Cystic fibrosis is caused by a recessive allele (let's call it "c"). If both parents are carriers, they have the genotype "Cc" (where "C" is the normal allele and "c" is the allele for cystic fibrosis).
To determine the probability that an offspring will have cystic fibrosis, we can use a Punnett square:
- Possible gametes from each parent: C and c
- The combinations from two parents:
C c
-------------
C | CC | Cc |
-------------
c | Cc | cc |
The genotypes of the offspring would be:
- CC (normal) - 1
- Cc (carrier but not affected) - 2
- cc (affected) - 1
From the Punnett square, we see that 1 out of 4 offspring (or 25%) will have cystic fibrosis (cc).
Therefore, the probability that an offspring will have cystic fibrosis is: b. 25%.