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Cystic fibrosis is a disease condition that affects the lungs. This disease may result in breathing difficulty and lung infections. The accumulation of mucus results in the blockage of respiratory pathways. In particular, cystic fibrosis is a genetic condition.

An allele is different for a particular gene in which some alleles may get expressed, and some get masked. The allele that is getting masked is known as the recessive allele. It may show its expression once it is transferred to the offspring.

The recessive allele is the reason for causing the cystic fibrosis condition. Beth and Tom鈥檚 siblings, who have cystic fibrosis, have a homozygous recessive condition in their genes.

There is a possibility that Beth and Tom can be the carriers of the disease due to a recessive allele. The probability of each parent(Tom) compared to their sibling is 2/3. Both parents are the carriers, so the possibility is 录.

The probability of a child inheriting this disease is calculated as follows:$=\frac{2}{3}脳\frac{2}{3}脳\frac{1}{4}=\frac{1}{9}$

Probability

Hence, both the parents have to be the carriers to cause the disease in the child.