Cystic fibrosis is a recessive disorder, meaning that an individual must inherit two copies of the defective gene (one from each parent) to manifest the disease. If both parents are carriers, they each have one normal allele (N) and one cystic fibrosis allele (c), represented as Nn.
To find the probability of their offspring having cystic fibrosis, we can set up a Punnett square:
| | N | n | |--------|--------|--------| | N | NN | Nn | | n | Nn | nn |
The possible offspring genotypes from this cross are:
- 25% NN (normal, not a carrier)
- 50% Nn (normal, carrier)
- 25% nn (affected with cystic fibrosis)
Based on this Punnett square, the probability that an offspring will have cystic fibrosis (nn) is 25%.