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A genetic counselor made the pedigree below to help a family understand its history of cystic fibrosis, a serious genetic disorder. couple 3–4 does not understand how they could have two children

Respuesta :

I assume here that they have two children that have CF. 

This would mean that both parents are carriers:

C= normal CFTR gene
c = damaged CFTR gene
CC= not a carrier
Cc=carrier 
cc=has CF (disease)

With 2 offsprings that are cc, both parents must be carriers:
Cc x Cc  gives CC (0.25 chance), Cc (0.50 chance), cc (0.25 chance). 
They were unlucky in that both times the child they had was cc. It's a game of chance.