The following image is a pedigree for a family with the genes for cystic fibrosis, which is an autosomal recessive trait. if the dominant allele is represented by a and the recessive allele is represented by a, what would be a possible genotype and phenotype of individual ii.3? a. genotype: aa phenotype: carrier b. genotype: aa phenotype: unaffected c. genotype: aa phenotype: unaffected d. genotype: aa phenotype: affected

The answer should be C, genotype as AA and unaffected by cystic fibrosis.

Since both parents are both carriers of the cystic fibrosis allele but is not affected, the allele of the disease causing gene must be recessive thus is represented by small letter a. Only if the two alleles are both recessive can cause the disease, therefore the genotype of the parents are both Aa, with one dominant normal allele. With that being said, their offsprings must be either AA, aa, or Aa.

From the 4 options given, option A is incorrect because if both alleles are the normal A dominant allele, there’s no disease causing allele (a) so the individual will not be a carrier.

Option b is incorrect because if both alleles are the disease causing recessive a, they’re have cystic fibrosis. Only in this case the individual will actually be affected.

Option d is also incorrect because A is dominant to a so A is the gene that will show in the phenotype of the individual, thus the individual should be normal and just a carrier instead of being affected.

Option c is the correct answer, as there’s no disease causing recessive allele a.