Question

cystic fibrosis is an inherited disease caused by a recessive defective gene that causes thick, sticky mucus to build up in the lungs and digestive tract. it is one of the most common genetic disorders in caucasians.
mr. and mrs. oconnor have three children, two sons and a daughter. after going through a series of genetic tests years ago during adolescence, it was conclusive that their eldest son michael had this defective gene. their daughter aileen was shown to be a carrier for the disorder, and their youngest son showed no signs of having the gene.
aileen eventually married and had two daughters, one of which was recently diagnosed with cystic fibrosis. using the space below, construct a pedigree that shows the inheritance of this recessive gene within the oconnor family.

1. oconnor family pedigree for cystic fibrosis:

1. make some adjustments to your pedigree by shading those that are most certainly heterozygous for cystic fibrosis.
2. provide evidence from your pedigree that this a recessive trait.
3. provide evidence from your pedigree that this is an autosomal (and not x - linked) trait:

Explanation:

Step1: Define pedigree symbols

Standard symbols: Square = male, Circle = female, Filled = affected, Half-filled = carrier, Horizontal line = marriage, Vertical line = parent-child.

Step2: Map first generation (F1)

Mr. O'Connor (□) and Mrs. O'Connor (○): Both are carriers (since they have an affected child and a carrier child, so genotypes $Cc$ each).

Step3: Map second generation (F2)

• Eldest son Michael (■, affected, genotype $cc$)
• Daughter Aileen (○ half-filled, carrier, genotype $Cc$)
• Youngest son (□, non-carrier, genotype $CC$)
• Aileen's spouse (□ half-filled, carrier, genotype $Cc$: required to have an affected child with Aileen)

Step4: Map third generation (F3)

• Aileen's affected daughter (●, $cc$)
• Aileen's unaffected daughter (○, either $CC$ or $Cc$)

Step5: Shade confirmed heterozygotes

Half-fill Mr. O'Connor, Mrs. O'Connor, Aileen, and Aileen's spouse (all confirmed $Cc$).

Step6: Justify recessive trait

Affected child has unaffected parents.

Step7: Justify autosomal trait

Affected female has unaffected father.

Answer:

Pedigree Structure (Text-Based):

          □ (Cc)  ○ (Cc)
            Mr. O'Connor  Mrs. O'Connor
                |   |   |
        ■ (cc)  ○(Cc)  □(CC)
       Michael  Aileen  Youngest Son
                     |
            □(Cc)  ○(Cc)
             Aileen's Spouse
                |       |
            ●(cc)      ○(CC/Cc)
        Affected Daughter  Unaffected Daughter

Confirmed Heterozygotes Shaded (Half-Filled):

Mr. O'Connor, Mrs. O'Connor, Aileen, Aileen's spouse

Evidence for Recessive Trait:

Aileen and her spouse (unaffected, carriers) have an affected daughter. Recessive traits only appear when an individual inherits two defective alleles, which matches this pattern of unaffected parents producing an affected child.

Evidence for Autosomal Trait:

The affected daughter has an unaffected father. For an X-linked recessive trait, an affected female would require an affected father (who would pass the defective X chromosome), which is not the case here, proving it is autosomal.