You discover a genetic disease among the Nibblonians that presents as a chronic

Question

You discover a genetic disease among the Nibblonians that presents as a chronic runny nose. You have determine that the disease is due to a single autosomal recessive allele expressed according to classic Mendelian rules. In the following family pedigree only individual 1 presents with a chronic runny nose. Individual 2 has no family history of the disease as we can assume is homozygous dominant for the disease gene. One out of every four randomly selected individuals in your study population is a carrier for the runny nose allele. Given that individuals 5 and 6 are unaffected by the disease probability what is the probability that individual 4 is a carrier?

Explanation / Answer

R: Dominannt: Normal nose
r: Recessive: Chronic runny nose

Parents: rr (1) X RR (2)
F1: Rr (3) X RR or Rr (4)
F2: RR or Rr (5 & 6)

Random chance that 4 can be Rr = 1/4
If 4 is a heterozygote, 1 out of four individual in F2 are going to be affected. But, the observed two individuals do not show phenotype. So, they can either be carriers or homozygous dominant.
So,
Probability that 4 is a carrier = 1/4 X 1/4 X 1/4
= 1/64

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