but because III.1 is affected, must be a carrier with the genotype Rr. Therefore, the parents of III.3 have are Rr x Rr. We can carry out a punnett square to assess probabilities
R r
R RR Rr
r Rr rr
The probability of him inheriting an r from either parent is 1/2 (as there is 50% chance of being Rr in the punnet square). However, we already know that he is not rr (he is unaffected and has a R allele), so that means 2/3 of the options involve him being a carrier
2) The probability III.4 is a carrier can be calculated because we have the genotypes of her parents, RR x Rr
R R
R RR RR
r Rr Rr
The probability III.4 is a carrier is 1/2 but with no possibility that she is affected.
3) If both parents were carriers, there would be a 1/4 chance that individual IV.1 would be affected, see punnet square below:
R r
R RR Rr
r Rr rr
Only rr genotype is affected, so the probability is 1:4. However, there is only 1/2 probability that their mother is affected and 1/2 that their father is. To work out their probability based on this we have to multiply the probabilities: 2/3 x 1/2 x 1/4 = 1/12
