In this case, Cystic fibrosis is generated by a recessive allele, which means that both copies must be recessive to present the disease, on the contrary, if one allele is dominant and the other recessive the person will be a carrier but it won't show the disease, and a completely healthy person will be a dominant homozygous, so even when Joshua and Bella are healthy, both are carriers which is the punnet square we will show here.
Alleles
A: Dominant, healthy
a: recessive, CF
Parental cross:
Aa x Aa
Punnet square:
[tex]\begin{bmatrix}{\placeholder{⬚}} & A & {a} \\ {A} & {AA} & {Aa} \\ {a} & {Aa} & {aa}\end{bmatrix}[/tex]
Frequencies
Phenotypic: 3:1 = 75% healty, 25% CF
Genotypic: 1:2:1 = 25% homozygous dominant, 50% Heterozygous carrier, 25% Homozygous recessive CF.
Next we have the matching pedegree
